9781118230725.pdf

descends, it pulls on a block via a

second rope, and the block slides

over a lab table. Again consider the

cylinder – rod – Earth system, similar

to that shown in Fig. 8-23b. Your

work on the system is 200 J. The sys-

tem does work of 60 J on the block.

Within the system, the kinetic

energy increases by 130 J and

the gravitational potential energy

decreases by 20 J. (a) Draw an “en-

ergy statement” for the system, as in

Fig. 8-23c. (b) What is the change in

the thermal energy within the system?

8 In Fig. 8-25, a block slides along a track that descends through

distanceh. The track is frictionless except for the lower section.

There the block slides to a stop in a certain distance Dbecause of

friction. (a) If we decrease h, will the block now slide to a stop in a

distance that is greater than, less than, or equal to D? (b) If, instead,

we increase the mass of the block, will the stopping distance now be

greater than, less than, or equal to D?

QUESTIONS 201

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

( 1 ) ( 2 ) ( 3 )

Figure 8-26Question 9.

h D

Figure 8-25Question 8.

Figure 8-21Question 4.

System's energies:

ΔK = +50 J

ΔUg = +20 J

ΔEth =?

W = +100 J

(a) ((b) c)

Cylinder

Earth

Rope

Rod

System

Work W

A

B

C D

Figure 8-22Question 5.

Figure 8-23Question 6.

( 1 )

0.5 m

1.5 m

3.0 m

2.5 m

3.5 m

(2)

(3)

(4)

Cylinder

Rod

Block

Rope

Figure 8-24Question 7.

5 In Fig. 8-22, a block slides from AtoCalong a frictionless ramp,
and then it passes through horizontal region CD, where a frictional
force acts on it. Is the block’s kinetic energy increasing, decreasing,
or constant in (a) region AB, (b) region BC, and (c) region CD?
(d) Is the block’s 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 the
rod, the cylinder slides along the rod with considerable friction.
Your force does work W100 J on the cylinder – rod – Earth
system (Fig. 8-23b). An “energy statement” for the system is shown
in Fig. 8-23c: the kinetic energy Kincreases by 50 J, and the gravita-
tional potential energy Ugincreases by 20 J. The only other change
in energy within the system is for the thermal energy Eth. What is
the change Eth?

10 Figure 8-27 shows three plums

that are launched from the same level

with the same speed. One moves

straight upward, one is launched at a

small angle to the vertical, and one is

launched along a frictionless incline.

Rank the plums according to their

speed when they reach the level of

the dashed line, greatest first.

11 When a particle moves from f

toiand from jtoialong the paths

shown in Fig. 8-28, and in the indi-

cated directions, a conservative

force does the indicated amounts

of work on it. How much work is

done on the particle by when the

particle moves directly from ftoj?

F:

F:

(1) (2) (3)

Figure 8-27Question 10.

f

i

j

20 J

20 J

Figure 8-28Question 11.

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

9 Figure 8-26 shows three situations involving a plane that is not

frictionless and a block sliding along the plane. The block begins with

the 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.