Ch 4 Homework Name - Los Angeles Mission College

Ch 4 Homework

Name:

Follow the instructions and show your work clearly.

1. (Conceptual question 11)

Identify the action-reaction pairs in the following situations.

(a) A man takes a step.

(b) A snowball hits a girl in the back

(c) A baseball player catches a ball

(d) A gust of wind strikes a window

2.(Problem 6)

A freight train has a mass of Kg. If the locomotive can exert a constant pull of N,

how long does it take to increase the speed of the train from rest to 80 Km/h.

(a) Draw a free-body diagram of the freight train.

(b) Calculate the acceleration. What is the unit of the acceleration?

(c) Write down the equation you will use in order to find the time to reach 80 Km/h.

(d) Calculate the time to reach 80 km/h from rest.

3. (Problem 17)

Find the tension in each cable supporting the 600-N cat burglar in the Figure below.

(a) Draw a free-body diagram of the cat burglar.

(b) Write down forces acting on the car burglar and Find the T1 (tension in the first cable). (Hint:

assign a variable to each force and write down x-components and y-components of the forces

acting on the person. Tension in the cable 1 is shown as an example. Complete the table below).

X-direction Y-direction

Force value(N) Force value(N)

Tension in the cable 1 T1x = 0N Tension in the cable 1 T1y

Net Force(Fnet x)

Net Force(Fnet y)

(c) Find the T1.

(d) Draw a free-body diagram at the point A in the diagram.

(e) Write down forces acting on the point A(hint use trigonometry to decompose vectors into x-and

y- directions).



Net Force(Fnet x)

Net Force(Fnet y)

(f) Find T2 and T3. Write down magnitudes and directions.

(g) Suppose the horizontal cable were reattached higher up on the wall. Would the tension in the

other cable increase, decrease or stay the same? Why?

4. (Problem 24)

The systems in Figure below are in equilibrium. If the spring scales are calibrated in newtons, what do

they read? Ignore the masses of the pulleys and springs and assume the pulleys and the incline in Figure

are frictionless.(define a coordinate system in each case.)

(a) Draw a free-body diagram of the scale in the top-left system and write down forces acting on

the scale. What does the scale read?



Net Force(Fnet x)

Net Force(Fnet y)

(b) Draw a free-body diagram of the scale in the top-right system and write down forces acting on

the scale. What does the scale read?



Net Force(Fnet x)

Net Force(Fnet y)

(c) Draw a free-body diagram of the scale in the bottom-right system and write down forces acting

on the scale. What does the scale read?



Net Force(Fnet x)

Net Force(Fnet y)

(d) Draw a free-body diagram of the scale in the bottom-left system and write down forces acting

on the scale. What does the scale read?



Net Force(Fnet x)

Net Force(Fnet y)

5. (Problem 28)

Two packing crates of masses 10.0 kg and 5.00 kg are connected by a light string that passes over a

frictionless pulley as in Figure below. The 5.00-kg crate lies on a smooth incline of angle 40°.

(a) Define coordinate systems for the 10.0-kg crate and the 5.00-kg crate separately and draw them

on the figure above.

(b) Draw a free-body diagram of the 10.0-kg crate.

(c) Draw a free-body diagram of the 5.00-kg crate.

(d) Write down the forces acting on the 10.0-kg crate.



Net Force(Fnet x)

Net Force(Fnet y)

(e) Write down the forces acting on the 5.00-kg crate.



Net Force(Fnet x)

Net Force(Fnet y)

(f) From (d) and (e), find the acceleration of the 5.00-kg crate.

(g) From (d) and (e), find the tension in the string.

6. (Problem 34)

In Figure below, the light, taut, unstretchable cord B joins block 1 and the larger-mass block 2. Cord A

exerts a force on block 1 to make it accelerate forward.

(a) Define coordinate systems on the figure above and label all the physical quantities, using letters

(i.e. Mass of block 1 = m1).

(b) Draw free-body diagrams of block 1 and 2.

(c) Write down the forces acting on the block 1.



Net Force(Fnet x)

Net Force(Fnet y)

(d) Write down the forces acting on the block 2.



Net Force(Fnet x)

Net Force(Fnet y)

(e) How does the magnitude of the force exerted by cord A on block 1 compare with the magnitude

of the force exerted by cord B on block 2(Hint: block 2 is more massive than block 1)?

(f) How does the acceleration of block 1 compared with the acceleration of block 2?

(g) Does cord B exert a force on block 1? Explain your answer.

7. (Problem 39)

A dockworker loading crates on a ship finds that a 20-kg crate, initially at rest on a horizontal surface,

requires a 75-N horizontal force to set it in motion. However, after the crate is in motion, a horizontal

force of 60 N is required to keep it moving with a constant speed. Find the coefficients of static and

kinetic friction between crate and floor.

(a) Label physical quantities by using symbols you choose.

(b) Draw a free-body diagram of the 20-kg crate.

(c) Write down the forces when the crate is at rest.



Net Force(Fnet x)

Net Force(Fnet y)

(d) Find the static friction coefficient.

(e) Write down the forces when the crate is moving at a constant speed.



Net Force(Fnet x)

Net Force(Fnet y)

(f) Find the kinetic friction coefficient.

8. (Problem 41)

A 1000-N crate is being pushed across a level floor at a constant speed by a force of 300 N at an angle

of 20.0° below the horizontal, as shown in the Figure below (diagram on the left).

(a) Define a coordinate system and assign variables to physical quantities in the problem.

(b) Draw a free-body diagram of the crate.

(c) Write down forces acting on the crate.



Net Force(Fnet x)

Net Force(Fnet y)

(d) What is the coefficient of kinetic friction between the crate and the floor?

(e) If the 300-N force is instead pulling the block at an angle of 20.0° above the horizontal, as shown

in the Figure above(diagram on the right side), what will be the acceleration of the crate?

Assume that the coefficient of friction is the same as that found in part (a).

a. Draw a free-body diagram.

b. Write down forces acting on the crate.



Net Force(Fnet x)

Net Force(Fnet y)

c. Calculate the acceleration.

9. (Problem 47)

The coefficient of static friction between the 3.00-kg crate and the 35.0° incline of Figure below is 0.300.

What minimum force must be applied to the crate perpendicular to the incline to prevent the crate

from sliding down the incline?

(a) Define a coordinate system and assign variables to the physical quantities.

(b) Draw a free-body diagram of the crate.

(c) Write down the forces acting on the crate.



Net Force(Fnet x)

Net Force(Fnet y)

(d) Find the force, .

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Ch 4 Homework Name - Los Angeles Mission College