Conceptual Questions

7.1 Work: The Scientific Definition


Give an example of something we think of as work in everyday circumstances that is not work in the scientific sense. Is energy transferred or changed in form in your example? If so, explain how this is accomplished without doing work.


Give an example of a situation in which there is a force and a displacement, but the force does no work. Explain why it does no work.


Describe a situation in which a force is exerted for a long time but does no work. Explain.

7.2 Kinetic Energy and the Work-Energy Theorem


The person in Figure 7.34 does work on the lawn mower. Under what conditions would the mower gain energy? Under what conditions would it lose energy?

A person pushing a lawn mower with a force F. Force is represented by a vector making an angle theta below the horizontal and distance moved by the mover is represented by vector d. The component of vector F along vector d is F cosine theta. Work done by the person, W, is equal to F d cosine theta.
Figure 7.34

Work done on a system puts energy into the system. Work done by a system removes energy from the system. Give an example for each statement.


When solving for speed in Example 7.4, we kept only the positive root. Why?

7.3 Gravitational Potential Energy


In Example 7.7, we calculated the final speed of a roller coaster that descended 20 m in height and had an initial speed of 5 m/s downhill. Suppose the roller coaster had had an initial speed of 5 m/s uphill instead, and it coasted uphill, stopped, and then rolled back down to a final point 20 m below the start. We would find in that case that its final speed is the same as its initial. Explain this result in terms of conservation of energy.


Does the work you do on a book when you lift it onto a shelf depend on the path taken? On the time taken? On the height of the shelf? On the mass of the book?

7.4 Conservative Forces and Potential Energy


What is a conservative force?


The force exerted by a diving board is conservative, provided the internal friction is negligible. Assuming friction is negligible, describe changes in the potential energy of a diving board as a swimmer dives from it, starting just before the swimmer steps on the board until just after his feet leave it.


Define mechanical energy. What is the relationship of mechanical energy to nonconservative forces? What happens to mechanical energy if only conservative forces act?


What is the relationship of potential energy to conservative force?

7.6 Conservation of Energy


Consider the following scenario. A car for which friction is not negligible accelerates from rest down a hill, running out of gasoline after a short distance. The driver lets the car coast farther down the hill, then up and over a small crest. He then coasts down that hill into a gas station, where he brakes to a stop and fills the tank with gasoline. Identify the forms of energy the car has, and how they are changed and transferred in this series of events (see Figure 7.35).

A car coasting downhill, moving over a crest then again moving downhill and finally stopping at a gas station. Each of these positions is labeled with an arrow pointing downward.
Figure 7.35 A car experiencing non-negligible friction coasts down a hill, over a small crest, then downhill again, and comes to a stop at a gas station.

Describe the energy transfers and transformations for a javelin, starting from the point at which an athlete picks up the javelin and ending when the javelin is stuck into the ground after being thrown.


Do devices with efficiencies of less than one violate the law of conservation of energy? Explain.


List four different forms or types of energy. Give one example of a conversion from each of these forms to another form.


List the energy conversions that occur when riding a bicycle.

7.7 Power


Most electrical appliances are rated in watts. Does this rating depend on how long the appliance is on? When off, the appliance is a zero-watt device. Explain your answer in terms of the definition of power.


Explain, in terms of the definition of power, why energy consumption is sometimes listed in kilowatt-hours rather than joules. What is the relationship between these two energy units?


A spark of static electricity, such as you might receive from a doorknob on a cold dry day, may carry a few hundred watts of power. Explain why you are not injured by such a spark.

7.8 Work, Energy, and Power in Humans


Explain why it is easier to climb a mountain on a zigzag path rather than climb straight up the side. Is your increase in gravitational potential energy the same in both cases? Is your energy consumption the same in both?


Do you do work on the outside world when you rub your hands together to warm them? What is the efficiency of this activity?


Shivering is an involuntary response to lowered body temperature. What is the efficiency of the body when shivering, and is this a desirable value?


Discuss the relative effectiveness of dieting and exercise in losing weight, noting that most athletic activities consume food energy at a rate of 400 to 500 W, while a single cup of yogurt can contain 1,360 kJ (325 kcal). Specifically, is it likely that exercise alone will be sufficient to lose weight? You may wish to consider that regular exercise may increase the metabolic rate, whereas protracted dieting may reduce it.

7.9 World Energy Use


What is the difference between energy conservation and the law of conservation of energy? Give some examples of each.


If the efficiency of a coal-fired electrical generating plant is 35 percent, then what do we mean when we say that energy is a conserved quantity?