Page 46 - 2023-bfw-physics-stewart-3e-new.indd

328 Chapter 7 Conservation of Energy and an Introduction to Energy and Work

How you define your system Earthexternal: W grav =− mgy f + mgy i = − (mgy − mgy ) i
f
matters: Only forces exerted Earthinsystem: − (U grav,f − U ) =∆ U (7-15)
by an object or a system grav,i grav
external to a system can
do work on that system.
Conservative interactions
can be described by work
if exerted by an object or
a system external to the
system, or as potential
energy if internal. These
two descriptions will always
be the same magnitude but
opposite signs.

Chapter 7 Review Problems

1. A satellite orbits Earth in a circular path at a high a horizontal track. Assume that the speed of the cart at
altitude. Explain why the gravitational force does zero position =x 0 is zero.
work on the satellite. (a) Calculate the velocity of the cart after 1.0 m.
2. An assistant for the football team carries a 30.0-kg (b) Calculate the velocity of the cart after 3.0 m.
cooler of water from the top row of the stadium,
which is 20.0 m above the field level, down to the 10
bench area on the field. 8
(a) If the speed of the cooler is constant throughout 6
the trip, calculate the work done by the assistant 4
on the cooler of water. Net force (N) 2
(b) How much work is done by the force of gravity on 0
the cooler of water? 22
3. A 1250-kg car moves at 20.0 m/s. How much work must 24
be done on the car to increase its speed to 30.0 m/s?
26
4. Bicycling to the top of a hill is much harder than 0 1 2 3
coasting down. Does the person–bike–Earth system Position (m)
have more potential energy at the top of the hill or at 9. You are pulling your younger sister on a sled up a hill
the bottom? Explain your answer. inclined at 10° with the horizontal. You exert a force
5. An 8500-metric-ton freight train is out of control and on the sled that makes an angle of 20° with the hill,
moving at 90 km/h on level track. How much work and has a magnitude of 120 N. You walk a distance
must a superhero do on the train to bring it to a halt? of 20.0 m along the hill. The mass of the sister–sled
system is 18.0 kg. Assume the friction between the hill
6. A 375-g toy boat is floating in a swimming pool. One and the sled is negligible.
child exerts a force of 1.85 N in the direction 30° north of
east on the boat. Another child exerts a force on the boat (a) Calculate the work done on the sister–sled system
of 0.75 N south. The average drag force of the water on by your pulling force as you move 20.0 m along
the boat is approximately one-half the net force (due to the hill.
the children) in the direction of motion, and is in the oppo- (b) Calculate the work done on the sister–sled system
site direction of this net force. If the boat starts from rest, by gravity during this motion.
how fast is it moving after it travels a distance of 5.0 m? (c) Calculate the work done by the normal force on
7. A catcher in a baseball game stops a pitched ball orig- the sister–sled system during this motion.
inally moving at 44.0 m/s at the moment it first came (d) Calculate the total work done on the sister–sled
in contact with the catcher’s glove. After contacting system during this motion.
the glove, the ball traveled an additional 12.5 cm 10. A 5.00-kg object is attached to one end of a horizontal
before coming to a complete stop. The mass of the ball spring that has a negligible mass and a spring constant
is 0.145 kg. What is the average force that the glove of 250 N/m. The other end of the spring is fixed to a
imparts to the ball during the catch? Comment on the wall. The spring is compressed by 10.0 cm from its
force exerted on the catcher’s hand during the catch. equilibrium position and released from rest.
8. The graph below shows the net force exerted on a (a) What is the speed of the object when it is 8.00 cm
low-friction cart of mass 500 grams as it moves along from equilibrium?

Uncorrected proofs have been used in this sample. Copyright © Bedford, Freeman & Worth Publishers.
Distributed by Bedford, Freeman & Worth Publishers. For review purposes only. Not for redistribution.

08_stewart3e_33228_ch07_284_333_8pp.indd 328 20/08/22 8:46 AM