7/25/2019 Potential Energy Problems

1/1

descends, it pulls on a block via asecond rope, and the block slidesover a lab table. Again consider thecylinder rod Earth system, similarto that shown in Fig. 8-23b. Yourwork on the system is 200 J. The sys-

tem does work of 60 J on the block.Within the system, the kineticenergy increases by 130 J andthe gravitational potential energydecreases by 20 J. (a) Draw an en-ergy statement for the system, as inFig. 8-23c. (b) What is the change inthe thermal energy within the system?

8 In Fig. 8-25, a block slides along a track that descends throughdistance h. The track is frictionless except for the lower section.There the block slides to a stop in a certain distance D because offriction. (a) If we decrease h, will the block now slide to a stop in adistance that is greater than, less than,or equal toD? (b) If, instead,we increase the mass of the block,will the stopping distance now begreater than, less than,or equal toD?

201QUESTIONS

top is (c) the centripetal acceleration of the block greatest and (d) thenormal force on the block least?

(1) (3)(2)

Figure 8-26 Question 9.

hD

Figure 8-25 Question 8.

Figure 8-21 Question 4.

System's energies:

K= +50 JUg= +20 J

Eth= ?

W= +100 J

(b)(a) (c)

Cylinder

Earth

RopeRod

System

Work W

A

B

C D

Figure 8-22 Question 5.

Figure 8-23 Question 6.

(1)

0.5 m

1.5 m

3.0 m

2.5 m

3.5 m

(2)

(3)

(4)

Cylinder

Rod

lock

Rope

Figure 8-24 Question 7.

5 In Fig. 8-22, a block slides fromA to Calong a frictionless ramp,and then it passes through horizontal region CD, where a frictionalforce acts on it. Is the blocks kinetic energy increasing, decreasing,or constant in (a) region AB, (b) region BC, and (c) region CD?(d) Is the blocks mechanical energy increasing, decreasing, or

constant in those regions?

6 In Fig. 8-23a, you pull upward on a rope that is attached to a

cylinder on a vertical rod. Because the cylinder fits tightly on therod, the cylinder slides along the rod with considerable friction.Your force does work W100 J on the cylinder rod Earthsystem (Fig.8-23b).An energy statement for the system is shownin Fig. 8-23c: the kinetic energyKincreases by 50 J, and the gravita-tional potential energyUg increases by 20 J.The only other changein energy within the system is for the thermal energy Eth. What isthe changeEth?

10 Figure 8-27 shows three plumsthat are launched from the same levelwith the same speed. One movesstraight upward, one is launched at asmall angle to the vertical, and one islaunched along a frictionless incline.Rank the plums according to theirspeed when they reach the level ofthe dashed line,greatest first.

11 When a particle moves from fto i and from j to i along the pathsshown in Fig. 8-28, and in the indi-cated directions, a conservativeforce does the indicated amountsof work on it. How much work isdone on the particle by when theparticle moves directly from f to j?

F:

F:

(1) (2) (3)Figure 8-27 Question 10.

f

i

j

20 J

20 J

Figure 8-28 Question 11.

7 The arrangement shown in Fig. 8-24 is similar to that inQuestion 6. Here you pull downward on the rope that is attachedto the cylinder, which fits tightly on the rod. Also, as the cylinder

9 Figure 8-26 shows three situations involving a plane that is notfrictionless and a block sliding along the plane.The block begins withthe same speed in all three situations and slides until the kinetic fric-tional force has stopped it. Rank the situations according to the in-crease in thermal energy due to the sliding,greatest first.