Study Questions

1. What is a conservation law?

2. What is an "isolated" system?

3. What is conserved in an elastic collision?

4. Give an example of a (nearly) elastic collision, besides billiard-ball collisions. Give an example of a very inelastic collision.

5. What is the difference between momentum and kinetic energy?

6. What is the integrated effect of a force acting over a distance on an isolated object?

7. What is the integrated effect of a force acting during a time interval on an isolated object?

8. Define work.

9. Why is potential energy referred to as "potential" energy?

10. What is meant by a "conservative" system? Give an example of one.

11. Why is it understandable that scientists first believed heat to be a fluid?

12. What were believed to be the characteristics of caloric fluid?

13. Why do higher pressures make a steam engine work better?

14. What was Watt's main improvement for steam engines?

15. Define heat capacity and specific heat.

16. Why can steam scald one more than boiling water?

17. How did the calorists explain heat produced by friction?

18. Why did Rumford question the validity of the caloric theory?

19. What are the two important results of Joule's experiment with a water churn?

20. What is heat, in terms of the kinetic-molecular model?

21. What is the difference between heat and temperature?

22. Explain phase changes, including latent heats, in terms of the kinetic-molecular model.

23. State the law of conservation of energy in your own words.

24. How does recognizing heat as a form of energy "save" the conservation of energy principle for a pendulum?

25. How is total mechanical energy lost in a system consisting of the Sun and a planet?

26. Name the five basic forms of energy.

27. What kinds of energy are present in the following systems? (a) a car battery, (b) a bullet, (c) a rock on a mountaintop, (d) burning gas.

28. From a practical viewpoint, what is energy?

Problems

1. A rock of mass 1.0 kg is dropped from a height of 10 meters. What will be its velocity just before it strikes the ground? What if the height is 100 meters? (Neglect air friction.)

2. A pendulum bob at the bottom of its swing has a velocity of 10 m/sec. How high will it rise above the lowest point of its swing? (Acceleration of gravity = g = 9.8 m/sec2.)

3. A person throws a ball of mass 0.5 kg straight up in the air with a velocity of 10 m/sec. How high will the ball rise? What will be the velocity of the ball just before it strikes the ground? (Neglect air friction.)

4. How much heat is required to raise 10 liters of water from 0° to 50° C?

5. How much heat is required to melt 1 kg of ice? (Latent heat of fusion of water is 80 cal/gm.) How much heat is required to boil away 1 kg of water already at 100° C? (Latent heat of vaporization is 539 cal/gm.)

6. How many gallons of gasoline must be burned in order to raise the temperature of 10 liters of water from 25° C to 50° C, assuming 50 percent efficiency in converting the heat released into heat in the water? (The heat of combustion of gasoline is 3.4 X 107 cal/gallon.)

7. Consider the example in the Appendix for the velocity of air molecules. If instead of air (70 percent nitrogen, 30 percent oxygen) the gas is hydrogen at normal temperature and pressure (density = M/V = 0.09 gm/liter), what is the velocity of the molecules?

8. If your body is 20 percent efficient in converting food intake to useful work, how many kcal of food must you eat in one day to do 25 X 105 joules of work without tapping any of your body's energy reserves?

9. If 60 percent of the potential energy of the water flowing over a dam can be converted to electricity, find the maximum power output, in kW, for a dam 50 m high where the water crosses at a rate of 300,000 kg/min (1 watt = 1 joule/sec, 1 kW = 1000 watts).

10. An automobile with mass 2000 kg is traveling at 20 m/sec. How much heat is developed in the brakes when the car is stopped?

11. A person of mass 70 kg walks up to the third floor of a building. This is a vertical height of 12 meters above the street. How many joules of work did the person do? By how much did the person's potential energy increase?

12. A bullet of mass 10 gm moving at 200 m/sec strikes and lodges in a block of mass 1 kg hanging from a string. It turns out that one percent of the kinetic energy of the bullet is transferred to the block. How high will the block rise as it swings on the string? What happened to the remaining 99 percent of the kinetic energy of the bullet?

References

Sanborn C. Brown, Benjamin Thompson, Count Rumford, MIT Press, Cambridge, Mass., 1979.

A personal and scientific biography of one of the first American scientists.

Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, Addison-Wesley, Reading, Mass., 1963, Vol. 1, Chapter 4. A more advanced undergraduate textbook with emphasis on understanding the physical principles involved. Uses elementary calculus.

Douglas C. Giancoli, The Ideas of Physics, Harcourt Brace Jovanovich, New York, 1974, Chapter 7.

An introductory text similar to this one with somewhat different emphases and topics.

Gerald Holton and Stephen G. Brush, Introduction to Concepts and Theories in Physical Science, 2nd ed., Addison-Wesley, Reading, Mass., 1973. Chapters 15-17, 22. See comment in references for Chapter 2.

Morton Mott-Smith, The Concept of Energy Simply Explained, Dover Publications, New York, 1964.

A reprint of a very interesting book originally published in 1934, and written in a descriptive style for the lay public. Because it was written 60 years ago, some of its assertions are not acceptable today.

C. J. Schneer, The Evolution of Physical Science, Grove Press, New York, 1964 (a reprint of The Search for Order, Harper & Row, New York, 1960). See comment in references for Chapter 2.

Sadi Carnot. (American Institute of Physics Niels Bohr Library.)

Chapter 5

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