fisica 05

Chapter 5 Torque and Rotational Equilibrium Physics, 6th Edition

Chapter 5. Torque and Rotational Equilibrium

Unit Conversions

5-1. Draw and label the moment arm of the force F about an axis at point A in Fig. 5-11a. What

is the magnitude of the moment arm?

Moment arms are drawn perpendicular to action line:

rA = (2 ft) sin 250 rA = 0.845 ft

5-2. Find the moment arm about axis B in Fig. 11a. (See figure above.)

rB = (3 ft) sin 250 rB = 1.27 ft

5-3. Determine the moment arm if the axis of rotation is at point A in Fig. 5-11b. What is the

magnitude of the moment arm?

rB = (2 m) sin 600 rB = 1.73 m

5-4. Find the moment arm about axis B in Fig. 5-11b.

rB = (5 m) sin 300 rB = 2.50 m

Torque

5-5. If the force F in Fig. 5-11a is equal to 80 lb, what is the resultant torque about axis A

neglecting the weight of the rod. What is the resultant torque about axis B?

Counterclockwise torques are positive, so that A is - and B is +.

(a) A = (80 lb)(0.845 ft) = -67.6 lb ft (b) B = (80 lb)(1.27 ft) = +101 lb ft

5-6. The force F in Fig. 5-11b is 400 N and the angle iron is of negligible weight. What is the

resultant torque about axis A and about axis B?

Counterclockwise torques are positive, so that A is + and B is -.

(a) A = (400 N)(1.732 m) = +693 N m; (b) B = (400 N)(2.50 m) = -1000 N m

36

3 ft

2 ftrB

BA

250

F

rA

250

2 m

5 m

rBrA

600 B

300

A

F


5-7. A leather belt is wrapped around a pulley 20 cm in diameter. A force of 60 N is applied to

the belt. What is the torque at the center of the shaft?

r = ½D = 10 cm; = (60 N)(0.10 m) = +6.00 N m

5-8. The light rod in Fig. 5-12 is 60 cm long and pivoted about point A. Find the magnitude and

sign of the torque due to the 200 N force if is (a) 900, (b) 600, (c) 300, and (d) 00.

= (200 N) (0.60 m) sin for all angles:

(a) = 120 N m (b) = 104 N m

(b) = 60 N m (d) = 0

5-9. A person who weighs 650 N rides a bicycle. The pedals move in a circle of radius 40 cm. If

the entire weight acts on each downward moving pedal, what is the maximum torque?

= (250 N)(0.40 m) = 260 N m

5-10. A single belt is wrapped around two pulleys. The drive pulley has a diameter of 10 cm,

and the output pulley has a diameter of 20 cm. If the top belt tension is essentially 50 N at

the edge of each pulley, what are the input and output torques?

Input torque = (50 N)(0.10 m) = 5 N m

Output torque = (50 N)(0.20 m) = 10 N m

Resultant Torque

5-11. What is the resultant torque about point A in Fig. 5-13. Neglect weight of bar.

= +(30 N)(6 m) - (15 N)(2 m) - (20 N)(3 m)

= 90.0 N m, Counterclockwise.

37

F

A200 N

r

60 cm

30 N

2 m

15 N

20 NA

4 m 3 m


5-12. Find the resultant torque in Fig. 5-13, if the axis is moved to the left end of the bar.

= +(30 N)(0) + (15 N)(4 m) - (20 N)(9 m)

= -120 N m, counterclockwise.

5-13. What horizontal force must be exerted at point A in Fig 5-11b to make the resultant torque

about point B equal to zero when the force F = 80 N?

= P (2 m) – (80 N)(5 m) (sin 300) = 0

2 P = 200 N; P = 100 N

5-14. Two wheels of diameters 60 cm and 20 cm are fastened together and turn on the same axis

as in Fig. 5-14. What is the resultant torque about a central axis for the shown weights?

r1 = ½(60 cm) = 0.30 m ; r2 = ½(30 cm) = 0.15 m

= (200 N)(0.30 m) – (150 N)(0.15 m) = 37.5 N m; = 37.5 N m, ccw

5-15. Suppose you remove the 150-N weight from the small wheel in Fig. 5-14. What new

weight can you hang to produce zero resultant torque?

= (200 N)(0.30 m) – W (0.15 m) = 0; W = 400 N

5-16. Determine the resultant torque about the corner A for Fig. 5-15.

= +(160 N)(0.60 m) sin 400 - (80 N)(0.20 m)

= 61.7 N m – 16.0 N m = 45.7 N m

R = 45.7 N m

5-17. Find the resultant torque about point C in Fig. 5-15.

= - (80 N)(0.20 m) = -16 N m

*5-18. Find the resultant torque about axis B in Fig. 5-15.

38

30 N

2 m

15 N

20 N

A 4 m 3 m

2 m

5 m

rB

B

300

P

F = 80 N

C

B

A

80 N

400

20 cm60 cm

r400

160 N

C

80 N

400

20 cm60 cm

r

160 N

Fx

Fy

B80 N

400

20 cm60 cm

160 N


Fx = 160 cos 400; Fy = 160 sin 400

= – (123 N)(0.2 m) + (103 N)(0.6 m) = 37.2 N m

Equilibrium

5-19. A uniform meter stick is balanced at its midpoint with a single support. A 60-N weight is

suspended at the 30 cm mark. At what point must a 40-N weight be hung to balance the

system? (The 60-N weight is 20 cm from the axis)

= 0; (60 N)(20 cm) – (40 N)x = 0

40 x = 1200 N cm or x = 30 cm: The weight must be hung at the 80-cm mark.

5-20. Weights of 10 N, 20 N, and 30 N are placed on a meterstick at the 20 cm, 40 cm, and 60

cm marks, respectively. The meterstick is balanced by a single support at its midpoint. At

what point may a 5-N weight be attached to produce equilibrium.

= (10 N)(30 cm) + (20 N)(10 cm)

– (30 N)(10 cm) – (5 N) x = 0

5 x = (300 + 200 –300) or x = 40 cm

The 5-N weight must be placed at the 90-cm mark

5-21. An 8-m board of negligible weight is supported at a point 2 m from the right end where a

50-N weight is attached. What downward force at the must be exerted at the left end to

produce equilibrium?

F (6 m) – (50 N)(2 m) = 0

6 F = 100 N m or F = 16.7 N

39

20 cmx

40 N60 N

10 cm

30 cm

10 N 20 N

x

5 N30 N

50 N

F6 m 2 m


5-22. A 4-m pole is supported at each end by hunters carrying an 800-N deer which is hung at a

point 1.5 m from the left end. What are the upward forces required by each hunter?

= A (0) – (800 N)(1.5 m) + B (4.0 m) = 0

4B = 1200 N or B = 300 N

Fy = A + B – 800 lb = 0; A = 500 N

5-23. Assume that the bar in Fig. 5-16 is of negligible weight. Find the forces F and A provided

the system is in equilibrium.

= (80 N)(1.20 m) – F (0.90 m) = 0; F = 107 N

Fy = F – A – 80 N = 0; A = 107 N – 80 N = 26.7 N

F = 107 N, A = 26.7 N

5-24. For equilibrium, what are the forces F1 and F2 in Fig. 5-17. (Neglect weight of bar.)

= (90 lb)(5 ft) – F2 (4 ft) – (20 lb)(5 ft) = 0;

F2 = 87.5 lb Fy = F1 – F2 – 20 lb – 90 lb = 0

F1 = F2 +110 lb = 87.5 lb + 110 lb, F1 = 198 lb

5-25. Consider the light bar supported as shown in Fig. 5-18. What are the forces exerted by the

supports A and B?

= B (11 m) – (60 N)(3 m) – (40 N)( 9 m) = 0;

B = 49.1 N Fy = A + B – 40 N – 60 N = 0

A = 100 N – B = 100 N – 49.1 N; B = 50.9 N

5-26. A V-belt is wrapped around a pulley 16 in. in diameter. If a resultant torque of 4 lb ft is

required, what force must be applied along the belt?

R = ½(16 in.) = 8 in. R = (8/12 ft) = 0.667 ft

F (0.667 ft) = 4 lb ft; F = 6.00 lb

40

F

800 N

BA

2.5 m1.5 m

Axis

80 N

F

A

90 cm30 cmAxis

20 lbF2

5 ft

Axis

1 ft

90 lb

F1

4 ft

B

3 m

Axis

40 N

2 m

60 N

A6 m


5-27. A bridge whose total weight is 4500 N is 20 m long and supported at each end. Find the

forces exerted at each end when a 1600-N tractor is located 8 m from the left end.

= B (20 m) – (1600 N)(8 m) – (4500 N)( 10 m) = 0;

B = 2890 N Fy = A + B – 1600 N – 4500 N = 0

A = 6100 N – B = 6100 N – 2890 N; B = 3210 N

5-28. A 10-ft platform weighing 40 lb is supported at each end by stepladders. A 180-lb painter

is located 4 ft from the right end. Find the forces exerted by the supports.

= B(10 ft) – (40 lb)(5 ft) – (180 lb)( 6 ft) = 0;

B = 128 lb Fy = A + B – 40 lb – 180 lb = 0

A = 220 lb – B = 220 lb – 128 lb; A = 92.0 lb

*5-29. A horizontal, 6-m boom weighing 400 N is hinged at the wall as shown in Fig. 5-19. A

cable is attached at a point 4.5 m away from the wall, and a 1200-N weight is attached to

the right end. What is the tension in the cable?

= 900 – 370 = 530; Ty = T sin 530

= (T sin 530)(4.5 m) – (400 N)(3 m) – (1200 N)(6 m) = 0;

3.59 T = 1200 N + 7200 N; T = 2340 N

*5-30. What are the horizontal and vertical components of the force exerted by the wall on the

boom? What is the magnitude and direction of this force?

Fx = H – Tx = 0; H – T cos 530 = 0; H = (2340 N) cos 530; H = 1408 N

Fy = V + T sin 530 – 400 N – 1200 N = 0; V = 1600 N – (2340 N) sin 530 = -269 N

Thus, the components are: H = 1408 N and V = -269 N. The resultant of these is:

R = 1434 N, 349.20

Center of Gravity

41

B

10 m

Axis

4500 N

2 m

1600 N

A8 m

B4 ft

Axis

180 lb

1 ft

40 lb

A5 ft

1.5 mH

Ty

Ty B1.5 m

Axis

1200 N400 N

V3 m


5-31. A uniform 6-m bar has a length of 6 m and weighs 30 N. A 50-N weight is hung from the

left end and a 20-N force is hung at the right end. How far from the left end will a single

upward force produce equilibrium?

Fy = F – 50 N – 30 N – 20 N = 0; F = 100 N

= F x – (30 N)(3 m) – (20 N)(6 m) = 0

(100 N) x = 210 N m; x = 2.10 m

5-32. A 40-N sphere and a 12-N sphere are connected by a light rod 200 mm in length. How far

from the middle of the 40-N sphere is the center of gravity?

Fy = F – 40 N – 12 N = 0; F = 52 N

= F x – (40 N)(0) – (12 N)(0.20 m) = 0

(52 N) x = 2.40 N m; x = 0.0462 m or x = 46.2 mm

5-33. Weights of 2, 5, 8, and 10 N are hung from a 10-m light rod at distances of 2, 4, 6, and 8 m

from the left end. How far from the left in is the center of gravity?

Fy = F – 10 N – 8 N – 5 N – 2 N = 0; F = 25 N

Fx – (2 N)(2 m) – (5 N)(4 m) – (8 N)(6 m) – (10 N)(8 m) = 0

(25 N) x = 152 N m; x = 6.08 m

5-34. Compute the center of gravity of sledgehammer if the metal head weighs 12 lb and the 32-

in. supporting handle weighs 2 lb. Assume that the handle is of uniform construction and

weight.

Fy = F – 2 lb – 12 lb = 0; F = 14 lb

Fx – (12 lb)(0) – (2 lb)(16 in.) = 0; Fx = 32 lb in.

(14 lb) x = 32 lb in.; x = 2.29 in. from head.

Challenge Problems

42

Axis

F

20 N30 N50 N

x

3 m3 m

F

12 N40 N

200 mm

x

10 N5 N 8 N2 N

2 m

2 m 2 m

2 m

2 m

x F

F

16 in. 16 in.

x

2 lb12 lb


5-35. What is the resultant torque about the hinge in Fig. 4-20? Neglect weight of the curved bar.

= (80 N)(0.6 m) – (200 N)(0.4 m) sin 400

= 48.0 N m – 51.4 N m; = – 3.42 N m

5-36. What horizontal force applied to the left end of the

bar in Fig. 4-20 will produce rotational equilibrium?

From Prob. 5-33: = - 3.42 N m.

Thus, if = 0, then torque of +3.42 N m must be added.

F (0.6 m) cos 400 = +3.45 N m; F = 7.45 N

5-37. Weights of 100, 200, and 500 lb are placed on a light board resting on two supports as

shown in Fig. 4-21. What are the forces exerted by the supports?

= (100 lb)(4 ft) + B(16 ft)

– (200 lb)(6 ft) – (500 lb)(12 ft) = 0; B = 425 lb

Fy = A + B – 100 lb – 200 lb – 500 lb = 0

A = 800 lb – B = 800 lb – 425 lb; A = 375 lb

The forces exerted by the supports are : A = 375 N and B = 425 N

5-38. An 8-m steel metal beam weighs 2400 N and is supported 3 m from the right end. If a

9000-N weight is placed on the right end, what force must be exerted at the left end to

balance the system?

= A (5 m) + (2400 N)(1 m) – (9000 N)( 3 m) = 0;

A = 4920 N Fy = A + B – 2400 N – 9000 N = 0

B = 11,400 N – A = 11,400 N – 4920 N; A = 6480 N

*5-39. Find the resultant torque about point A in Fig. 5-22.

43

400500

F

200 N

60 cm

40 cmr 400

400

80 N

200 N60 cm40 cm

r 400

80 N

Axis

100 lb 200 lb 500 lb

A B

6 ft6 ft 4 ft4 ft

A9000 N

F4 m 3 m1 m

2400 N

r

r

70 N

50 N

B

5 cm

A

16 cm

500

550


= (70 N)(0.05 m) sin 500 – (50 N)(0.16 m) sin 550

= 2.68 N m – 6.55 N m = –3.87 N m

= –3.87 N m

*5-40. Find the resultant torque about point B in Fig. 5-22.

= (70 N)(0) – (50 N)(a + b) ; First find a and b.

a = (0.05 m) cos 500 = 0.0231 m; b = (0.16 m) sin 550 = 0.131 m

= – (50 N)(0.0231 m + 0.131 m) = –8.16 N m

= –8.16 N m

Critical Thinking Questions

*5-41. A 30-lb box and a 50-lb box are on opposite ends of a 16-ft board supported only at its

midpoint. How far from the left end should a 40-lb box be placed to produce equilibrium?

Would the result be different if the board weighed 90 lb? Why, or why not?

= (30 lb)(8 ft) + (40 lb)(x) – (50 lb)(8 ft) = 0;

x = 4.00 ft Note that the weight acting at the center

of the board does NOT contribute to torque about

the center, and therefore, the balance point is not affected, regardless of the weight.

5-42. On a lab bench you have a small rock, a 4-N meterstick and a single knife-edge support.

Explain how you can use these three items to find the weight of the small rock.

Measure distances a and b; determine F and then

calculate the weight W from equilibrium methods.

*5-43. Find the forces F1, F2, and F3 such that the system drawn in Fig. 5-23 is in equilibrium.

44

0.5 mF

4 N W

ba

b

a70 N

50 N

B

5 cm

16 cm

500

550

x

F

W40 lb

8 ft8 ft

50 lb30 lb

50 lb2 ft

5 ft

2 ft6 ft

3 ft 2 ft300 lb

200 lb

F3F2

F1

R’’

R


Note action-reaction forces R and R’.

First, lets work with top board:

(about R) = 0; Force R is upward.

R = (300 lb)(6 ft) – (50 lb)(2 ft) – F1(8 ft) = 0

F1 = 213 lb Now, Fy = 0 gives: 213 lb + R –300 lb – 50 lb = 0; R = 138 lb = R’

Next we sum torques about F2 with R’ = 138 lb is directed in a downward direction:

F = (138 lb)(3 ft) + F3(7 ft) – (200 lb)(5 ft) = 0; From which: F3 = 83.9 lb

Fy = 0 = F2 + 83.9 lb – 138 lb – 200 lb; F2 = –254 lb

The three unknown forces are: F1 = 213 lb, F2 = –254 lb, F3 = 83.9 lb

*5-44. (a) What weight W will produce a tension of 400 N in the rope attached to the boom in

Fig. 5-24?. (b) What would be the tension in the rope if W = 400 N? Neglect the weight

of the boom in each case.

(a) m) sin 300) – W (6 m) cos 300 = 0

W = 154 N

(b) = T(4 m) sin 300 – (400 N)(6 m) cos 300 = 0

T = 600 N

*5-45. Suppose the boom in Fig. 5-24 has a weight of 100 N and the suspended weight W is

equal to 400 N. What is the tension in the cord?

m) sin 300) – (400 N)(6 m) cos 300 – (100 N)(3 m) cos 300 = 0

T = 1169 N

*5-46. For the conditions set in Problem 5-5, what are the horizontal and vertical components

of the force exerted by the floor hinge on the base of the boom?

45

Axis

300

4 m

2 m400 N

W

300

100 NAxis

300

4 m

2 mT

W

300

V

H

100 NAxis

300

4 m

2 m1169 N

400 N

300


Fx = H – 1169 N = 0; or H = 1169 N

Fy = V – 100 N – 400 N = 0; or V = 500 N

H = 1169 N and V = 500 N

**5-47. What is the tension in the cable for Fig. 5-25. The weight of the boom is 300 N but its

length is unknown. (Select axis at wall, L cancels.)

T sin 750 = 75.0 N + 273 N; T = 360 N

**5-48. What are the magnitude and direction of the force exerted by

the wall on the boom in Fig. 5-25? Again assume that the weight of the board is 300 N.

Refer to the figure and data given in Problem 5-7 and recall that T = 360 N.

Fx = H - (360 N) cos 450 = 0; H = 255 N

Fy = V + (360 N) sin 450 – 300 N – 546 N = 0; V = 591 N

H = 255 N and V = 591 N

*5-49. An car has a distance of 3.4 m between front and rear axles. If 60 percent of the weight

rests on the front wheels, how far is the center of gravity located from the front axle?

= 0.6W(0) + 0.4W(3.4 m) – F x = 0

But F = W: 1.36 W – W x = 0

x = 1.36 m from front axle

Chapter 6. Uniform Acceleration

Problems:

46

600

300

450

T = 360 N

T

H

546 N

L

r

750

300 N

450

300

V

0.4W

xF

0.6W3.4 m

Axis


Speed and Velocity

6-1. A car travels a distance of 86 km at an average speed of 8 m/s. How many hours were

required for the trip?

t = 2.99 h

6-2. Sound travels at an average speed of 340 m/s. Lightning from a distant thundercloud is

seen almost immediately. If the sound of thunder reaches the ear 3 s later, how far away is

the storm?

t = 58.8 ms

6-3. A small rocket leaves its pad and travels a distance of 40 m vertically upward before

returning to the earth five seconds after it was launched. What was the average velocity for

the trip?

v = 16.0 m/s

6-4. A car travels along a U-shaped curve for a distance of 400 m in 30 s. It’s final location,

however is only 40 m from the starting position. What is the average speed and what is the

magnitude of the average velocity?

Average speed: v = 13.3 m/s

Average velocity: v = 1.33 m/s, E

6-5. A woman walks for 4 min directly north with a average velocity of 6 km/h; then she

walks eastward at 4 km/h for 10 min. What is her average speed for the trip?

47

s = 400 m

D = 40 m

6 km/h,4 min

4 km/h, 10 min

D

s1

s2

CB

A E


t1 = 4 min = 0.0667 h; t2 = 10 min = 0.167 h

s1 = v1t1 = (6 km/h)(0.0667 h) = 0.400 km

s1 = v2t2 = (4 km/h)(0.167 h) = 0.667 km

v = 4.57 km/h

6-6. What is the average velocity for the entire trip described in Problem 6-5?

D = 0.778 km, 31.00

v = 3.33 km/h, 31.00

6-7. A car travels at an average speed of 60 mi/h for 3 h and 20 min. What was the distance?

t = 3 h + 0.333 h = 3.33 h; s = vt = (60 mi/h)(3.33 h); s = 200 mi

6.8 How long will it take to travel 400 km if the average speed is 90 km/h?

t = 4.44 h

*6-9. A marble rolls up an inclined ramp a distance of 5 m, then stops and returns to a point 5

m below its starting point. The entire trip took only 2 s. What was the average speed

and what was the average velocity? (s1 = 5 m, s2 = -10 m)

speed = v = 7.50 m/s

velocity = v = – 2.5 m/s, down plane.

Uniform Acceleration

48

D

s2s1


6-10. The tip of a robot arm is moving to the right at 8 m/s. Four seconds later, it is moving to

the left at 2 m/s. What is the change in velocity and what is the acceleration.

v = vf - vo = (–2 m/s) – (8 m/s) v = –10 m/s

a = –2.50 m/s2

6-11. An arrow accelerates from zero to 40 m/s in the 0.5 s it is in contact with the bow string.

What is the average acceleration?

a = 80.0 m/s2

6-12. A car traveling initially at 50 km/h accelerates at a rate of 4 m/s2 for 3 s. What is the

final speed?

vo = 50 km/h = 13.9 m/s; vf = vo + at

vf = (13.9 m/s) + (4 m/s2)(3 s) = 25.9 m/s; vf = 25.9 m/s

6-13. A truck traveling at 60 mi/h brakes to a stop in 180 ft. What was the average acceleration

and stopping time?

vo = 60 mi/h = 88.0 ft/s 2as = vf2 – vo

2

a = – 21.5 ft/s2

t = 4.09 s

49


6-14. An arresting device on a carrier deck stops an airplane in 1.5 s. The average acceleration

was 49 m/s2. What was the stopping distance? What was the initial speed?

vf = vo + at; 0 = vo + (– 49 m/s2)(1.5 s); vo = 73.5 m/s

s = vf t - ½at2 ; s = (0)(1.5 s) – ½(-49 m/s2)(1.5 s)2; s = 55.1 m

6-15. In a braking test, a car traveling at 60 km/h is stopped in a time of 3 s. What was the

acceleration and stopping distance? ( vo = 60 km/h = 16.7 m/s)

vf = vo + at; (0) = (16.7 m/s) + a (3 s); a = – 5.56 m/s2

; s = 25.0 m

6-16. A bullet leaves a 28-in. rifle barrel at 2700 ft/s. What was its acceleration and time in the

barrel? (s = 28 in. = 2.33 ft)

2as = vo2 - vf

2 ; ; a = 1.56 x 106 m/s2

; t = 1.73 ms

6-17. The ball in Fig. 6-13 is given an initial velocity of 16 m/s at the bottom of an inclined

plane. Two seconds later it is still moving up the plane, but with a velocity of only 4

m/s. What is the acceleration?

vf = vo + at; ; a = -6.00 m/s2

6-18. For Problem 6-17, what is the maximum displacement from the bottom and what is the

velocity 4 s after leaving the bottom? (Maximum displacement occurs when vf = 0)

2as = vo2 - vf

2; ; s = +21.3 m

vf = vo + at = 16 m/s = (-6 m/s2)(4 s); vf = - 8.00 m/s, down plane

50


6-19. A monorail train traveling at 80 km/h must be stopped in a distance of 40 m. What average

acceleration is required and what is the stopping time? ( vo = 80 km/h = 22.2 m/s)

2as = vo2 - vf

2; ; a = -6.17 m/s2

; t = 3.60 m/s

Gravity and Free-Falling Bodies

6-20. A ball is dropped from rest and falls for 5 s. What are its position and velocity?

s = vot + ½at2; s = (0)(5 s) + ½(-9.8 m/s2)(5 s)2 ; s = -122.5 m

vf = vo + at = 0 + (-9.8 m/s2)(5 s); v = -49.0 m/s

6-21. A rock is dropped from rest. When will its displacement be 18 m below the point of

release? What is its velocity at that time?

s = vot + ½at2; (-18 m) = (0)(t) + ½(-9.8 m/s2)t2 ; t = 1.92 s

vf = vo + at = 0 + (-9.8 m/s2)(1.92 s); vf = -18.8 m/s

6-22. A woman drops a weight from the top of a bridge while a friend below measures the time

to strike the water below. What is the height of the bridge if the time is 3 s?

s = vot + ½at2 = (0) + ½(-9.8 m/s2)(3 s)2; s = -44.1 m

6-23. A brick is given an initial downward velocity of 6 m/s. What is its final velocity after

falling a distance of 40 m?

2as = vo2 - vf

2 ; ;

v = 28.6 m/s; Since velocity is downward, v = - 28.6 m/s

51


6-24. A projectile is thrown vertically upward and returns to its starting position in 5 s. What

was its initial velocity and how high did it rise?

s = vot + ½at2; 0 = vo(5 s) + ½(-9.8 m/s2)(5 s)2 ; vo = 24.5 m/s

It rises until vf = 0; 2as = vo2 - vf

2 ; ; s = 30.6 m

6-25. An arrow is shot vertically upward with an initial velocity of 80 ft/s. What is its

maximum height? (At maximum height, vf = 0; a = g = -32 ft/s2)

2as = vo2 - vf

2; ; s = 100 ft

6-26. In Problem 6-25, what are the position and velocity of the arrow after 2 s and after 6 s?

s = vot + ½at2 = (80 ft/s)(2 s) + ½(-32 ft/s2)(2 s)2 ; s = 96 ft

vf = vo + at = (80 ft/s) + (-32 ft/s2)(2 s); vf = 16 ft/s

s = vot + ½at2 = (80 ft/s)(6 s) + ½(-32 ft/s2)(6 s)2 ; s = -96 ft

vf = vo + at = (80 ft/s) + (-32 ft/s2)(6 s); vf = -112 ft/s

6-27. A hammer is thrown vertically upward to the top of a roof 16 m high. What minimum

initial velocity was required?

2as = vo2 - vf

2 ; ; vo = 17.7 m/s

Horizontal Projection

6-28. A baseball leaves a bat with a horizontal velocity of 20 m/s. In a time of 0.25 s, how far

will it have traveled horizontally and how far has it fallen vertically?

x = vox t = (20 m/s)(2.5 s) ; x = 50.0 m

y = voy + ½gt2 = (0)(2.5 s) + ½(-9.8 m/s2)(0.25 s)2 y = -0.306 m

52

0


6-29. An airplane traveling at 70 m/s drops a box of supplies. What horizontal distance will the

box travel before striking the ground 340 m below?

First we find the time to fall: y = voy t + ½gt2

t = 8.33 s ; x = vox t = (70 m/s)(8.33 s) ; x = 583 m

6-30. At a lumber mill, logs are discharged horizontally at 15 m/s from a greased chute that is

20 m above a mill pond. How far do the logs travel horizontally?

y = ½gt2; ; t = 2.02 s

x = vox t = (15 m/s)(8.33 s) ; x = 30.3 m

6-31. A steel ball rolls off the edge of a table top 4 ft above the floor. If it strikes the floor 5 ft

from the base of the table, what was its initial horizontal speed?

First find time to drop 4 ft: ; t = 0.500 s

x = vox t ; ; vox = 10.0 ft/s

6-32. A bullet leaves the barrel of a weapon with an initial horizontal velocity of 400 m/s. Find

the horizontal and vertical displacements after 3 s.

x = vox t = (400 m/s)(3 s) ; x = 1200 m

y = voy + ½gt2 = (0)(3 s) + ½(-9.8 m/s2)(3 s)2 y = -44.1 m

6-33. A projectile has an initial horizontal velocity of 40 m/s at the edge of a roof top. Find

the horizontal and vertical components of its velocity after 3 s.

vx = vox = 40 m/s vy = voy t + gt = 0 + (-9.8 m/s2)(3s); vy = -29.4 m/s

53


The More General Problem of Trajectories

6-34. A stone is given an initial velocity of 20 m/s at an angle of 580. What are its horizontal

and vertical displacements after 3 s?

vox = (20 m/s) cos 580 = 10.6 m/s; voy = (20 m/s) sin 580 = 17.0 m/s

x = voxt = (10.6 m/s)(3 s); x = 31.8 m

y = voyt + ½gt2 = (17.0 m/s)(3 s) +½(-9.8 m/s2)(3 s)2; y = 6.78 m

6-35. A baseball leaves the bat with a velocity of 30 m/s at an angle of 300. What are the

horizontal and vertical components of its velocity after 3 s?


vx = vox = 26.0 m/s ; vx = 26.0 m/s

vy = voy + gt = (15 m/s) + (-9.8 m/s2)(3 s) ; vy = -14.4 m/s

6-36. For the baseball in Problem 6-33, what is the maximum height and what is the range?

ymax occurs when vy = 0, or when: vy = voy + gt = 0 and t = - voy/g

; Now we find ymax using this time.

ymax = voyt + ½gt2 = (15 m/s)(1.53 s) + ½(-9.8 m/s2)(1.53 s)2; ymax = 11.5 m

The range will be reached when the time is t’ = 2(1.53 s) or t’ = 3.06 s, thus

R = voxt’= (30 m/s) cos 300 (3.06 s); R = 79.5 m

6-37. An arrow leaves the bow with an initial velocity of 120 ft/s at an angle of 370 with the

horizontal. What are the horizontal and vertical components of is displacement two

seconds later?

vox = (120 ft/s) cos 370 = 104 ft/s; voy = (120 ft/s) sin 300 = 60.0 ft/s

54


6-37. (Cont.) The components of the initial velocity are: vox = 104 ft/s; voy = 60.0 ft/s

x = voxt = (104 ft/s)(2 s); x = 208 ft

y = voyt + ½gt2 = (60.0 m/s)(2 s) +½(-32 ft/s2)(2 s)2; y = 56.0 ft

*6-38. In Problem 6-37, what are the magnitude and direction of arrow’s velocity after 2 s?

vx = vox = 104 ft/s ; vx = 104 ft/s

vy = voy + gt = (60 m/s) + (-32 ft/s2)(2 s) ; vy = -4.00 ft/s

*6-39. A golf ball in Fig. 6-14 leaves the tee with a velocity of 40 m/s at 650. If it lands on a

green located 10 m higher than the tee, what was the time of flight, and what was the

horizontal distance to the tee?


y = voyt + ½gt2: 10 ft = (36.25 m/s) t + ½(-9.8 m/s2)t2

Solving quadratic (4.9t2 – 36.25t + 10 = 0) yields: t1 = 0.287 s and t2 = 7.11 s

The first time is for y = +10 m on the way up, the second is y = +10 m on the way down.

Thus, the time from tee to green was: t = 7.11 s

Horizontal distance to tee: x = voxt = (16.9 m/s)(7.11 s); x = 120 m

*6-40. A projectile leaves the ground with a velocity of 35 m/s at an angle of 320. What is the

maximum height attained.


ymax occurs when vy = 0, or when: vy = voy + gt = 0 and t = - voy/g

; Now we find ymax using this time.

ymax = voyt + ½gt2 = (18.55 m/s)(1.89 s) + ½(-9.8 m/s2)(1.89 s)2; ymax = 17.5 m

55


*6-41. The projectile in Problem 6-40 rises and falls, striking a billboard at a point 8 m above

the ground. What was the time of flight and how far did it travel horizontally.


y = voyt + ½gt2: 8 m = (18.55 m/s) t + ½(-9.8 m/s2)t2

Solving quadratic (4.9t2 – 18.55t + 8 = 0) yields: t1 = 0.497 s and t2 = 3.36 s

The first time is for y = +8 m on the way up, the second is y = +8 m on the way down.

Thus, the time from tee to green was: t = 3.29 s

Horizontal distance to tee: x = voxt = (29.7 m/s)(3.29 s); x = 97.7 m

Challenge Problems

6-42. A rocket travels in space at 60 m/s before it is given a sudden acceleration. It’s velocity

increases to 140 m/s in 8 s, what was its average acceleration and how far did it travel in

this time?

; a = 10 m/s2

; t = 800 s

6-43. A railroad car starts from rest and coasts freely down an incline. With an average

acceleration of 4 ft/s2, what will be the velocity after 5 s? What distance does it travel?

vf = vo + at = 0 + (4 ft/s2)(5 s); vf = 20 ft/s

s = vot + ½at2 = 0 + ½(4 ft/s2)(5 s)2; s = 50 ft

56


*6-44. An object is projected horizontally at 20 m/s. At the same time, another object located

12 m down range is dropped from rest. When will they collide and how far are they

located below the release point?

A: vox = 20 m/s, voy = 0; B: vox = voy = 0

Ball B will have fallen the distance y at the same time t as ball A. Thus,

x = voxt and (20 m/s)t = 12 m; t = 0.600 s

y = ½at2 = ½(-9.8 m/s2)(0.6 s)2 ; y = -1.76 m

6-45. A truck moving at an initial velocity of 30 m/s is brought to a stop in 10 s. What was the

acceleration of the car and what was the stopping distance?

; a = -3.00 m/s2

; s = 150 m

6-46. A ball is thrown vertically upward with an initial velocity of 23 m/s. What are its

position and velocity after 2s, after 4 s, and after 8 s?

Apply s = vot + ½at2 and vf = vo + at for time of 2, 4, and 8 s:

(a) s = (23 m/s)(2 s) + ½(-9.8 m/s2)(2 s)2 ; s = 26.4 m

vf = (23 m/s) + (-9.8 m/s2)(2 s) ; vf = 3.40 m/s

(b) s = (23 m/s)(4 s) + ½(-9.8 m/s2)(4 s)2 ; s = 13.6 m

vf = (23 m/s) + (-9.8 m/s2)(4 s) ; vf = -16.2 m/s

(c) s = (23 m/s)(8 s) + ½(-9.8 m/s2)(8 s)2 ; s = -130 m

vf = (23 m/s) + (-9.8 m/s2)(8 s) ; vf = -55.4 m/s

57

y

BA

12 m


6-47. A stone is thrown vertically downward from the top of a bridge. Four seconds later it

strikes the water below. If the final velocity was 60 m/s. What was the initial velocity of

the stone and how high was the bridge?

vf = vo + at; v0 = vf – at = (-60 m/s) - (-9.8 m/s)(4 s); vo = -20.8 m/s

s = vot + ½at2 = (-20.8 m/s)(4 s) + ½(-9.8 m/s)(4 s)2; s = 162 m

6-48. A ball is thrown vertically upward with an initial velocity of 80 ft/s. What are its

position and velocity after (a) 1 s; (b) 3 s; and (c) 6 s

Apply s = vot + ½at2 and vf = vo + at for time of 2, 4, and 8 s:

(a) s = (80 ft/s)(1 s) + ½(-32 ft/s2)(1 s)2 ; s = 64.0 ft

vf = (80 ft/s) + (-32 ft/s2)(2 s) ; vf = 16.0 ft/s

(b) s = (80 ft/s)(3 s) + ½(-32 ft/s2)(3 s)2 ; s = 96.0 ft

vf = (80 ft/s) + (-32 ft/s2)(3 s) ; vf = -16.0 ft/s

(c) s = (80 ft/s)(6 s) + ½(-32 ft/s2)(6 s)2 ; s = 64.0 ft

vf = (80 ft/s) + (-32 ft/s2)(6 s) ; vf = -96.0 ft/s

6-49. An aircraft flying horizontally at 500 mi/h releases a package. Four seconds later, the

package strikes the ground below. What was the altitude of the plane?

y = ½gt2 = ½(-32 ft/s2)(4 s)2; y = -256 ft

*6-50. In Problem 6-49, what was the horizontal range of the package and what are the

components of its final velocity?

vo = 500 mi/h = 733 ft/s; vx = vox = 733 ft/s; voy = 0; t = 4 s

x = vxt = (733 ft/s)(4 s); x = 2930 ft

vy = voy + at = 0 + (-32 ft/s)(4 s); vy = -128 ft/s; vx = 733 m/s

58


*6-51. A putting green is located 240 ft horizontally and 64 ft vertically from the tee. What

must be the magnitude and direction of the initial velocity if a ball is to strike the green

at this location after a time of 4 s?

x = voxt; 240 ft = vox (4 s); vox = 60 m/s

s = vot + ½at2; 64 ft = voy(4 s) + ½(-32 ft/s2)(4 s)2; voy = 80 ft/s

; v = 100 ft/s, = 53.10


6-52. A long strip of pavement is marked off in 100-m intervals. Students use stopwatches to

record the times a car passes each mark. The following data is listed:

Distance, m0 10 m 20 m 30 m 40 m 50 m

Time, s0 2.1 s 4.3 s 6.4 s 8.4 s 10.5 s

Plot a graph with distance along the y-axis and time along the x-axis. What is the

significance of the slope of this curve? What is the average speed of the car? At what

instant in time is the distance equal to 34 m? What is the acceleration of the car?

Data taken directly from the graph (not drawn): Ans. Slope is v, 4.76 m/s, 7.14 s, 0.

6-53. An astronaut tests gravity on the moon by dropping a tool from a height of 5 m. The

following data are recorded electronically.

Height, m5.00 m 4.00 m 3.00 m 2.00 m 1.00 m 0 m

Time, s0 1.11 s 1.56 s 1.92 s 2.21 s 2.47 s

59


6-53. (Cont.) Plot the graph of this data. Is it a straight line? What is the average speed for the

entire fall? What is the acceleration? How would you compare this with gravity on earth?

Data taken directly from the graph (not drawn): Ans. Slope is v, 4.76 m/s, 7.14 s, 0.

*6-54. A car is traveling initially North at 20 m/s. After traveling a distance of 6 m, the car

passes point A where it's velocity is still northward but is reduced to 5 m/s. (a) What

are the magnitude and direction of the acceleration of the car? (b) What time was

required? (c) If the acceleration is held constant, what will be the velocity of the car

when it returns to point A?

(a) vo = 20 m/s, vf = 5 m/s, x = 6 m

2as = vo2 - vf

2; ; a = -31.2 m/s2

(b) ; t = 0.480 s

(c) Starts at A with vo = + 5 m/s then returns to A with zero net displacement (s = 0):

2as = vo2 - vf

2; 0 = (5 m/s)2 – vf2; ; vf = - 5 m/s

*6-55. A ball moving up an incline is initially located 6 m from the bottom of an incline and has

a velocity of 4 m/s. Five seconds later, it is located 3 m from the bottom. Assuming

constant acceleration, what was the average velocity? What is the meaning of a negative

average velocity? What is the average acceleration and final velocity?

vo = + 4 m/s; s = -3 m; t = 5 s Find vavg

s = vavg t; vavg = -0.600 m/s

Negative average velocity means that the velocity was down the plane most of the time.

60

x = 6 mx = 0

A v = 5 m/sv = 20 m/s

4 m/s6 m

3 m

s = 0


*6-55. (Cont.) s = vot + ½at2; -3 m = (4 m/s)(5 s) + ½a (5 s)2; a = -1.84 m/s2

vf = vo + at = 4 m/s + (-1.84 m/s2)(5 s); vf = -5.20 m/s

*6-56. The acceleration due to gravity on an distant planet is determined to be one-fourth its

value on the earth. Does this mean that a ball dropped from a height of 4 m above this

planet will strike the ground in one-fourth the time? What are the times required on the

planet and on earth?

The distance as a function of time is given by: s = ½at2 so that

one-fourth the acceleration should result in twice the drop time.

te = 0.904 s tp = 1.81 s

*6-57. Consider the two balls A and B shown in Fig. 6-15. Ball A has a constant acceleration of

4 m/s2 directed to the right, and ball B has a constant acceleration of 2 m/s2 directed to the

left. Ball A is initially traveling to the left at 2 m/s, while ball B is traveling to the left

initially at 5 m/s. Find the time t at which the balls collide. Also, assuming x = 0 at the

initial position of ball A, what is their common displacement when they collide?

Equations of displacement for A and B:

s = so + vot + ½at2 (watch signs)

For A: sA = 0 + (-2 m/s)t + ½(+4 m/s2) t2

For B: sB = 18 m + (-5 m/s)t + ½(-2 m/s2) t2; Next simplify and set sA = sB

- 2t + 2t2 = 18 – 5t - t2 3t2 + 3t – 18 = 0 t1 = - 3 s, t2 = +2 s

Accept t = +3 s as meaningful answer, then substitute to find either sA or sB:

sA = -2(2 s) + 2(2 s)2; x = + 4 m

61

v = - 5 m/s2

v = - 2 m/s

+

aa = +4 m/s2

x = 18 mx = 0

A B

ab = -2 m/s2


*6-58. Initially, a truck with a velocity of 40 ft/s is located distance of 500 ft to the right of a

car. If the car begins at rest and accelerates at 10 ft/s2, when will it overtake the truck?

How far is the point from the initial position of the car?

Equations of displacement for car and truck:

s = so + vot + ½at2 (watch signs)

For car: sC = 0 + ½(+10 ft/s2) t2 ; Truck: sT = 500 ft + (40 ft/s)t + 0;

Set sC = sT 5t2 = 500 + 40t or t2 – 8t –100 = 0; t1 = -6.77 s; t2 = +14.8 s

Solve for either distance: sC = ½(10 ft/s2)(14.8 s)2; s = 1092 ft

*6-59. A ball is dropped from rest at the top of a 100-m tall building. At the same instant a

second ball is thrown upward from the base of the building with an initial velocity of 50

m/s. When will the two balls collide and at what distance above the street?

For A: sA = 100 m + v0At + ½gt2 = 100 m + 0 + ½(-9.8 m/s2) t2

For B: sB = 0 + (50 m/s)t + ½(-9.8 m/s2) t2 Set sA = sB

100 – 4.9 t2 = 50 t – 4.9 t2; 50 t = 100; t = 2.00 s

Solve for s: sA = 100 m – (4.9 m/s2)(2 s)2; s = 80.4 m

*6-60. A balloonist rising vertically with a velocity of 4 m/s releases a sandbag at the instant

when the balloon is 16 m above the ground. Compute the position and velocity of the

sandbag relative to the ground after 0.3 s and 2 s. How many seconds after its release

will it strike the ground?

The initial velocity of the bag is that of the balloon: voB = + 4 m/s

62

v = 0

s = 0 s = 500 ft

v = 40 ft/s+

A

Bs = 0

s = 100 m


From ground: s = soB + voBt + ½gt2; s = 18 m + (4 m/s)t + ½(-9.8 m/s2)t2

s = 18 m + (4 m/s)(0.3 s) – (4.9 m/s2)(0.3 s)2 ; s = 16.8 m

*6-61. An arrow is shot upward with a velocity of 40 m/s. Three seconds later, another arrow is

shot upward with a velocity of 60 m/s. At what time and position will they meet?

Let t1 = t be time for first arrow, then t2 = t - 3 for second arrow.

s1 = (40 m/s)t1 + ½(-9.8 m/s2)t12 ; s1 = 40t – 4.9t2

s2 = (60 m/s)t2 + ½(-9.8 m/s2)t22 ; s2 = 60(t – 3) - 4.9(t – 3)2

s1 = s2; 40t – 4.9t2 = 60t – 180 – 4.9(t2 – 6t + 9)

The solution for t gives: t = 4.54 s

Now find position: s1 = s2 = (40 m/s)(4.54 s) – (4.9 m/s2)(4.54 s)2; s = 80.6 m

*6-62. Someone wishes to strike a target, whose horizontal range is 12 km. What must be the

velocity of an object projected at an angle of 350 if it is to strike the target. What is the

time of flight?

y = voyt + ½gt2 = 0; ( vo sin 350)t = (4.9 m/s2)t2 or

R = voxt = 12 km; (vo cos 350)t = 12,000 m; Set t = t

; From which vo = 354 m/s and t = 41.4 s

*6-63. A wild boar charges directly toward a hunter with a constant speed of 60 ft/s. At the

instant the boar is 100 yd away, the hunter fires an arrow at 300 with the ground. What

must be the velocity of the arrow if it is to strike its target?

y = 0 = (v0 sin 300)t + ½(-32 ft/s2)t2; Solve for t

63

s1 = s2

60 m/s

40 m/s

v = -60 ft/s

s1 = s2 s = 300 fts = 0

vo

300


; t = 0.03125 vo

s1 =( v0 cos 300) t = (0.866 vo)(0.03125 vo); s1 = 0.0271 vo2

*6-63. (Cont.) s1 = 0.0271 vo2 ; t = 0.03125 vo

vB = - 60 ft/s; soB = 300 ft

s2 = soB + vBt = 300 ft + (-60 ft/s)t

s2 = 300 – 60 (0.03125 vo) = 300 – 1.875 vo Now, set s1 = s2 and solve for vo

0.0271 vo2 = 300 – 1.875 vo or vo

2 + 69.2 vo – 11,070 = 0

The quadratic solution gives: vo = 76.2 ft/s

hapter 7. Newton’s Second Law

Newton’s Second Law

7-1. A 4-kg mass is acted on by a resultant force of (a) 4 N, (b) 8 N, and (c) 12 N. What are the

resulting accelerations?

(a) 1 m/s2 (b) 2 m/s2 (c) 3 m/s2

7-2. A constant force of 20 N acts on a mass of (a) 2 kg, (b) 4 kg, and (c) 6 kg. What are the

resulting accelerations?

(a) 10 m/s2 (b) 5 m/s2 (c) 3.33 m/s2

7-3. A constant force of 60 lb acts on each of three objects, producing accelerations of 4, 8, and

12 N. What are the masses?

64

s1 = s2 s = 300 fts = 0

vo

300


15 slugs 7.5 slugs 5 slugs

7-4. What resultant force is necessary to give a 4-kg hammer an acceleration of 6 m/s2?

F = ma = (4 kg)(6 m/s2); F = 24 N

7-5. It is determined that a resultant force of 60 N will give a wagon an acceleration of 10 m/s2.

What force is required to give the wagon an acceleration of only 2 m/s2?

; F = ma = (6 slugs)(2 m/s2); F = 12 N

7-6. A 1000-kg car moving north at 100 km/h brakes to a stop in 50 m. What are the magnitude

and direction of the force? Convert to SI units: 100 km/h = 27.8 m/s

F = ma = (1000 kg)(7.72 m/s2); F = 772 N, South.

The Relationship Between Weight and Mass

7-7. What is the weight of a 4.8 kg mailbox? What is the mass of a 40-N tank?

W = (4.8 kg)(9.8 m/s2) = 47.0 N ; = 4.08 kg

7-8. What is the mass of a 60-lb child? What is the weight of a 7-slug man?

= 1.88 slugs ; W = (7 slugs)(32 ft/s2) = 224 lb

7-9. A woman weighs 180 lb on earth. When she walks on the moon, she weighs only 30 lb.

What is the acceleration due to gravity on the moon and what is her mass on the moon? On

the Earth?

Her mass is the same on the moon as it is on the earth, so we first find the constant mass:

65


mm = me = 5.62 slugs ;

Wm = mmgm ; gm = 5.33 ft/s2

7-10. What is the weight of a 70-kg astronaut on the surface of the earth. Compare the resultant

force required to give him or her an acceleration of 4 m/s2 on the earth with the resultant

force required to give the same acceleration in space where gravity is negligible?

On earth: W = (70 kg)(9.8 m/s2) = 686 N ; FR = (70 kg)(4 m/s2) = 280 N

Anywhere: FR = 280 N The mass doesn’t change.

7-11. Find the mass and the weight of a body if a resultant force of 16 N will give it an

acceleration of 5 m/s2.

= 3.20 kg ; W = (3.20 kg)(9.8 m/s2) = 31.4 N

7-12. Find the mass and weight of a body if a resultant force of 200 lb causes its speed to

increase from 20 ft/s to 60 ft/s in a time of 5 s.

= 25.0 slugs

W = mg = (25.0 slugs)(32 ft/s2); W = 800 lb

7-13. Find the mass and weight of a body if a resultant force of 400 N causes it to decrease its

velocity by 4 m/s in 3 s.

; m = 300 kg

W = mg = (300 kg)(9.8 m/s2); W = 2940 N

66


Applications for Single-Body Problems:

7-14. What horizontal pull is required to drag a 6-kg sled with an acceleration of 4 m/s2 if a

friction force of 20 N opposes the motion?

P – 20 N = (6 kg)(4 m/s2); P = 44.0 N

7-15. A 2500-lb automobile is speeding at 55 mi/h. What resultant force is required to stop the

car in 200 ft on a level road. What must be the coefficient of kinetic friction?

We first find the mass and then the acceleration. (55 mi/h = 80.7 m/s)

F = ma = (78.1 slugs)(-16.3 ft/s2); F = -1270 lb

k = 0.508

7-16. A 10-kg mass is lifted upward by a light cable. What is the tension in the cable if the

acceleration is (a) zero, (b) 6 m/s2 upward, and (c) 6 m/s2 downward?

Note that up is positive and that W = (10 kg)(9.8 m/s2) = 98 N.

(a) T – 98 N = (10 kg)(0 m/sand T = 98 N

(b) T – 98 N = (10 kg)(6 m/sand T = 60 N + 98 N or T = 158 N

(c) T – 98 N = (10 kg)(-6 m/sand T = - 60 N + 98 N or T = 38.0 N

7-17. A 64-lb load hangs at the end of a rope. Find the acceleration of the load if the tension in

the cable is (a) 64 lb, (b) 40 lb, and (c) 96 lb.

(a) ; a = 0

67

6 kg20 N P

W = mg

+T

10 kg

W

+T

m = W/g


(b) ; a = -12.0 ft/s2

(b) ; a = 16.0 ft/s2

7-18. An 800-kg elevator is lifted vertically by a strong rope. Find the acceleration of the

elevator if the rope tension is (a) 9000 N, (b) 7840 N, and (c) 2000 N.

Newton’s law for the problem is: T – mg = ma (up is positive)

(a) 9000 N – (800 kg)(9.8 m/s2) = (800 kg)a ; a = 1.45 m/s2

(a) 7840 N – (800 kg)(9.8 m/s2) = (800 kg)a ; a = 0

(a) 2000 N – (800 kg)(9.8 m/s2) = (800 kg)a ; a = -7.30 m/s2

7-19. A horizontal force of 100 N pulls an 8-kg cabinet across a level floor. Find the

acceleration of the cabinet if k = 0.2.

F = kN = k mg F = 0.2(8 kg)(9.8 m/s

100 N – F = ma; 100 N – 15.7 N = (8 kg) a; a = 10.5 m/s2

7-20. In Fig. 7-10, an unknown mass slides down the 300 inclined plane.

What is the acceleration in the absence of friction?

Fx = max; mg sin 300 = ma ; a = g sin 300

a = (9.8 m/s2) sin 300 = 4.90 m/s2, down the plane

7-21. Assume that k = 0.2 in Fig 7-10. What is the acceleration?

Why did you not need to know the mass of the block?

Fx = max; mg sin 300 - kN = ma ; N = mg cos 300

mg sin 300 - k mg cos 300 = ma ; a = g sin 300 - k g cos 300

a = (9.8 m/s2)(0.5) – 0.2(9.8 m/s2)(0.866); a = 3.20 m/s2, down the plane.

68

+

mmg

T

mg

F 100 NN

mg

N

300

300

F

mg

N

300

300


*7-22. Assume that m = I 0 kg and k = 0. 3 in Fig. 7- 10. What push P directed up and along

the incline in Fig.7-10 will produce an acceleration of 4 m/s2 also up the incline?

F = kN = kmg cos 300; F = 0.3(10 kg)(9.8 m/s2)cos 300 = 25.5 N

Fx = ma; P – F – mg sin 300 = ma

P – 25.5 N – (10 kg)(9.8 m/s2)(0.5) = (10 kg)(4 m/s2)

P – 25.5 N – 49.0 N = 40 N; P = 114 N

*7-23. What force P down the incline in Fig. 7-10 is required to cause the acceleration DOWN

the plane to be 4 m/s2? Assume that in = IO kg and k = 0. 3.

See Prob. 7-22: F is up the plane now. P is down plane (+).

Fx = ma; P - F + mg sin 300 = ma ; Still, F = 25.5 N

P - 25.5 N + (10 kg)(9.8 m/s2)(0.5) = (10 kg)(4 m/s2)

P - 25.5 N + 49.0 N = 40 N; P = 16.5 N

Applications for Multi-Body Problems

7-24. Assume zero friction in Fig. 7-11. What is the acceleration of the system? What is the

tension T in the connecting cord?

Resultant force = total mass x acceleration

80 N = (2 kg + 6 kg)a; a = 10 m/s2

To find T, apply F = ma to 6-kg block only: 80 N – T = (6 kg)(10

m/s

7-25. What force does block A exert on block B in Fig, 7-12?

F = mTa; 45 N = (15 kg) a; a = 3 m/s2

Force ON B = mB a = (5 kg)(3 m/s2); F = 15 N

69

+

+

P

F

mg

N

300

300

F

300P

mg

N

300

T

6 kg2 kg 80 N

45 N5 kg10 kg

A B


*7-26. What are the acceleration of the system and the tension in the

connecting cord for the arrangement shown in Fig. 7-13?

Assume zero friction and draw free-body diagrams.

For total system: m2g = (m1 + m2)a (m1g is balanced by N)

; a = 5.88 m/s2 Now, to find T, consider only m1

F = m1a T = m1a = (4 kg)(5.88 m/s

*7-27. If the coefficient of kinetic friction between the table and the 4 kg block is 0.2 in

Fig. 7-13, what is the acceleration of the system. What is the tension in the cord?

Fy = 0; N = m1g; F = kN = km1g

For total system: m2g - km1g = (m1 + m2)a

*7-27. (Cont.) or a = 5.10 m/s2

To find T, consider only m2 and make down positive:

Fy = m2a ; m2g – T = m2a; T = m2g – m2a

(6 kg)(9.8 m/s2) – (6 kg)(5.10 m/s2); T = 28.2 N

*7-28. Assume that the masses m1 = 2 kg and m2 = 8 kg are connected by a cord

that passes over a light frictionless pulley as in Fig. 7-14. What is the

acceleration of the system and the tension in the cord?

Resultant force = total mass of system x acceleration

70

N

m1 g

m2 g

T

T +a

Fk

N

m1 g

m2 g

T

T +a

Fk

N

m1 g

m2 g

T

T +a

+a

m2g m1g

TT


m2g – m1g = (m1 + m2)a

a = 5.88 m/s Now look at m1

alone:

T - m1g = m1 a; T = m1(g + a) = (2 kg)(9.8 m/s2 – 5.88 m/s2); T = 31.4 N

*7-29. The system described in Fig. 7-15 starts from rest. What is the

acceleration assuming zero friction? (assume motion down plane)

Fx = mT a; m1g sin 320 – m2g = (m1 + m2) a

(10 kg)(9.8 m/s2)sin 320 – (2 kg)(9.8 m/s2) = (10 kg + 2 kg)a

a = 2.69 m/s2

*7-30. What is the acceleration in Fig. 7-15 as the 10-kg block moves down the plane against

friction (k = 0.2). Add friction force F up plane in figure for previous problem.

m1g sin 320 – m2g – F = (m1 + m2) a ; Fy = 0 ; N = m1g cos 320

*7-30. (Cont.) m1g sin 320 – m2g – F = (m1 + m2) a ; F = kN = k m1g cos 320

m1g sin 320 – m2g – k m1g cos 320 = (m1 + m2) a ; a = 1.31 m/s2

*7-31 What is the tension in the cord for Problem 7-30? Apply F = ma to mass m2 only:

T – m2g = m2 a; T = m2(g + a) = (2 kg)(9.8 m/s2 + 1.31 m/s2); T = 22.2 N

Challenge Problems

7-32. A 2000-lb elevator is lifted vertically with an acceleration of 8 ft/s2.

Find the minimum breaking strength of the cable pulling the elevator?

71

T

320

m1g

N

320m2g

T

+a

+aT

2000 lb


Fy = ma;

T – mg = ma; T = m(g + a); T = (62.5 slugs)(32 ft/s2 + 8 ft/s2); T = 2500 lb

7-33. A 200-lb worker stands on weighing scales in the elevator of Problem 7-32.

What is the reading of the scales as he is lifted at 8 m/s?

The scale reading will be equal to the normal force N on worker.

N – mg = ma; N = m(g + a); T = (62.5 slugs)(32 ft/s2 + 8 ft/s2); T = 2500 lb

7-34. A 8-kg load is accelerated upward with a cord whose breaking strength is 200 N. What is

the maximum acceleration?

Tmax – mg = ma

a = 15.2 m/s2

7-35. For rubber tires on a concrete road k = 0.7. What is the horizontal stopping distance for a

1600-kg truck traveling at 20 m/s? The stopping distance is determined by the

acceleration from a resultant friction force F = kN, where N = mg:

F = -kmg = ma; a = -kg = - (0.7)(9.8 m/s2); a = -6.86m/s2

Recall that: 2as = vo2 - vf

2; ; s = 29.2 km

*7-36. Suppose the 4 and 6-kg masses in Fig. 7-13 are switched so that the larger mass is on the

table. What would be the acceleration and tension in the cord neglecting friction?

For total system: m2g = (m1 + m2); m1 = 6 kg; m2 = 4 kg

72

+a

200 lb

N

8 kg

+aT = 200 N

mg

N

m1 g

m2 g

T

T +a


; a = 3.92 m/s2

F = m1a T = m1a = (6 kg)(5.88 m/s

*7-37. Consider two masses A and B connected by a cord and hung over a single pulley. If

mass A is twice that of mass B, what is the acceleration of the system?

mA = 2mB ; If the left mass B is m, the right mass A will be 2m.

2mg – mg = (2m + m)a mg = 3ma

a = 3.27 m/s

*7-38. A 5-kg mass rests on a 340 inclined plane where k = 0.2. What push

up the incline, will cause the block to accelerate at 4 m/s2?

F = kN = kmg cos 340; F = 0.2(5 kg)(9.8 m/s2)cos 340 = 8.12 N

Fx = ma; P – F – mg sin 340 = ma

P – 8.12 N – (5 kg)(9.8 m/s2) sin 340 = (5 kg)(4 m/s2) P = 47.4 N

*7-39. A 96-lb block rests on a table where k = 0.4. A cord tied to this block passes over a

light frictionless pulley. What weight must be attached to the free end if the system is to

accelerated at 4 ft/s2?

F = kN = 0.2 (96 lb); F = 19.2 lb

W – 19.2 lb = 12 lb + 0.125 W; W = 35.7 lb


73

BA2m m

+a

2mg mg

TT

F340

P

mg

N

340

+

F

N

96 lb

W

T

T +a

Fo

rce

, N

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9

Acceleration, m/s2

F

a


7-40. In a laboratory experiment, the acceleration of a small car is measured by the separation of

spots burned at regular intervals in a paraffin-coated tape. Larger and larger weights are

transferred from the car to a hanger at the end of a tape that passes over a light frictionless

pulley. In this manner, the mass of the entire system is kept constant. Since the car moves

on a horizontal air track with negligible friction, the resultant force is equal to the weights

at the end of the tape. The following data are recorded:

Weight, N 2 4 6 8 10 12

Acceleration, m/s2 1.4 2.9 4.1 5.6 7.1 8.4

Plot a graph of weight (force) versus acceleration. What is the significance of the slope of

this curve? What is the mass?

The slope is the change in Force over

the change in acceleration, which is

the mass of the system. Thus, the mass

is found to be: m = 1.42 kg

7-41. In the above experiment, a student

places a constant weight of 4 N at the free end of the tape. Several runs are made,

increasing the mass of the car each time by adding weights. What happens to the

acceleration as the mass of the system is increased? What should the value of the product

of mass and acceleration be for each run? Is it necessary to include the mass of the

constant 4 N weight in these experiments?

The acceleration increases with increasing mass. According to Newton’s second law, the

product of the total mass of the system and the acceleration must always be equal to the

74


resultant force of 4 N for each run. It is necessary to add the mass of the 4-N weight to

each of the runs because it is part of the total mass of the system.

7-42. An arrangement similar to that described by Fig. 7-13 is set up except that the masses are

replaced. What is the acceleration of the system if the suspended

mass is three times that of the mass on the table and k = 0.3.

Fy = 0; N = mg; F = kN = kmg

For total system: 3mg - kmg = (3m + m)a ; (3 - k)mg = 4 ma

a = 6.62 m/s2

7-43. Three masses, 2 kg, 4 kg, and 6 kg, are connected (in order) by strings and

hung from the ceiling with another string so that the largest mass is in the lowest

position. What is the tension in each cord? If they are then detached from the

ceiling, what must be the tension in the top string in order that the system

accelerate upward at 4 m/s2? In the latter case what are the tensions in the

strings that connect masses?

7-43. (Cont.) The tension in each string is due only to the weights BELOW the string. Thus,

TC = (6 kg)(9.8 m/s2) = 58.8 N ; TB = (6 kg + 4 kg)(9.8 m/s2) = 98.0 N ;

TA = (6 kg + 4 kg + 2 kg)(9.8 m/s2) = 118 N

Now consider the upward acceleration of 4 m/s

Fy = 0; TA = (2 kg + 4 kg + 6 kg)(4 m/s2); TA = 48 N

TB = (4 kg + 6 kg)(4 m/s2) = 40 N ; TC = (6 kg)(4 m/s2) = 24 N

75

Fk

N

mg

3mg

T

T +a

C

B

A

4 kg

6 kg

2 kg

a = + 4 m/s2

C

B

A

4 kg

6 kg

2 kg


7-44. An 80-kg astronaut on a space walk pushes against a 200-kg solar panel that has become

dislodged from a spacecraft. The force causes the panel to accelerate at 2 m/s2. What

acceleration does the astronaut receive? Do they continue to accelerate after the push?

The force on the solar panel Fp is equal and opposite that on the astronaut Fa.

Fp = mpap; Fa = maaa ; Thus, mpap = - maaa ; solve for aa:

; a = - 5 m/s2

Acceleration exists only while a force is applied, once the force is removed, both astronaut

and solar panel move in opposite directions at the speeds obtained when contact is broken..

7-45. A 400-lb sled slides down a hill (k = 0.2) inclined at an angle of 600. What is the normal

force on the sled? What is the force of kinetic friction? What is the resultant force down

the hill? What is the acceleration? Is it necessary to know the weight of the sled to

determine its acceleration?

Fy = 0; N – W cos 600 = 0; N = (400 lb)cos 600 = 200 lb ;

F = kN = (0.2)(200 lb); F = 40 lb

Fx = W sin 600 – F = (400 lb)sin 600 – 40 lb; FR = 306 lb

7-45 (Cont.) Since FR = ma; we note that: W sin 600 - kW = (W/g)a; Thus, the weight

divides out and it is not necessary for determining the resultant acceleration.

*7-46. Three masses, m1 = 10 kg, m2 = 8 kg, and m3 = 6 kg, are connected as shown in Fig. 7-

16. Neglecting friction, what is the acceleration of the system? What are the tensions in

the cord on the left and in the cord on the right? Would the acceleration be the same if

the middle mass m2 were removed?

Total mass of system = (10 + 8 +6) = 24 kg

76

W = mg = 400 lb

+a

N F

600

600

N

10 kg 6 kg

++

TBTA

TBTA

m3gm1g

m2g

8 kg


Resultant Force on system = m1g – m3g

The normal force N balances m2g; F = mTa

m1g – m3g = (m1 + m2 +m3)a ; (10 kg)(9.8 m/s2) – (6 kg)(9.8 m/s2) = (24 kg) a

(24 kg)a = 98.0 N – 58.8 N; a = 1.63 m/s2 ; The acceleration is not affected by m2.

To find TA apply F = m1a to 10-kg mass: m1g – TA = m1a ; TA = m1g – m1a

TA = m1(g – a) = (10 kg)(9.8 m/s1.63 m/s2); TA = 81.7 N

Now apply to 6-kg mass: TB – m3g = m3a; TB = m3g + m3a

TB = (6 kg)(9.8 m/s1.63 m/s2) ; TB = 68.6 N

*7-47. Assume that k = 0.3 between the mass m2 and the table in Fig. 7-16. The masses m2 and

m3 are 8 and 6 kg, respectively. What mass m1 is

required to cause the system to accelerate to the

left at 2 m/s2? ( F = km2g acts to right. )

Apply F = mTa to total system, left is positive.

m1g – F – m3g = (m1 + m2 +m3)a ; F = km2g = 0.3(8 kg)(9.8 m/s2); F = 23.5 N

m1(9.8 m/s2) – 23.5 N - (6 kg)(9.8 m/s2) = (m1 + 14 kg)(2 m/s2)

9.8 m1 – 23.5 kg – 58.8 kg = 2m1 + 28 kg ; m1 = 14.1 kg

*7-48. A block of unknown mass is given a push up a 400 inclined plane and then released. It

continues to move up the plane (+) at an acceleration of –9 m/s2.

What is the coefficient of kinetic friction?

Since block is moving up plane, F is directed down plane.

F = kN ; Fy = 0; N = mg cos 400; F = kmg cos 400

Fx = ma; -F - mg sin 400 = ma; -kmg cos 400 - mg sin 400 = ma

77

N

6 kg

++

TBTA

TBTA

m3gm1g

m2g

8 kg

F

+F

F

mg

400

400


a = -kg cos 400 - g sin 400 ; -9 m/s2 = -k(9.8 m/s2) cos 400 - (9.8 m/s2) sin 400

Solving for k we obtain: k = 0.360

*7-49. Block A in Fig. 7-17 has a weight of 64 lb. What must be the weight of block B if Block

A moves up the plane with an acceleration of 6 ft/s2. Neglect friction.

Fx = ma: WB – WA sin 600 = (mA + mB) a

;

WB – (64 lb)(0.866) = 0.188(64 lb + WB)

WB – 55.4 lb = 12.0 lb + 0.188WB ; WB = 83.0 lb

*7-50. The mass of block B in Fig. 7-17 is 4 kg. What must be the mass of block A if it is to

move down the plane at an acceleration of 2 m/s2? Neglect friction.

Fx = ma: mAg sin 600 - mBg = (mA + mB) a

(9.8 m/s2)(0.866)mA – (4 kg)(9.8 m/s2) = mA(2 m/s2) + (4 kg)(2 m/s2)

8.49 mA – 39.2 kg = 2 mA + 8 kg; mA = 7.28 kg.

*7-51. Assume that the masses A and B in Fig. 7-17 are 4 kg and 10 kg, respectively. The

coefficient of kinetic friction is 0.3. Find the acceleration if (a) the system is initially

moving up the plane, and (b) if the system is initially moving down the plane?

(a) With upward initial motion, F is down the plane.

F = kN ; Fy = 0; N = mAg cos 600; F = kmAg cos 600

Resultant force on entire system = total mass x acceleration

mBg – mAg sin 600 - kmAg cos 600 = (mA + mB)a

(10 kg)(9.8 m/s2) – (4 kg)(9.8 m/s2)(0.866) – 0.3(10 kg)(9.8 m/s2)(0.5) = (14 kg)a

98 N – 33.9 N – 14.7 N = (14 kg)a; or a = 3.53 m/s2

78

mBg

T

T

600

mAg

+aN

600

T

T

WB

600

WA = 64 lb

+aN

600

F

mBg

T

T

600

mAg

+aN

600


(b) If initial motion is down the plane, then F is up the plane, but the resultant force is

still down the plane. The block will side until it stops and then goes the other way.

Fx = ma; mBg – mAg sin 600 + kmAg cos 600 = (mA + mB)a

98 N – 33.9 N + 14.7 N = (14 kg)a

a = 5.63 m/s2 The greater acceleration results from

the fact that the friction force is increasing the resultant force

instead of decreasing it as was the case in part (a).

Chapter 8. Work, Energy, and Power

Work

8-1. What is the work done by a force of 20 N acting through a parallel distance of 8 m? What

force will do the same work through a distance of 4 m?

Work = (20 N)(8 m) = 160 J ; F (4 m) = 160 J; F = 40.0 N

8-2. A worker lifts a 40 lb weight through a height of 10 ft. How many meters can a 10-kg

block be lifted by the same amount of work?

Work = (20 lb)(10 ft) = 200 ft lb;

Work = Fs = mgs; ; s = 2.77 m

8-3. A tugboat exerts a constant force of 4000 N on a ship, moving it a distance of 15 m. What

work is done?

Work = (4000 N)(15 m); Work = 60,000 J

8-4. A 5-kg hammer is lifted to a height of 3 m. What is the minimum required work?

79

F

mBg

T

T

600

mAg

+aN

600


Work = Fs = (5 kg)(9.8 m/s2)(3 m); Work = 147 J

8-5. A push of 30 lb is applied along the handle of a lawn mower producing a horizontal

displacement of 40 ft. If the handle makes an angle of 300 with the ground, what work was

done by the 30-lb force?

Work = (F cos )s = (30 lb) cos 300 (40 ft)

Work = 1040 ft lb

8-6. The trunk in Fig. 8-10 is dragged a horizontal distance of 24 m by a rope that makes an

angle with the floor. If the rope tension is 8 N, what works are done for the following

angles: 00, 300, 600, 900?

Work = (F cos )s = (8 N) cos 00 (24 m) = 192 J

Work = (8 N) cos 300 (24 m) = 166 J ; Work60 = 96 J ; Work90 = 0 J

8-7. A horizontal force pushes a 10-kg sled along a driveway for a distance of 40 m. If the

coefficient of sliding friction is 0.2, what work is done by the friction force?

Work = (F cos )s = (F) (cos 1800)s = - F s; but F = kN = k mg

Work = kmg s = (0.2)(10 kg)(9.8 m/s2)(40 m); Work = –784 J

*8-8. A sled is dragged a distance of 12.0 m by a rope under constant tension of 140 N. The

task requires 1200 J of work. What angle does the rope make with the ground?

Work = (F cos )s;

cos = 0.714; = 44.40

Resultant Work

80

8 N

s = 40 ft

P

Fk

N

300

W

140 N

12 m


8-9. An average force of 40 N compresses a coiled spring a distance of 6 cm. What is the work

done by the 40-N force? What work done by the spring? What is the resultant work?

Work40 = (40 N)(0.06 m) = 2.40 J, (positive work)

Worksp = (-40 N)(0.06 m) = -2.40 J, (negative work)

Resultant work = (works) = 2.4 J – 2.4 J = 0 J

Work is positive when force is with displacement, negative when against displacement.

8-10. A horizontal force of 20 N drags a small sled 42 m across the ice at constant speed. Find

the work done by the pulling force and by the friction force. What is the resultant force?

Work40 = (20 N)(24 m) = 2.40 J, (positive work)

Worksp = (-20 N)(24 m) = -2.40 J, (negative work)

Resultant force and, hence, resultant work are zero.

*8-11. A 10-kg block is dragged 20 m by a parallel force of 26 N. If k = 0.2, what is the

resultant work and what acceleration results.

F = kN = kmg F = 0.2(10 kg)(9.8 m/s2) = 19.6 N

Work = FR s = (P – F)s; Work = (26 N – 19.6 N)(20 m) Work = 128 J

FR = (26 N – 19.6 N) = 6.40 N; ; a = 0.640 m/s2

*8-12. A rope making an angle of 350 drags a 10-kg toolbox a horizontal distance of 20 m. The

tension in the rope is 60 N and the constant friction force is 30 N. What work is done by

the rope? What work is done by friction? What is the resultant work?

(Work)rope = (60 N) cos 350 (20 m); (Work)r = 983 J

(Work)F = (-30 N)(20 m) = -600 J; (Work)F = -600 J

Resultant Work = (works) = 983 J – 600 J; Resultant Work = 383 J

81

0.06 m

40 N

F42 m 20 N

F20 m 26 N

60 NN

mg

350F20 m


Extra work can show that for this example, k = 0.472

*8-13. For the example described in Problem 8-12, what is the coefficient of friction between the

toolbox and the floor. (Refer to figure and information given in previous problem.)

Fy = 0; N + (60 N) sin 350 – (10 kg)(9.8 m/s2) = 0 ; and N = 63.6 N

k = 0.472

*8-14. A 40-kg sled is pulled horizontally for 500 m where k = 0.2. If the resultant work is

50 kJ, what was the parallel pulling force?

F = kN = k mg = 0.2(40 kg)(9.8 m/s2); F = 78.4 N

(P – F) s = 50 kJ; (P – 78.4 N)(500 m) = 50,000 J; P = 178 N

*8-15. Assume that m = 8 kg in Fig. 8-11 and k = 0. What minimum work is required by the

force P to reach the top of the inclined plane? What work is required to lift the 8 kg

block vertically to the same height?

Minimum work is for P = W sin 400 with zero acceleration.

; W = mg = 78.4 N

WorkP = P s =(W sin 400) s = (78.4 N) sin 400 (18.67 m); WorkP = 941 J

(Work)V = W h = (78.4 N)(12 m); (Work)V = 941 J

*8-16. What is the minimum work by the force P to move the 8-kg block to the top of the incline

if k = 0.4. Compare this with the work to lift it vertically to the same height.

Fy = 0; N = mg cos 400; N = (78.4 N) cos 400 = 60.06 N

F = kN = (0.4)(60.06 N); F = 24.0 N

Fx = 0; P – F – mg sin 400 = 0; P = F + W sin 400

82

500 m N

F

mg

P

PN

400

400

12 m

F

PN

400

400

12 m


P = 24.0 N + (78.4 N) sin 400; P = 74.4 N; Recall that s = 18.67 m from Prob. 8-15.

WorkP = (74.4 N)(18.67 m) ; WorkP = 1390 J

From Prob. 8-15, the work to lift vertically is: (Work)V = 941 J

*8-17. What is the resultant work when the 8-kg block slides from the top to the bottom of the

incline in Fig. 8-11. Assume that k = 0.4.

The resultant work is the work of the resultant force:

FR = mg sin 400 – F = (78.4 N)sin 400 – 0.4(78.4 N)cos 400

FR = 26.4 N; Work = (26.4 N)(18.67 m) = 492 J

Work and Kinetic Energy

8-18. What is the kinetic energy of a 6-g bullet at the instant its speed is 190 m/s? What is the

kinetic energy of a 1200-kg car traveling at 80 km/h? (80 km/h = 22.2 m/s)

Ek = ½mv2 = ½(0.006 kg)(190 m/s)2; Ek = 217 J

Ek = ½mv2 = ½(1200 kg)(22.2 m/s)2; Ek = 296 kJ

8-19. What is the kinetic energy of a 2400-lb automobile when its speed is 55 mi/h? What is

the kinetic energy of a 9-lb ball when its speed is 40 ft/s? (55 mi/h = 80.7 ft/s)

Ek = ½mv2 = ½(75 slugs)(80.7 ft/s)2; Ek = 244,000 ft lb

Ek = ½mv2 = ½(0.281 slugs)(40 ft/s)2; Ek = 225 ft lb

83

mg

FN

400

400

12 m s = 18.67 m


8-20. What is the change in kinetic energy when a 50-g ball hits the pavement with a velocity of

16 m/s and rebounds with a velocity of 10 m/s?

Consider the upward direction as positive, then vo = -10 m/s and vf = -16 m/s.

Ek = ½mvf 2- ½mvo

2 = ½(0.05 kg)(10 m/s)2 - ½(0.05 kg)(-16 m/s)2

Ek = 2.50 J – 6.40 J = -3.90 J ; The change represents a loss of kinetic energy.

*8-21. A runaway, 400-kg wagon enters a cornfield with a velocity of 12 m/s and eventually

comes to rest. What work was done on the wagon?

Work = ½mvf2 - ½mvo

2 =(0) - ½(400 kg)(12 m/s)2; Work = -28.8 kJ

*8-22. A 2400-lb car increases its speed from 30 mi/h to 60 mi/h? What resultant work was

required? What is the equivalent work in joules?

vo = 30 mi/h = 44 ft/s; vf = 60 mi/h = 88 ft/s;


2 = ½(75 slugs)(88 ft/s)2 - ½(75 slugs)(44 ft/s)2;

Work = 217,800 ft lb ; Work = = 295 kJ

*8-23. A 0.6-kg hammer head is moving at 30 m/s just before striking the head of a spike. Find

the initial kinetic energy. What work can be done by the hammer head?

Ek = ½mv2 = ½(0.6 kg)(30 m/s)2; Ek = 270 J

Work = Ek = 0 – 200 J; Work = -270 J

*8-24. A 12-lb hammer moving at 80 ft/s strikes the head of a nail moving it into the wall a

distance of ¼ in. What was the average stopping force?

s = 0.250 in. (1 ft/12 in.) = 0.0203 ft; vo = 80 ft/s

84


Fs = ½mvf2 - ½mvo

2; F (0.0203 ft) = 0 - ½(0.375 slugs)(80 ft/s)2; F = 57,600 ft lb

8-25. What average force is needed to increase the velocity of a 2-kg object from 5 m/s to

12 m/s over a distance of 8 m?

Fs = ½mvf2 - ½mvo

2; F(8 m) = ½(2 kg)(12 m/s)2 - ½(2 kg)(5 m/s)2; F = 14.9 N

*8-26. Verify the answer to Problem 8-25 by applying Newton’s second law of motion.

To apply F = ma, we need to find a: 2as = vf2 – vo

2

F = (2 kg)(-7.44 m/s2) = -14.9 N

*8-27. A 20-g projectile strikes a mud bank in Fig. 8-12, penetrating a distance of 6 cm before

stopping. Find the stopping force F if the entrance velocity is 80 m/s.

Fs = ½mvf2 - ½mvo

2; F (0.06 m) = 0 - ½(0..02 kg)(80 m/s)2

F = -1070 N

*8-28. A 1500-kg car is moving along a level road at 60 km/h. What work is required to stop the

car? If k = 0.7, what is the stopping distance? (60 km/h = 16.67 m/s)


2 ; Work = 0 - ½(1500 kg)(16.67 m/s)2; Work = -208,300 J

The work is done by friction: F = kN = k mg and (Work)F= -(k mg)s

-(k mg)s = -208,300 J; ; s = 20.2 m

Potential Energy

8-29. A 2-kg block rests on top of a table 80 cm from the floor. Find the potential energy of the

book relative to (a) the floor, (b) the seat of a chair 40 cm from the floor, and (c) relative

to the ceiling 3 m from the floor?

850.4 m

0.8 m3 m


For Ep = mgh, the height h is measured from reference point:

For floor, h = 0.8 m; for seat, h = 0.4 m; for table, h = - 2.2 m

(a) Ep = (2 kg)(9.8 m/s2)(0.8 m) = 15.7 J

(b) Ep = (2 kg)(9.8 m/s2)(0.4 m) = 7.84 J ; (c) Ep = (2 kg)(9.8 m/s2)(-2.2 m) = -43.1 J

8-30. A 1.2 kg brick is held a distance of 2 m above a manhole. The bottom of the manhole is

3 m below the street. Relative to the street, what is the potential energy at each location?

What is the change in potential energy?

Ep = (1.2 kg)(9.8 m/s2)(2 m) = 23.5 J for held brick.

Ep = (1.2 kg)(9.8 m/s2)(-3 m) = -35.3 J for brick in hole

Ep = Ef – Eo = -35.3 J – (23.5 J); Ep = -58.8 J

8-31. At a particular instant a mortar shell has a velocity of 60 m/s. If its potential energy at that

point is one-half of its kinetic energy, what is its height above the earth?

Ek = ½mv2 and Ep = mgh; At the instant in question, Ep = ½Ek

and

h = 91.8 m

*8-32. A 20-kg sled is pushed up a 340 slope to a vertical height of 140 m. A constant friction

force of 50 N acts for the entire distance. What external work was required? What was

the change in potential energy?

External work done by force P acting for distance s:

Fx = 0: P – mg sin 340 – F = 0; F = 50 N, m = 20 kg

P = (5 kg)(9.8 m/s2) sin 340 + 50 N; P = 77.4 N

86

P

F

N

mg

340

340

140 m

600 N


; (Work)P = (77.4 N)(250 m); (Work)P = 19.400 J

Ep = mgh = (5 kg)(9.8 m/s2)(140 m); Ep = 6860 J

The difference: 19,400 J – 6860 J = 12,540 J is the work done against friction.

*8-33. An average force of 600 N is required to compress a coiled spring a distance of 4 cm.

What work is done BY the spring? What is the change in potential energy of the

compressed spring?

Work done BY spring is opposite to compressing force.

Work = (-600 N)(0.04 m) = -24.0 J Work by spring = - 24.0 J

Now, EP = 0 initially, so that Ep = -(Work)SP = -(-24 J); Ep = +24.0 J

Conservation of Energy (No Friction)

8-34. A 64-lb weight is lifted to a height of 10 ft and then released to fall freely. What is the

potential energy, the kinetic energy and the total energy at (a) the highest point, (b) 3 ft

above the ground, and (c) at the ground? (W = 64 lb, g = 32 ft/s

(a) EP = Wh = (64 lb)(10 ft) = 640 ft lb; EP = 640 ft lb

Ek = ½mv2 = 0 ( vo = 0); ET = EP + Ek = 640 ft lb + 0

At 10 ft: EP = 640 ft lb; Ek = 0; and ET = 640 ft lb

(b) EP = Wh = (64 lb)(3 ft) = 192 ft lb; EP = 192 ft lb

Ek = ET – EP; Ek = 640 ft lb – 192 ft lb; Ek = 448 ft lb

At 3 ft: EP = 192 ft lb; Ek = 448 ft lb; and ET = 640 ft lb

(c) At 0 ft h = 0 and ET is same: EP = 0 ft lb; Ek = 640 ft lb; and ET = 640 ft lb

87

0.04 m

h = 10 ft

h = 3 ft


8-35. A 4-kg hammer is lifted to a height of 10 m and dropped? What are the potential and

kinetic energies of the hammer when it has fallen to a point 4 m from the earth?

At 10 m: Ek = 0 and EP = mgh = (4 kg)(9.8 m/s2)(10 m); ET = 0 + 392 J = 392 J

Ep = mgh = (4 kg)(9.8 m/s2)(4 m); Ep = 157 J; Ek = ET – EP = 392 J – 157 J

Thus at h = 4 m: EP = 157 J and Ek = 235 J

*8-36. What will be the velocity of the hammer in Problem 8-35 just before striking the ground?

What is the velocity at the 4-m location?

At bottom, Ep = 0 and ET = 392 J so that Ek = 392 J – 0 = 392 J;

Ek = ½mv2 = 392 J v = 14.0 m/s

*8-37. What initial velocity must be given to a 5-kg mass if it is to rise to a height of 10 m?

What is the total energy at any point in its path?

In absence of friction, total energy at bottom must equal total energy at top:

ET = ½mvo2 + 0 = 0 + mgh; vo

2 = 2 gh

v0 = 14.0 m/s2 ET = Ep + Ek at any point

At top: ET = 0 + mgh = (5 kg)(9.8 m/s2)(10 m); ET = 490 J

*8-38. A simple pendulum 1 m long has an 8-kg bob. How much work is needed to move the

pendulum from its lowest point to a horizontal position? From energy considerations

find the velocity of the bob as it swings through the lowest point.

A force F equal to the weight mg must act through a distance equal to length of string:

Work = mgh = (8 kg)(9.8 m/s2)(1 m); Work = 78.4 J

The total energy at top(mgh) must be equal to total energy at bottom.(½mv2):

mgh = ½mv2 v = 4.43 m/s

88


*8-39. A ballistic pendulum is illustrated in Fig. 8-13. A 40-g ball is caught by a 500-g

suspended mass. After impact, the two masses rise a vertical distance of 45 mm. Find

the velocity of the combined masses just after impact? (See Figure next page)

Total mass M = 40 g + 500 g = 540 g; M = 540 g = 0.540 kg

*8-39. (Cont.) Find vo of total mass M such that M rises h = 0.045 m:

Energy conservation: ½Mv2 + 0 = 0 + Mgh;

v = 0.939 m/s

*8-40. A 100-lb sled slides from rest at the top of a 370 inclined plane. The original height is

80 ft. In the absence of friction, what is the velocity of the sled when it reaches the

bottom of the incline? (Not dependent on either angle or weight.)

Energy conservation: 0 + mgh = ½mv2 + 0;

v = 71.6 ft/s

*8-41. An 8-kg block in Fig. 8-14 has an initial downward velocity of 7 m/s. Neglecting

friction, find the velocity when it reaches point B?

½mvo2 + mgho = ½mvf

2 + mghf

vo2 + 2gho = vf

2 + 0 ; vf2 = vo

2 + 2gho

; vf = 21.0 m/s

*8-42. What is the velocity of the 8-kg block at point C in Problem 8-39? (Note hf 0 this time)

½mvo2 + mgho = ½mvf

2 + mghf ; vo2 + 2gho = vf

2 + 2ghf ; vf2 = vo

2 + 2gho – 2ghf

; vf = 16.9 m/s

89

h

h

C

B

7 m/s

8 m

20 m


*8-43. An 80-lb girl sits in a swing of negligible weight. If she is given an initial velocity of 20

ft/s, to what height will she rise?

Energy conservation: 0 + mgh = ½mv2 + 0;

h = 6.25 ft

Energy and Friction Forces

*8-44. A 60-kg sled slides to the bottom of a 250 slope of length 30 m. A 100-N friction force

acts for the entire distance. What is the total energy at the top of the slope and at the

bottom? What is the velocity of the sled at the bottom?

h = (20 m)sin 25m; At top: EP = mgh; Ek = 0

ET = EP + Ek = mgh + 0; ET = (60 kg)(9.8 m/s2)(8.45 m)

Total energy at top, ET = 4969 J Cons. of E: ET(top) = Ekf (bottom)+Losses

Loss = (Work)F = F s; Loss = (100 N)(30 m) = 3000 J

Cons. of E: 4960 J = ½(60 kg)v2 + 3000 J; From which: v = 8.10 m/s

*8-45. A 500-g block is released from the top of a 300 inline and slides 160 cm to the bottom. A

constant friction force of 0.9 N acts the entire distance. What is the total energy at the

top? What work is done by friction? What is the velocity at the bottom?

W = mg = (0.5 kg)(9.8 m/s2) = 4.90 N;

h = (1.60 m) sin 300 = 0.800 m; ET = EP + Ek

ET = Wh + 0; ET = (4.90 N)(0.80 m); ET = 3.92 J

(Work)F = F s = (-0.900 N)(1.60 m); (Work)F = -1.44 J (negative work)

Total energy at top = total energy at bottom + work done against friction

90

h

mg

FN

250

250

h s = 20 m

W

FN

300

300

h s = 160 m


3.92 J = ½mv2 + F s ; 3.92 J = ½(0.5 kg) v2 + 1.44 J

Solving for v, we obtain: v = 3.15 m/s

Note that the work done BY friction is negative, but when applying conservation of

energy we use the work AGAINST friction (+1.44 J) to account for the LOSS

*8-46. What initial velocity must be given to the 500-g block in Problem 8-43 if it is to just reach

the top of the same slope? (See previous problem)

From Prob. 8-43: F = 0.9 N, h =0.8 m, W = 4.90 N

½mvo2 = Whf + F s = 3.92 J + 1.44 J ; ½mvo

2 =5.36 J

vo = 4.63 m/s

*8-47. A 64-lb cart starts up a 370 incline with an initial velocity of 60 ft/s. If it comes to rest

after a moving a distance of 70 ft, how much energy was lost to friction?

W = 64 lb; m = (64/32) = 2 slugs; h = 70 sin 37m

½mvo2 = Whf + Loss; Loss = ½mvo

2 - Whf

Loss = ½(2 slugs)(60 ft/s)2 – (64 lb)(42.1 ft)

Loss = 3600 ft lb - 2240 ft lb; Loss = 904 ft lb

*8-48. A 0.4-kg ball drops a vertical distance of 40 m and rebounds to a height of 16 m. How

much energy was lost in collision with the floor? Conservation of energy.

mgho = mghf + Loss; Loss = mgho – mghf = mg(ho – hf)

Loss = (0.4 kg)(9.8 m/s2)(40 m – 16 m); Loss = 94.1 J

*8-49. A 4-kg sled is given an initial velocity of 10 m/s at the top of a 340 slope. If k = 0.2,

how far must the sled travel until its velocity reaches 30 m/s?

91

F

s = 160 m

W

N

300

300

h

W = 64 lb

vo = 60 ft/sF

s = 70 ft

N

300

300

h

s

10 m/s

FN

mg

340

340

ho

30 m/s


mgho + ½mvo2 = 0 + ½mvf

2 + F s and ho = s sin 340

mg(s sin 340) - F s = ½mvf2 - ½mvo

2 ; F = k mg cos 340

mg(s sin 340) – (kmg cos 340)s = ½mvf2 - ½mvo

2

*8-49. (Cont.) (g sin 340 - kg cos 340)s = ½vf2 - ½vo ;

s = 104 m

*8-50. Assume in Fig. 8-14 that the sliding mass is 6-kg and that 300 J of energy is lost doing

work against friction. What is the velocity when the mass reaches point C?

½mvo2 + mgho = ½mvc

2 + mghf + 300 J

½mvc2 = ½mvo

2 + mgho – mghc – 300 J

vc2 = vo

2 + 2g(ho – hc) -

vc = 13.6 m/s

*8-51. A bus slams on brakes to avoid an accident. The tread marks of the tires are 80 feet long.

If k = 0.7, what was the speed before applying brakes?

F = kmg; Work = F s = kmgs

Work = Ek ; -kmgs = ½mvf2 - ½mvo

2

vo = 59.9 ft/s

Power

92

0

C

B

7 m/s

8 m

20 m

vf = 0vo = ? F = kN

s = 80 ft


8-52. A power-station conveyor belt lifts 500 tons of ore to a height of 90 ft in one hour. What

average horsepower is required? (W = 500 tons = 1 x 106 lb; 1 hp = 550 ft lb/s)

; P = 25,000 ft lb/s = 45.5 hp

8-53. A 40-kg mass is lifted through a distance of 20 m in a time of 3 s. Find average power.

; P = 2610 W

8-54. A 300-kg elevator is lifted vertical distance of 100 m in 2 min. What is the output power?

; P = 2.45 kW

8-55. A 90 kW engine is used to lift a 1200-kg load. What is the average velocity of the lift?

; ; v = 7.65 m/s

8-56. To what height can a 400 W engine lift a 100-kg mass in 3 s?

; h = 0.122 m

8-57. An 800-N student runs up a flight of stairs rising 6 m in 8 s. What is the average power?

; P = 600 W

*8-58. A speedboat must develop a 120 hp in order to move at a constant speed of 15 ft/s through

the water. What is the average resistive force due to the water?

93


; F = 4400 lb

Challenge Problems

*8-59. A worker lifts a 20-kg bucket from a well at constant speed and does 8 kJ of work. How

deep is the well?

Work = Fs ; ; s = 40.8 m

*8-60. A horizontal force of 200 N pushes an 800-N crate horizontally for a distance of 6 m at

constant speed. What work is done by the 200-N force. What is the resultant work?

Work = Fs = (200 N)(6 m); Work = 1200 J

Speed is constant, so FR = 0, and Resultant work = 0 J

*8-61. A 10-kg mass is lifted to a height of 20 m, and released. What is the total energy of the

system? What is the velocity of the mass when it is located 5 m from the floor?

In absence of friction, total energy is constant, so that: ET (Top) = ET(5 m)

ET = mgh + 0 = (10 kg)(9.8 m/s2)(20 m); ET = 1960 J

When h = 5 m, (10 kg)(9.8 m/s2)(5 m) + ½(10 kg)vf2 = 1960 J

490 J + (5 kg)vf2 = 1960 J; vf = 17.1 m/s

*8-62. A crate is lifted at a constant speed of 5 m/s by an engine whose output power is 4 kW.

What is the mass of the crate?

94

0


; m = 81.6 kg

*8-63. A roller coaster boasts a maximum height of 100 ft. What is the maximum speed in miles

per hour when it reaches its lowest point? (Conservation of Energy)

mgh + 0 = 0 + ½mv2 ; ; v = 80.0 ft/s or 54.4 mi/h

*8-64. A 20-N force drags an 8-kg block a horizontal distance of 40 m by a rope at an angle of

370 with the horizontal. Assume k = 0.2 and that the time required is one minute. What

resultant work is done?

Resultant work = work of the resultant force

FR = (20 N)cos 37 - F) F = kN Fy = 0

N + 20 sin370 – (8 kg)9.8 m/s2) = 0; N = 66.4 N; F = 0.2(66.4 N) = 13.3 N

FR = 20 cos 370 – 13.3 N = 2.70 N; Work = FRs = (2.70 N)(40 m); Work = 108 J

*8-65. What is the velocity of the block in Problem 8-60 at the end of the trip? What resultant

power was expended? (Assume block starts from rest, then apply work-energy theorem.)


2 ; 108 J = ½(8 kg)v2 + 0; ; v = 5.20 m/s

P = 1.80 W

*8-66. A 70-kg skier slides down a 30 m slope that makes an angle of 280 with the horizontal.

Assume that k = 0.2. What is the velocity of the skier at the bottom of the slope?

mgho + ½mvo2 = 0 + ½mvf

2 + F s and ho = s sin 280

95

20 NN

mg

F37

40 m

30 m

FN

mg

280

280

ho

vf


mg(s sin 280) - F s = ½mvf2 - ½mvo

2 ; F = k mg cos 280

mg(s sin 280) – (kmg cos 280)s = ½mvf2

*8-66. (Cont.) (g sin 280 - kg cos 280)s = ½vf2;

vf = 13.1 m/s

*8-67. A 0.3 mg flea can jump to a height of about 3 cm. What must be the takeoff speed? Do

you really need to know the mass of the flea?

½mvo2 = mgh ; ;

v = 0.767 m/s ; The mass is not needed.

*8-68. A roller coaster goes through a low point and barely makes the next hill 15 m higher.

What is the minimum speed at the bottom of the loop?

½mvo2 = mgh ; ; v = 17.1 m/s

*8-69. The hammer of a pile driver weighs 800 lb and falls a distance of 16 ft before striking the

pile. The impact drives the pile 6 in. deeper into the ground. What was the average force

driving the pile? m = W/g = (800/32) = 25.0 slugs; s = 6 in. = 0.5 ft

The work done by the pile driver Fs is determine from the change in kinetic energy, so

we need to find the velocity of the driver just before striking the stake:

½mvf2 = mgho ; ; v = 32.0 ft/s

Work to stop driver = change in kinetic energy of driver

Fs = ½mvf2 - ½mvo

2; F = -25,600 lb

96



*8-70. An inclined board is used to unload boxes of nails from the back of a truck. The height

of the truck bed is 60 cm and the board is 1.2 m in length. Assume that k = 0.4 and the

boxes are given an initial push to start sliding. What is their speed when they reach the

ground below. What initial speed would they need at the bottom in order to slide back

into the truck bed? In the absence of friction would these two questions have the same

answer? [ h = 0.6 m; s = 1.2 m ; sin = 0.6/1.2; = 300 ]

(a) (Work)F = F s = kN = k mg cos 300 s

mgh = ½mv2 + k mg cos 300 s; 2gh = v2 +2kgs cos 300

v2 = 2gh – 2gk s cos 300 = 2(9.8 m/s2)(0.6 m) – 2(9.8 m/s2)(0.4)(1.2 m)(0.866)

v2 = 11.76 – 8.15 = 3.61 m2/s2; ; v = 1.90 m/s

(b) Going up the plane, the initial speed must provide the energy to overcome the

friction force which would now be directed DOWN the plane.

½mv2 = mgh + kmg cos 300 s (Note the difference in this equation from that above.)

v2 = 2gh +2kgs cos 300 = (2)(9.8)(0.6) + 2(0.4)(9.8)(1.2)(0.866)

v2 = 11.76 + 8.15 = 19.9 m2/s2; ; v = 4.46 m/s

In the downhill case, the initial potential energy was lost to friction and what little

remained appeared in the form of a small velocity at the bottom. In the uphill case, the

initial kinetic energy (high velocity) was used to gain the height h, but more energy was

needed to overcome friction. In the absence of friction, height is transferred into velocity

97

mg

FN

h=0.6 m s = 1.20 m


going down, and velocity is transferred to height going up. Thus, absent friction, the

same velocities would be found for each of the above cases. (v = 3.43 m/s)

*8-71. A 96-lb safe is pushed with negligible friction up a 300 incline for a distance of 12 ft.

What is the increase in potential energy? Would the same change in potential energy

occur if a 10-lb friction force opposed the motion up the incline? Why? Would the same

work be required? [ h = 12 sin 300 = 6.00 ft ]

(a) Ep = Wh = (96 lb)(6 ft) Ep = 576 ft lb

(b) Ep is a function only of weight and height, so the same change

in potential energy occurs regard less of friction or the path taken.

(c) With a 10-lb friction force, a work of (10 lb)(12 ft) = 120 ft lb is needed in addition

to the work of 576 ft lb just to lift the weight. The total work is 696 ft lb.

*8-72. A 2-kg ball is suspended from a 3-m cable attached to a spike in the wall. The ball is

pulled out, so that the cable makes an angle of 700 with the wall, and then released. If 10 J

of energy are lost during the collision with the wall, what is the maximum angle between

the cable and the wall after the first rebound?

yo = (3 m) cos 700 = 1.026 m

ho = 3 m – 1.026 m; ho = 1.974 m

mgho = mghf + 10 J; mghf = mgho – 10 J

; hf = 1.464 m ; yf = 3 m – 1.464 m = 1.536 m

; = 59.20

98

mgF

N

300

300

h=6 ft

s = 12 ft

ho

700

hf

3 m3 myo

yf


*8-73. A 3-kg ball dropped from a height of 12 m has a velocity of 10 m/s just before hitting the

ground. What is the average retarding force due to the air? If the ball rebounds from the

surface with a speed of 8 m/s, what energy was lost on impact? How high will it rebound

if the average air resistance is the same as before?

First apply E conservation to the falling portion of the problem:

mgho = ½mv2 + F s ; F s = mgho - ½mv2

F (12 m) = (3 kg)(9.8 m/s2)(12 m) - ½(3 kg)(10 m/s)2; F = 16.9 N

The loss or work done on impact equals the change in Ek: Loss = Work = ½mvf2 - ½mvo

2

Work = ½(3 kg)(8 m/s)2 - ½(3 kg)(10 m/s)2; Work = - 54 J; Impact loss = 54 J

To find rebound height, apply conservation of energy with losses to air and to impact:

mgho = mghf + (F s)Air + Impact loss; ( s = 12 m + hf )

(3 kg)(9.8 m/s2)(12 m) = (3kg)(9.8 m/s2)hf + (16.9 N)(12 m + hf) + 54 J

353 J = (29.4 N)hf + 203 J + (16.9 N)hf + 54 J; hf = 2.07 m

*8-74. Consider a roller coaster where the first hill is 34 m high? If the coaster losses only 8%

of its energy between the first two hills, what is the maximum height possible for the

second hill?

mgho = mghf + 0.08 mgho; hf = (1 – 0.08)ho = 0.92 (34 m) hf = 31.3 m

99

hf12 m

8 m/s

10 m/s


*8-75. A 4-kg block is compressed against a spring at the bottom the inclined plane in Fig. 8-15.

A force of 4000 N was required to compress the spring a distance of 6 cm. If it is then

released and the coefficient of friction is 0.4, how far up the incline will the block move?

Work to compress spring = (4000 N)(0.06 m) = 240 J = Ep

Ep(spring) = mgh + F s; F s = kmg cos 300 s

F s = (0.4)(4 kg)(9.8 m/s2)(0.866) s = (13.6 N) s

240 J = (4 kg)(9.8 m/s2)h + (13.6 N) s h = s sin 300 = 0.5 s

240 J = (4 kg)(9.8 m/s2)(2 s) + (13.6 N) s; s = 2.61 m

Chapter 9. Impulse and Momentum 9-1. A 0.5-kg wrench is dropped from a height of 10 m. What is its momentum just before it

strikes the floor? (First find the velocity from conservation of energy.)

mgh = ½mv2; v = 14.0 m/s

p = mv = (0.5 kg)(14 m/s); p = 7.00 kg m/s, down

9-2. Compute the momentum and kinetic energy of a 2400-lb car moving north at 55 mi/h.

v = 55 mi/h = 80.7 ft/s

p = mv = (75 slugs)(80.7 ft/s); p = 6050 slug ft/s

K = ½mv2 = ½(75 slugs)(80.66 ft/s)2; K = 244,000 ft lb

100

F

N

mg

340

340

h


9-3. A 2500-kg truck traveling at 40 km/h strikes a brick wall and comes to a stop in 0.2 s. (a)

What is the change in momentum? (b) What is the impulse? (c) What is the average force

on the wall during the crash? Take + to be toward the wall. ( 40 km/h = 11.1 m/s)

p = mvf – mvo = 0 - (2500 kg)(11.1 m/s); p = - 27,800 kg m/s

Impulse = p; F t = -27,800 kg m/s

Force ON truck: F = -139,000 N

Force on wall is opposite, so F = + 139,000 N

9-4. What is the momentum of a 3-kg bullet moving at 600 m/s in a direction 300 above the

horizontal? What are the horizontal and vertical components of this momentum?

p = mv = (3 kg)(600 m/s); p = 1800 kg m/s, 300

px = 1800 cos 300 and py = 1800 sin 300; px = 1560 kg m/s; py = 900 kg m/s

*9-5. A 0.2-kg baseball traveling to the left at 20 m/s is driven in the opposite direction at 35

m/s when it is hit by a bat. The average force on the ball is 6400 N. How long was it in

contact with the bat? ( Impulse = change in momentum. )

F t = mvf – mvo = (0.2 kg)(35 m/s) – (0.2 kg)(-20 m/s)

(6400 N) t = 11 kg m/s; t = 1.72 ms

*9-6. A bat exerts an average force of 248 lb on 0.6-lb ball for 0.01 s. The incoming velocity of

the ball was 44 ft/s. If it leaves in the opposite direction what is its velocity?

Choose positive + as direction away from the bat, making incoming ball velocity negative:

F t = mvf – mvo ; F t = mvf – mvo ;

(240 lb)(0.01 s) = (0.01875 slugs)vf - (0.01875 slugs)(-44 ft/s)

101

t-20 m/s

35 m/s

+

300

600 m/s


0.01875 vf = 2.4 lb s – 0.825; vf = 84.0 ft/s

*9-7. A 500-g ball travels from left to right at 20 m/s. A bat drives the ball in the opposite

direction with a velocity of 36 m/s. The time of contact was 0.003 s. What was the

average force on the ball? ( m = 0.5 kg, vo = +20 m/s, vf = -36 m/s, t = 0.003 s )

F t = mvf – mvo ; F(0.003 s) = (0.5 kg)(-36 m/s) – (0.5 kg)(20 m/s)

; F = 9330 N

*9-8. A 400-g rubber ball is dropped a vertical distance of 12 m onto the pavement. It is in

contact with the pavement for 0.01 s and rebounds to a height of 10 m. What is the total

change in momentum? What average force is exerted on the ball?

To apply the impulse-momentum theorem, we need to first find the velocities

just before and just after impact with the ground.

(Ep)Beginning = (Ek)Ground ; mgho = ½mvo2;

vo = - 15.3 m/s

½mvf2 = mghf ; vf = + 14 m/s

Ft = mvf – mvo; F(0.01 s) = (0.4 kg)(14 m/s) – (0.4 kg)(-15.3 m/s); F = 1170 N

*9-9. A cue stick strikes an eight-ball with an average force of 80 N over a time of 12 ms. If the

mass of the ball is 200 g, what will be its velocity?

Ft = mvf – mvo; (80 N)(0.012 s) = (0.2 kg)vf – 0; v = 4.80 m/s

9-10. A golfer hits a 46 g golf ball with an initial velocity of 50 m/s at 300. What are the x- and

y-components of the momentum imparted to the ball?

vx = (50) cos 300 = 43.3 m/s ; vy = (50) sin 300 = 25.0 m/s

102

vf

10 mhf

12 m

vo

vy

vx300

50 m/s


px = (0.046 kg)(43.3 m/s); py = (0.046 kg)(25 m/s) px = 1.99 kg m/s; py = 1.15 m/s

*9-11. The face of the club in Problem 9-10 is in contact with the ball for 1.5 ms. What are the

horizontal and vertical components of the average force on the ball?

We need to treat horizontal and vertical impulses and momenta separately:

From previous problem: po = 0, pf = 1.99 kg m/s, pfy = 1.15 kg m/s

Fx t = pfx – pox =1.99 kg m/s; ; Fx = 1330 N

Fx t = pfx – pox =1.15 kg m/s; ; Fy = 767 N

Conservation of Momentum

9-12. A spring is tightly compressed between a 6-kg block and a 2–kg block and then tied with a

string. When the string breaks, the 2-kg block moves to the right with a velocity of 9

m/s. What is the velocity of the 6-kg block?

Total momentum is zero before and after the event.

0 + 0 = m1v1 + m2v2 ; ; v1 = - 3.00 m/s

9-13. Two masses, one three times that of the other, are compressed against a spring and then

tied together on a frictionless surface as shown in Fig. 9-8. The connecting string

breaks and sends the smaller mass to the left with a velocity of 10 m/s. What was the

velocity of the larger mass?

Momentum zero before and after: 0 + 0 = m1v1 + m2v2

103

v1

m2 = 2 kgm1 = 6 kg kg

v2

v1

v210 m/s3m

m


; v1 = - 3.33 m/s

9-14. A 70-kg person standing on a frictionless surface throws a football forward with a velocity

of 12 m/s. If the person moves backward at 34 cm/s, what was the mass of the football?

Momentum zero before and after: 0 + 0 = m1v1 + m2v2

; m2 = 1.98 m/s

9-15. A 20-kg child is at rest in a wagon. The child jumps forward at 2 m/s, sending

the wagon backward at 12 m/s. What is the mass of the wagon?

0 = m1v1 + m2v2 ; ; m1 = - 3.33 kg

9-16. Two children, weighing 80 and 50 lb, are at rest on roller skates. The larger child pushes

so that the smaller moves away at 6 mi/h. What is the velocity of the larger child?

0 = m1v1 + m2v2 ; ; v1 = - 3.75 ft/s

(Here were able to use the weight because it is proportional to the mass)

9-17. A 60-g firecracker explodes, sending a 45-g piece to the left and another to the right with a

velocity of 40 m/s. What is the velocity of the 45-g piece?

The two pieces add to 60 g: m1 + m2 = 60 g. Thus, m1 = 45 g, m2 = 15 g

0 = m1v1 + m2v2 ; ; v1 = - 13.3 m/s

*9-18. A 24-g bullet is fired with a muzzle velocity of 900 m/s from a 5-kg rifle. Find the recoil

velocity of the rifle and the ratio of the kinetic energy of the bullet to that of the rifle?

104


0 = m1v1 + m2v2 ; ; v1 = - 4.32 m/s

; Ratio = 208

*9-19. A 6-kg bowling ball collides head on with 1.8-kg pin. The pin moves forward at 3 m/s

and the ball slows to 1.6 m/s. What was the initial velocity of the bowling ball?

mbub + 0 = mbvb + mpvp; (6 kg)ub = (6 kg)(1.6 m/s) + (1.8 kg)(3 m/s)

6ub = 9.6 m/s + 5.4 m/s; ub = 2.50 m/s

*9-20. A 60-kg man on a lake of ice catches a 2-kg ball. The ball and man each move at 8 cm/s

after the ball is caught. What was the velocity of the ball before it was caught? What

energy was lost in the process? (A completely inelastic collision: vc = vm = vb = 8 cm/s)

mbub + mmum = (mb + mm)vc ; (2 kg)ub + 0 = (2 kg + 60 kg)(0.08 m/s)

2ub = 4.96 m/s; ub = 2.48 m/s

½mbub2 + 0 =(mb + mm)vc

2; ½(2 kg)(2.48 m/s)2 = ½(62 kg)(0.08 m/s)2 + Loss

Loss = 6.15 J – 0.198 J; Loss = 5.95 J

*9-21. A 200-g rock traveling south at 10 m/s strikes a 3-kg block initially at rest. (a) If the two

stick together on collision, what will be their common velocity? (b) What energy was

lost in the collision?

mrur + mbub = (mr + mb)vc ; (0.2 kg)(10 m/s) + 0 = (0.2 kg + 3 kg)vc

2 m/s = 3.2 vc ; vc = 0.625 m/s

105


½mrur2 + 0 =(mr + mb)vc

2; ½(0.2 kg)(10 m/s)2 = ½ (3.2 kg)(0.625 m/s)2 + Loss

Loss =10.0 J – 0.625 J; Loss = 9.38 J

Elastic and Inelastic Collisions

9-22. A car traveling at 8 m/s crashes into a car of identical mass stopped at a traffic light. What

is the velocity of the wreckage immediately after the crash, assuming the cars stick

together? ( u1 = 8.00 m/s; u2 = 0, m1 = m2 = m)

mu1 + mu2 = (m + m)vc ; mu1 = 2mvc

; vc = 4.00 m/s

9-23. A 2000-kg truck traveling at 10 m/s crashes into a 1200-kg car initially at rest. What is the

common velocity after the collision if they stick together? What is the loss in energy?

m1u1 + m2u2 = (m1 + m2)vc ; (2000 kg)(10 m/s) + 0 = (2000 kg + 1200 kg)vc

20,000 m/s = 3200 vc ; vc = 6.25 m/s

½m1u12 + 0 =(m1 + m2)vc

2; ½(2000 kg)(10 m/s)2 = ½(3200 kg)(6.25 m/s)2 + Loss

Loss = 100,000 J – 62,500 J; Loss = 37,500 J

9-24. A 30-kg child stands on a frictionless surface. The father throws a 0.8-kg football with a

velocity of 15 m/s. What velocity will the child have after catching the football?

m1u1 + 0 = m1v1 + m2v2; (0.8 kg)(15 m/s) = (30 kg + 0.8 kg)vc

(30.8 kg)vc = 12 m/s; vc =0.390 m/s

*9-25. A 20-g object traveling to the left at 8 m/s collides head on with a 10-g object traveling to

the right at 5 m/s. What is their combined velocity after impact?

106

0


m1u1 + m2u2 = (m1 + m2)vc ; (20 g)(-8 m/s) + (10 g)(5 m/s) = (20 g + 10 g)vc

-110 m/s = 30 vc ; vc = -3.67 m/s, to left

*9-26. Find the percent loss of energy for the collision in Problem 9-25.

Conservation of Energy: ½m1u12 + ½m2u2

2 =½(m1 + m2)vc2 + Loss

½(20 g)(-8 m/s)2 + ½(10 g)(5 m/s)2 = ½(30 g)(-3.67 m/s)2 + Loss

765 J = 202 J + Loss; Loss = 563 J; %Loss = = 73.6%

*9-27. A 2-kg block of clay is tied to the end of a string as shown in

Fig. 9-9. A 500-g steel ball embeds itself into the clay causing both

to rise a height of 20 cm. Find the entrance velocity of the ball?

Before applying momentum conservation, we need to know the common velocity of the

clay and ball after the collision. Energy is conserved : ½(m1 + m2) vc2 = (m1 + m2) gh

; vc = 1.98 m/s

m1u1 + 0 = (m1 + m2) vc ; (0.5 kg) u1 = (0.5 kg + 2 kg)(1.98 m/s)

(0.5 kg)u1 = 4.95 m/s; u1 = 9.90 m/s

*9-28. In Problem 9-27, suppose the 500-g ball passes entirely through the clay an emerges with

a velocity of 10 m/s. what must be the new entrance velocity if the block is to raise to the

same height of 20 cm?

We must find the velocity v2 of the clay (m2) after collision:

½(m1 + m2) v22 = (m1 + m2) gh

107

10 m/s

h

h


; v2 = 1.98 m/s;

Momentum is conserved: m1u1 + 0 = m1v1 + m2v2;

(0.5 kg)u1 = (0.5 kg)(10 m/s) + (2 kg)(1.98 m/s); u1 = 17.9 m/s

*9-29. A 9-g bullet is embedded into a 2.0 kg ballistic pendulum (see Fig. 8-13). What was the

initial velocity of the bullet if the combined masses rise to a height of 9 cm?

½(m1 + m2) vc2 = (m1 + m2) gh

; vc = 1.33 m/s

m1u1 + 0 = (m1 + m2) vc ; (0.009 kg) u1 = (0.009 kg + 2 kg)(1.33 m/s)

(0.009 kg)u1 = 2.68 m/s; u1 = 297 m/s

*9-30. A billiard ball moving to the left at 30 cm/s collides head on with another ball moving to

the right at 20 cm/s. The masses of the balls are identical. If the collision is perfectly

elastic, what is the velocity of each ball after impact? (Consider right as +.)

Momentum: m1u1 + m2u2 = m1v1 + m2v2 Given: m1 = m2 = m, v1 = -30 cm/s, v2 = 0

m(-30 cm/s) + 0 = mv1 + mv2 ; v1 + v2 = (-30 cm/s) + (20 cm/s); v1 + v2 = -10 cm/s

Energy (e = 1): v2 – v1 = u1 – u2 = (-30 cm/s) – (20 cm/s); v2 – v1 = - 50 cm/s

From second equation: v2 = v1 – 50 cm/s; Substituting this for v2, we obtain:

v1 + (v1 - 50 cm/s) = - 10 cm/s; and v1 = 20 cm/s, to right

And, v2 = v1 – 50 cm/s = (20 cm/s) – 50 cm/s; v2 = -30 cm/s, to left

9-31. The coefficient of restitution for steel is 0.90. If a steel ball is

dropped from a height of 7 m, how high will it rebound?

108

h2

h1 7 m


; h2 =5.67 m

*9-32. What is the time between the first contact with the surface and the second contact for

Problem 9-31? (We need to know vo to rise to 5.67 m, then find t.)

; vo = 10.54 m/s

;

t = 1.07 s; T = 2t; T = 2.15 s

*9-33. A ball dropped from rest onto a fixed horizontal plate rebounds to a height that is 81

percent of its original height. What is the coefficient of restitution? What is the required

velocity on the first impact to cause the ball to rebound to a height of 8 m.

e = 0.900

; v2 = u2 = 0; ; u1 = -1.11v1

; v1 = -12.5 m/s

u1 = -1.11v1; u1 = -1.11(-12.5 m/s); u1 = 13.9 m/s

*9-34. A 300-g block moving north at 50 cm/s collides with a 200-g block moving south at 100

cm/s. If the collision is completely inelastic, what is their common velocity after sticking

together? What is the loss in energy? (Consider north as positive)

109

+h1h2


Momentum: m1u1 + m2u2 = m1v1 + m2v2 ; v1 = v2 = vc for inelastic collision

(300 g)(50 cm/s) + (200 g)(-100 cm/s) = (300 g + 200 g)vc

15,000 g cm/s – 20,000 g cm/s = (500 g)vc; vc = -10 cm/s, south

(Note: When working with energy, it is necessary to use kg for the mass unit.)


2 =½(m1 + m2)vc2 + Loss

½(0.3 kg)(-8 m/s)2 + ½(0.2 kg)(5 m/s)2 = ½(0.3 kg + 0.2 kg)(-3.67 m/s)2 + Loss

Solving for “loss”, we obtain: Loss = 0.135 J

*9-35. Suppose the collision in Prob. 9-34 is perfectly elastic. Find the velocities after impact.

m1u1 + m2u2 = m1v1 + m2v2; m1 = 300 g, m2 = 200 g, u1 = 50 cm/s, u2 = - 100 cm/s

(300 g)(50 cm/s) + (200 g)(-100 cm/s) = (300 g)v1 + (200 g)v2

Dividing each term by 100 g: 3 v 1 + 2 v2 = - 50 cm/s

Energy (e = 1): v2 – v1 = u1 – u2 = (50 cm/s) – (-100 cm/s); v2 – v1 = 150 cm/s

Substitute v2 = v1 + 150 cm/s into the earlier equation and solve for v1:

3 v1 + 2 (v1 + 150 cm/s) = - 50 cm/s; v1 = -80 cm/s, to left

v2 – (-80 cm/s) = 150 cm/s; v2 = 70 cm/s, to right

*9-36. A 5-lb and a 12-lb object approach each other with equal velocities of 25 ft/s. What will

be their velocities after impact if the collision is (a) completely inelastic or (b) perfectly

elastic? Since weight is proportional to mass, we will use the weights instead.

Momentum: W1u1 + W2u2 = W1v1 + W2v2 ; v1 = v2 = vc for inelastic collision

(5 lb)(25 ft/s) + (12 lb)(-25 ft/s) = (5 lb + 12 lb)vc; vc = -10.3 ft/s

Elastic case: (5 lb)(25 ft/s) + (12 lb)(-25 ft/s) = (5 lb)v1 + (12 lb)v2 ;

Dividing each term by 5 lb: v1 + 2.4 v 2 = - 35 ft/s

110


Energy (e = 1): v2 – v1 = u1 – u2 = (25 ft/s) – (–25 ft/s); v2 – v1 = 50 ft/s

Substitute v1 = v2 - 50 ft/s into the earlier equation and solve for v1:

(v2 – 50 ft/s) + 2.4 v2 = - 35 ft/s; v2 = 4.41 ft/s

v1 = v2 – 50 ft/s = 4.41 ft/s – 50 ft/s; v1 = -45.6 ft/s

Challenge Problems

9-37. An average force of 4000 N acts on a 400-g piece of metal causing it to move from rest to

a velocity of 30 m/s. What was the time of contact for this force?

F t = mvf – mvo = (0.4 kg)(30 m/s) – 0;

(4000 N)t = 12 kg m/s; t = 3.00 ms

*9-38. An 600-g object whose velocity is initially 12 m/s, collides with a wall and rebounds

with half of its original kinetic energy. What impulse was applied by the wall?

½mv02 = 2(½mvf

2) ;

F t = mvf – mvo = (0.6 kg)( –2.45 m/s) – (0.6 kg)(12 m/s); F t = -12.3 N m

*9-39. A 10-kg block at rest on a horizontal surface is struck by a 20-g bullet moving at 200

m/s. The bullet passes entirely through the block and exits with a velocity of 10 m/s.

What is the velocity of the block?

m1u1 + 0 = m1v1 + m2v2; m1 = 0.02 kg

(0.02 kg)(200 m/s) = (0.02 kg)(10 m/s) + (10 kg)v2 ; v2 = 0.380 m/s

9-40. In Problem 9-39, how much kinetic energy was lost?

111

v2 = ? v1 = 10 m/s


Conservation of Energy: ½m1u12 + 0 = m1v1

2 + m2v22 + Loss

½(0.2 kg)(200 m/s)2 = ½(0.2 kg)(10 m/s)2 + ½(10 kg)(0.380 m/s)2 + Loss

Solving for “loss”, we obtain: Loss =3990 J

*9-41. A 60-g body having an initial velocity of 100 cm/s to the right collides with a 150-g body

moving to the left at 30 cm/s. The coefficient of restitution is 0.8. What are the

velocities after impact. What percent of the energy is lost in collision?

m1u1 + m2u2 = m1v1 + m2v2; m1 = 60 g, m2 = 150 g, u1 = 100 cm/s, u2 = - 30 cm/s

(60 g)(100 cm/s) + (150 g)(-30 cm/s) = (60 g)v1 + (150 g)v2

Divide each term by 60 g and simplify: v1 + 2.5 v2 = 25 cm/s

v2 – v1 = e(u1 – u2); v2 – v1 = 0.8[100 cm/s – (-30 cm/s)]; v2 – v1 = 104 cm/s

Solve for v1: v1 = v2 – 104 cm/s; Now substitute to find v2.

*9-41. (Cont.) (v2 – 104 cm/s) + 2.5 v2 = 25 cm/s; v2 = 36.9 cm/s, to right

v1 = v2 – 104 cm/s = (36.9 cm/s) – 104 cm/s; v1 = -67.1 cm/s, to left


2 = m1v12 + m2v2

2 + Loss

For energy we must use SI units with mass in “kg” and velocity in ”m/s.”

Eok = ½(0.06 kg)(1 m/s)2 + ½(0.15 kg)(-0.3 m/s)2; Eok = 0.03675 J

Efk = ½(0.06 kg)(-0.671 m/s)2 + ½(0.15 kg)(0.369 m/s)2; Efk = 0.0237 J

%Loss = 35.5%

*9-42. The block in Fig. 9-6 weighs 6 lb. How high will it rise if a 0.4-lb projectile with an

initial velocity of 90 ft/s embeds itself into the block?

Momentum is conserved: m1u1 + 0 = m1v1 + m2v2; m W

(0.4 lb)(90 ft/s2) = (0.4 kg + 6 lb)vc vc = 5.625 ft/s

112

h


Now, we can find h using conservation of energy and the common initial velocity vc:

½(m1 + m2) v22 = (m1 + m2) gh ; The mass divides out.

; h = 0.494 ft; h = 5.93 in.

*9-43. A single railroad car moving north at 10 m/s strikes two identical, coupled cars initially

moving south at 2 m/s. If all three couple together after colliding, what is their common

velocity? m1 = m2 = m3 = m; u1 = 10 m/s; u2 = u3 = -2 m/s; v1 = v2 = v3 =vc

Momentum is conserved: mu1 + m(u2 + u3) = (3m)vc (Mass divides out.)

10 m/s – 2 m/s – 2 m/s = 3 vc; vc = 2.00 m/s, north

*9-44. An atomic particle of mass 2.00 x 10-27 kg moves with a velocity of 4.00 x 106 and

collides head on with a particle of mass 1.20 x 10-27 kg initially at rest. Assuming a

perfectly elastic collision, what is the velocity of the incident particle after the collision?

m1u1 + 0 = m1v1 + m2v2; m1 = 2 x 10-27 kg, m2 = 1.2 x 10-27 kg, u1 = 4 x 106 m/s

(2 x 10-27 kg)( 4 x 106 m/s) = (2 x 10-27 kg)v1 + (1.2 x 10-27 kg )v2

Dividing each term by 2 x 10-27 kg: v 1 + 0.6 v 2 = 4 x 10 6 m/s

Energy (e = 1): v2 – v1 = u1 – u2 = 4 x 106 m/s – 0 ; v2 – v1 = 4 x 10 6 m/s

Substitute v2 = ( v1 + 4 x 106 m/s) into the earlier equation and solve for v1:

v1 + 0.6 (v1 + 4 x 106 m/s) = 4 x 106 m/s; v1 = 1.00 x 106 m/s

*9-45. A bat strikes a 400-g softball moving horizontally to the left at 20 m/s. It leaves the bat

with a velocity of 60 m/s at an angle of 300 with the horizontal. What are the horizontal

and vertical components of the impulse imparted to the ball?

First find components of velocity: v1x = - 20 m/s; v1y = 0

113

v1x-20 m/s

v2y

v2x300

60 m/s


v2x = (60 cos 300) = 52.0 m/s; v2y = (60 sin 300) = 30 m/s

Fx t = J = mv2x – mv1x ; Jx = (0.4 kg)(52.0 m/s) – (0.4 kg)(-20 m/s); Jx = 28.8 N s

Fy t = J = mv2y – mv1y ; Jy = (0.4 kg)(30 m/s) – 0 ; Jy = 12.0 N s

*9-46. If the bat in Problem 9-45 was in contact with the ball for 5 ms, what was the magnitude

of the average force on the softball?

;

; F = 6240 N

*9-47. A cart A has a mass of 300 g and moves on a frictionless air track at 1.4 m/s when it hits

a second cart B at rest. The collision is perfectly elastic and the 300-g cart’s velocity is

reduced to 0.620 m/s after the collision. What was the mass of the other cart and what

was its velocity after the collision? m1 = 300 g; u1 = 1.4 m/s; v1 = 0.620 m/s

m1u1 + 0 = m1v1 + m2v2; (300 g)(1.4 m/s) = (300 g)(0.620 m/s) + m2v2

m2v2 = 234 g m/s; Elastic collision: v2 – v1 = u1 – u2 = (1.4 m/s) – 0

v2 = v1 + 1.4 m/s = 0.620 m/s + 1.4 m/s; v2 = 2.02 m/s

m2 = 116 g

*9-48. If the collision in Fig. 9-10, assume that the collision of the two masses is completely

inelastic. What is the common velocity after the collision and what is the ratio of the

final kinetic energy to the initial kinetic energy?

m1u1 + 0 = (m1 + m2)vc ; u1 = 15 m/s

(1 kg)(15 m/s) + 0 = (1 kg + 2 kg)vc vc = 5.00 m/s

114

15 m/s

2 kg1 kg


; Ratio = 0.333

*9-49. Assume the collision in Problem 9-48 is perfectly elastic. What is the velocity of each

mass after the collision?

Elastic: m1u1 + 0 = m1v1 + m2v2 and v2 – v 1 = u1 – u2

(1 kg)(15 m/s) = (1 kg)v1 + (2 kg)v2 ; v1 + 2v2 = 15 m/s ; v1 = 15 m/s – 2 v2

v2 – v 1 = u1 – u2 = (15 m/s) – 0; v2 = 15 m/s + v1

v2 = 15 m/s + (15 m/s – 2v2); v2 = 10 m/s ; v1 = 15 m/s – 2(10 m/s ) = -5 m/s

v1 = -5 m/s and v2 = 10 m/s

*9-50. A 2-kg mass moves to right at 2 m/s and collides with a 6-kg mass moving to the left at 4

m/s. (a) If the collision is completely inelastic, what is their common velocity after

colliding, and how much energy is lost in the collision?

Momentum: m1u1 + m2u2 = m1v1 + m2v2 ; v1 = v2 = vc for inelastic collision

(2 kg)(2 m/s) + (6 kg)(-4 m/s) = (2 kg + 6 kg)vc

4 kg m/s – 24 kg m/s = (8 kg)vc; vc = -2.50 m/s


2 = ½(m1 + m2)vc2 + Loss

½(2 kg)(2 m/s)2 + ½(6 kg)(-4 m/s)2 = ½(2 kg + 6 kg)(-2.50 m/s)2 + Loss

Solving for “loss”, we obtain: Loss = 27.0 J

**9-51. In Problem 9-50, assume the collision is perfectly elastic. What are the velocities after

the collision?

m1u1 + m2u2 = m1v1 + m2v2; m1 = 2 kg, m2 = 6 kg, u1 = 2 m/s, u2 = - 4 m/s

(2 kg)(2 m/s) + (6 kg)(-4 m/s) = (2 kg)v1 + 6 kg)v2

Dividing each term by 2 kg: v1 + 3 v 2 = - 10 m/s

115


Energy (e = 1): v2 – v1 = u1 – u2 = (2 m/s) – (-4 m/s); v2 – v1 = 6 m/s

Substitute v2 = v1 + 6 m/s into the earlier equation and solve for v1:

v1 + 3(v1 + 6 m/s) = - 10 m/s; v1 = -7.00 m/s

v2 – (-7.00 m/s) = 6 m/s; v2 = -1.00 m/s


*9-52. An astronaut in orbit outside a capsule uses a revolver to control motion. The astronaut

with gear weighs 200 lb on the earth. If the revolver fires 0.05-lb bullets at 2700 ft/s, and

10 shots are fired, what is the final velocity of the astronaut? Compare the final kinetic

energy of the ten bullets with that of the astronaut. Why is the difference so great?

0 + 0 = Wava + Wbvb; ; va = -0.675 ft/s

Each shot changes va by –0.675 m/s: vf = 10(0.675 m/s); vf = - 6.75 ft/s

We need masses: ;

Ekb = 10 (½mvb2) = (5)(0.00156 slugs)(2700 ft/s)2; Ekb = 56,950 ft lb

Eka = ½mava2 = ½(6.25 slugs)(6.75 ft/s)2 ; Eka = 142 ft lb

The kinetic energy of the bullets is much larger because when finding the kinetic

energy, one must deal with the square of velocity. The speeds dominate.

116


*9-53. In applying conservation of momentum for colliding objects to find final velocities, could

we use the weight of the objects instead of the mass? Why, or why not? Verify your

answer by applying it to one of the examples in the text.

Since weight is proportional to mass: W = mg, and since mass appears in every term

involving conservation of momentum, the weight can be used instead of the mass to

calculate either velocities or weights of colliding objects. For example, see Prob. 9-36.

m1u1 + m2u2 = m1v1 + m2v2

W1u1 + W2u2 = W1v1 + W2v2

*9-54. A 20-g bullet, moving at 200 m/s, strikes a 10-kg wooden block and passes entirely

through it, emerging with a velocity of 10 m/s. What is the velocity of the block after

impact? How much energy was lost?

m1u1 + 0 = m1v1 + m2v2

(0.020 kg)(200 m/s) = (0.020 kg)(10 m/s) + (10 kg)v2 ; v2 = 0.380 m/s

Conservation of Energy: ½m1u12 + 0 = ½(m1 + m2)vc

2 + Loss

½(0.02 kg)(200 m/s)2 = ½(10.02 kg)(0.38 m/s)2 + Loss Loss = 399 J

*9-55. A 0.30-kg baseball moving horizontally at 40 m/s is struck by a bat. The ball is in contact

with the bat for a time of 5 ms, and it leaves with a velocity of 60 m/s at an angle of 300,

what are the horizontal and vertical components of the average force acting on the bat?

First find components of velocity: v1x = - 20 m/s; v1y = 0

v2x = (60 cos 300) = 52.0 m/s; v2y = (60 sin 300) = 30 m/s

Fx t = mv2x – mv1x = (0.3 kg)(52.0 m/s) – (0.3 kg)(-40 m/s);

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v1x-40 m/s

v2y

v2x300

60 m/s


Fx(0.005 s) = 27.6 N s ; Fx = 5520 N

Fy t = mv2y – mv1y = (0.3 kg)(30 m/s) – 0 ; Fy(0.005 s) = 9.00 N s ; Fy = 1800 N

*9-56. When two masses collide they produce equal but opposite impulses. The masses do not

change in the collision, so the change in momentum of one should be the negative of the

change for the other. Is this true whether the collision is elastic or inelastic. Verify your

answer by using the data in Problems 9-50 and 9-51.

Momentum is conserved whether energy is lost in collision or not. Therefore, equal but

opposite impulses should always produce equal and opposite changes in momentum.

Prob. 9-50: The test is whether: m1v1 – m1u1 = -( m2u2 – m2v2); v1 = v2 = -2.5 m/s

*9-56. (Cont.) (2 kg)(-2.50 m/s) - (2 kg)(2 m/s) = -[(6 kg )(-2.50 m/s) - (6 kg)(-4 m/s)]

- 9 kg m/s = - 9 kg m/s; It works for inelastic collisions.

Prob. 9-51: Same test: m1v1 – m1u1 = -( m2v2 – m2u2); v1 = -7 m/s; v2 = -1 m/s

(2 kg)(-7 m/s) - (2 kg)(2 m/s) = -[(6 kg )(-1 m/s) - (6 kg)(-4 m/s)]

- 18 kg m/s = - 18 kg m/s; It also works for elastic collisions.

*9-57. Two toy cars of masses m and 3m approach each other, each traveling at 5 m/s. If they

couple together, what is their common speed afterward? What are the velocities of each

car if the collision is perfectly elastic? ( m1 = m, m2 = 3m, u1 = 5 m/s, u2 = - 5 m/s )

m1u1 + m2u2 = (m1 + m2 ) vc ; m(5 m/s) + 3m(-5 m/s) = (m + 3m) vc

-10 m/s = 4 vc; vc = -2.50 m/s For inelastic case

m1u1 + m2u2 = m1v1 + m2v2 ; m(5 m/s) + 3m(-5 m/s) = mv1 + 3mv2

v1 + 3 v 2 = - 10 m/s ; Now for elastic: v2 – v1 = u1 – u2

v2 – v1 = 5 m/s – (–5 m/s) = 10 m/s; v1 = v2 – 10 m/s

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(v2 – 10 m/s) + 3 v2 = -10 m/s; v2 = 0

v1 = (0) – 10 m/s = - 10 m/s; v1 = -10 m/s

*9-58. An 8-g bullet is fired horizontally at two blocks resting on a frictionless surface. The first

block has a mass of 1-kg and the second has a mass of 2-kg. The bullet passes

completely through the first block and lodges into the second. After the collisions, the 1-

kg block moves with a velocity of 1 m/s and the 2-kg block moves with 2 m/s. What is

the velocity of the bullet before and after emerging from the first block?

--Observe the figure on the next page --

*9-58. (Cont.) Total momentum at start = total momentum at finish

(0.008 kg) v1 = (1 kg)(1 m/s) + (2.008 kg)(2 m/s); v1 = 627 m/s

To find velocity emerging from 1-kg mass, we apply conservation to first block only:

(0.008 kg)(627 m/s) = (0.008 kg)ve + (1 kg)(1 m/s) ; ve = 502 m/s

*9-59. A 1-kg mass A is attached to a support by a cord of length 80 cm, and it is held

horizontally as in Fig. 9-11. After release it swings downward striking the 2-kg mass B

which is at rest on a frictionless tabletop. Assuming that the collision is perfectly elastic

what are the velocities of each mass immediately after impact?

First find uA from energy of fall: ½mv2 = mgh

; v = 3.96 m/s

mAuA + 0 = mAvA + mBvB; mA = 1 kg; uA = 3.96 m/s

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8 g2 m/s1 m/s2 kg1 kg

mA = 2 kg

mB = 2 kg

L = 80 cm


(1 kg)(3.96 m/s) = (1 kg)vA + (2 kg) vB

vA + 2 v B = 3.96 m/s ; Elastic: vB – vA = uA – uB = 3.96 m/s – 0

vB – vA = 3.96 m/s ; vA = vB – 3.96 m/s ; Substitute for vA in the other equation.

(vB – 3.96 m/s) + 2 vB = 3.96 m/s; From which: vB = 2.64 m/s

vA = vB – 3.96 m/s = 2.64 m/s – 3.96 m/s; vA = -1.32 m/s

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