Work and Energy Conceptual Questions

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?

Power is the rate at which work is done. Power increases in work is done faster.

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.

The car only has potential energy when it's at the top of the hill. When it coasts up the crest it has less potential energy than before because it converts to kinetic energy. Friction has removed some of the total energy. When it coasts down the hill, it continues to lose potential energy and converting to kinetic energy. Friction continues to remove energy until the car runs out of potential and kinetic energy.

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

A book on a table. Even though the gravitational force is pulling down the book and normal force of the table is pushing up, it's not doing any work.

What is a conservative force?

A conservative force is one, like the gravitational force, for which work done by or against it depends only on the starting and ending points of a motion and not on the path taken.

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.

An example of work in everyday circumstances is writing something down. In this instance, energy is transferred from your hand to the pencil, to the sheet of paper.

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

Doesn't depend on how long it's used

List the energy conversions that occur when riding a bicycle.

Chemical energy to mechanical energy (kinetic energy) in order to push the pedals

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

Electrical (ex. electric motor electric to mechanical energy) Mechanical (ex. rubbing your hands together) Thermal (ex. solar panels / turbine??) Chemical (ex. burning something)

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.

If you carry something, the force is vertical but the displacement is horizontal. Because they aren't in the same direction (perpendicular) there is no work.

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

Mechanical energy is the energy an object has because of its motion or position. The amount of work done by nonconservative forces adds to the mechanical energy on an object. If conservative forces only act, the mechanical energy stays constant.

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

No, they just transfer into forms of less useful energy.

The person in Figure 7.32 does work on the lawn mower. Under what conditions would the mower gain energy? Under what conditions would it lose energy?

The mower would gain energy if there's no friction. It would lose energy if the friction was greater than the pushing force.

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 speed. Explain in terms of conservation of energy.

The reason why it would have the same speed is because the total energy is the same at the initial point and final point.

A spark of static electricity, such as that 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.

The spark lasts for a tiny fraction of a second, so it contains very little energy.

What is the relationship of potential energy to conservative force?

The work done against a conservative force to reach the final depends on the potential energy added.

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.

When the javelin is on the ground PE and KE are zero. When the athlete picks up the javelin, KE and PE increase. When the athlete throws the javelin, PE decreases while KE increases until the javelin hits the ground.

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.

When the swimmer jumps on the diving board, the diving board stores up potential energy and converts that into kinetic 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?

Work depends on the height of the shelf because of displacement as well as the mass of the book. Work doesn't depend on the path nor the time take. Time isn't part of the work formula, nor does the path change the displacement.

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

ex. 1 Throwing a ball ex. 2 catching a ball