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3. Whenever you do work on something, you give it energy.
QUESTIONS FOR THOUGHT
Analyze how you would know for sure that this is a true
statement.
1. How is work related to energy?
4. Simple machines are useful because they are able to trade force
2. What is the relationship between the work done while moving for distance moved. Describe a conversation between yourself
a book to a higher bookshelf and the potential energy that the and another person who believes that you do less work when you
book has on the higher shelf? use a simple machine.
3. Does a person standing motionless in the aisle of a moving bus 5. Use the equation for kinetic energy to prove that speed is more
have kinetic energy? Explain. important than mass in bringing a speeding car to a stop.
4. A lamp bulb is rated at 100 W. Why is a time factor not included 6. Describe at least several examples of negative potential energy
in the rating? and how each shows a clear understanding of the concept.
5. Is a kWh a unit of work, energy, power, or more than one of 7. The forms of energy are the result of fundamental forces—

these? Explain. gravitational, electromagnetic, and nuclear—and objects that are
6. If energy cannot be destroyed, why do some people worry about interacting. Analyze which force is probably involved with each
the energy supplies? form of energy.
7. A spring clamp exerts a force on a stack of papers it is holding 8. Most technological devices convert one of the five forms of

together. Is the spring clamp doing work on the papers? Explain. energy into another. Try to think of a technological device
8. Why are petroleum, natural gas, and coal called fossil fuels? that does not convert an energy form to another. Discuss the

9. From time to time, people claim to have invented a machine significance of your fi nding.
that will run forever without energy input and will develop more 9. Are there any contradictions to the law of conservation of energy
energy than it uses (perpetual motion). Why would you have in any area of science?
reason to question such a machine?

10. Define a joule. What is the difference between a joule of work

and a joule of energy? INVITATION TO INQUIRY
11. Compare the energy needed to raise a mass 10 m on Earth to
the energy needed to raise the same mass 10 m on the Moon. New Energy Source?
Explain the difference, if any. Is waste paper a good energy source? There are 103 U.S. waste-to- energy

12. What happens to the kinetic energy of a falling book when the plants that burn solid garbage, so we know that waste paper would be
book hits the fl oor? a good source, too. The plants burn solid garbage to make steam that
is used to heat buildings and generate electricity. Schools might be able
to produce a pure waste paper source because waste paper accumulates
FOR FURTHER ANALYSIS near computer print stations and in offices. Collecting waste paper
from such sources would yield 6,800 Btu/lb, which is about one-half
the heat value of coal.
1. Evaluate the requirement that something must move whenever
work is done. Why is this a requirement? If you accept this invitation, start by determining how much waste
paper is created per month in your school. Would this amount produce
2. What are the significant similarities and diff erences between

work and power? enough energy to heat buildings or generate electricity?

PARALLEL EXERCISES

The exercises in groups A and B cover the same concepts. Solutions to group A exercises are located in appendix E.
Note: Neglect all frictional forces in all exercises.
Group A Group B
1. A force of 200 N is needed to push a table across a level classroom 1. How much work is done when a force of 800.0 N is exerted while

floor for a distance of 3 m. How much work was done on the table? pushing a crate across a level floor for a distance of 1.5 m?
2. A 880 N box is pushed across a level floor for a distance of 5.0 m 2. A force of 400.0 N is exerted on a 1,250 N car while moving it a

with a force of 440 N. How much work was done on the box? distance of 3.0 m. How much work was done on the car?
3. How much work is done in raising a 10.0 kg backpack from the 3. A 5.0 kg textbook is raised a distance of 30.0 cm as a student

floor to a shelf 1.5 m above the fl oor? prepares to leave for school. How much work did the student do
on the book?
4. If 5,000 J of work is used to raise a 102 kg crate to a shelf in a 4. An electric hoist does 196,000 J of work in raising a 250.0 kg
warehouse, how high was the crate raised? load. How high was the load lift ed?
5. A 60.0 kg student runs up a 5.00 m high stairway in a time of 5. What is the horsepower of a 1,500.0 kg car that can go to the top
3.92 seconds. How many watts of power did she develop? of a 360.0 m high hill in exactly 1.00 minute?

3-23 CHAPTER 3 Energy 83