91影视

A car has the same kinetic energy when traveling north as when it turns around and travels south. Is the momentum of the car the same in both cases?

Two lumps of clay with equal and opposite momenta have a head-on collision and come to rest. Is momentum conserved? Is kinetic energy conserved? Why are your answers the same or different?

An inefficient machine is said to 鈥渨aste energy.鈥� Does this mean that energy is actually lost? Explain.

The energy we require to live comes from the chemically stored potential energy in food, which is transformed into other energy forms during the metabolism process. What happens to a person whose combined work and heat output is less than the energy consumed? What happens when the person鈥檚 work and heat output is greater than the energy consumed? Can an undernourished person perform extra work without extra food? Defend your answers.

Your discussion partner is confused about ideas discussed in Chapter 4 that seem to contradict ideas discussed in this chapter. For example, in Chapter \(4,\) we learned that the net force is zero for a car traveling along a level road at constant velocity, but in this chapter, we learned that work is done in such a case. Your discussion friend asks, "How can work be done when the net force equals zero?" Discuss your explanation.

In the absence of air resistance, a ball thrown vertically upward with a certain initial \(\mathrm{KE}\) will return to its original level with the same \(\mathrm{KE}\) . When air resistance is a factor affecting the ball, will it return to its original level with the same, less, or more KE? Does your answer contradict the law of energy conservation?

You鈥檙e on a rooftop and you throw one ball downward to the ground below and another upward. The second ball, after rising, falls and also strikes the ground below. If air resistance can be ignored, and if your downward and upward initial speeds are the same, how will the speeds of the balls compare upon striking the ground? (Use the idea of energy conservation to arrive at your answer.)

When a driver applies the brakes to keep a car going downhill at constant speed and constant kinetic energy, the potential energy of the car decreases. Where does this energy go? In contrast, discuss where most of it goes with a hybrid vehicle.

Can something have energy without having momentum? Explain. Can something have momentum without having energy? Discuss and defend your answer.

Consider a bob attached by a string, a simple pendulum, that swings to and fro. (a) Why doesn鈥檛 the tension force in the string do work on the pendulum? (b) Explain, however, why the force due to gravity on the pendulum at nearly every point does do work on the pendulum. (c) What is the single position of the pendulum where 鈥渘o work by gravity鈥� occurs?