Section 11 - Work and gravitational potential energy
11.1 You lower a 2.50 kg textbook (remember when textbooks used to be made out of paper instead of being digital?) from a
height of 1.85 m to 1.50 m. What is its change in potential energy?
J
11.2 The reservoir behind the Grand Coulee dam in the state of Washington holds water with a total gravitational potential energy
of 1×10^16 J, where the reference point for zero potential energy is taken as the height of the base of the dam. (a) Suppose
that the dam released all its water, which flowed to form a still pool at the base of the dam. What would be the change in the
gravitational potential energy of the Earth-dam-water system? (b) What work was done by the gravitational force? (Part of this
work is ordinarily used to turn electric generators.) (c) How much work would it take to pump all the water back up into the
reservoir?
(a) J
(b) J
(c) J
11.3 What is the change in the gravitational potential energy of a Boeing 767 jet as it soars from the runway up to a cruising
altitude of 10.2 km? Assume its mass is a constant 2.04×10^5 kg.
J
11.4 A weightlifter raises a 115 kg weight from the ground to a height of 1.95 m in 1.25 seconds. What is the average power of this
maneuver?
W
11.5 A small domestic elevator has a mass of 454 kg and can ascend at a rate of 0.180 m/s. What is the average power that must
be supplied for the elevator to move at this rate?
WSection 14 - Work and energy
14.1 A firm bills at the rate of $1.00 per 125 J of work. It bills you $45.00 for carrying a sofa up some stairs. Their workers moved
the sofa up 3 flights of stairs, with each flight being 4.50 meters high. What is the mass of the sofa?kg
14.2 An engine supplies an upward force of 9.00 N to an initially stationary toy rocket, of mass 54.0 g, for a distance of 25.0 m.
The rocket rises to a height of 339 meters before falling back to the ground. What was the magnitude of the average force of
air resistance on the rocket during the upward trip?
N
14.3 On an airless moon, you drop a golf ball of mass 106 g out of a skyscraper window. After it has fallen 125 meters, it is moving
at 19.0 m/s. What is the rate of freefall acceleration on this moon?
m/s^2
14.4 You are pulling your sister on a sled to the top of a 16.0 m high, frictionless hill with a 10.0° incline. Your sister and the sled
have a total mass of 50.0 kg. You pull the sled, starting from rest, with a constant force of 127 N at an angle of 45.0° to the
hill. If you pull from the bottom to the top, what will the speed of the sled be when you reach the top?
m/sSection 16 - Conservation of energy
16.1 A large block of ice is moving down the hill toward you at 25.0 m/s. Its mass is 125 kg. It is sliding down a slope that makes a
30.0 degree angle with the horizontal. In short: Think avalanche. Assume the block started stationary and moves down the hill
with zero friction. How many meters has it been sliding?
mSection 19 - Interactive problem: conservation of energy
19.1 Use the information given in the interactive problem in this section to answer the following question. What initial height is
required for the soapbox car to make it through the hoop? Test your answer using the simulation.
m(^144) Copyright 2007 Kinetic Books Co. Chapter 6 Problems