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Chapter 7: Q4Q (page 316)
Electricity is billed in kilowatt-hours. Is this energy or power? How much is one kilowatt-hour in standard physics units? (The typical cost of one kilowatt-hour is 5 to 10 cents.)
One kilowatt-hour in standard physics units is
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A man sits with his back against the back of a chair, and he pushes a block of mass straight forward on a table in front of him, with a constant force F=30 N, moving the block a distance D=0.3 m. The block starts from rest and slides on a low-friction surface. (a) How much work does the man do on the block? (b) What is the final kinetic energy K of the block? (c) What is the final speed V of the block? (d) How much time does this process take? (e) Consider the system of the man plus the block: how much work does the chair do on the man? (f) What is the internal energy of the man?
Now suppose that the man is sitting on a train that is moving in a straight line with speed , and you are standing on the ground as the train goes by, moving to your right. From your perspective (that is, in your reference frame), answer the following questions: (g) What is the initial speed localid="1657950350828" of the block? (h) What is the final speedof the block? (i) What is the initial kinetic energy of the block? (j) What is the final kinetic energyof the block? (k) What is the change in kinetic energy, how does this compare with the change in kinetic energy in the man’s reference frame? (l) How far does the block move? (m) How much work does the man do on the block, and how does this compare with the work done by the man in his reference frame and with in your frame? (n) How far does the chair move? (o) Consider the system of the man plus the block: how much work does the chair do on the man, and how does this compare with the work done by the chair in the man’s reference frame? (p) What is the internal energy change of the man, and how does this compare with the internal energy change in his reference frame?
Describe a situation in which it would be appropriate to neglect the effects of air resistance.
When a falling object reaches terminal speed, its kinetic energy reaches a constant value. However, the gravitational energy of the system consisting of object plus Earth continues to decrease. Does this violate the principle of conservation of energy? Explain why or why not.
Design a “bungee jump” apparatus for adults. A bungee jumper falls from a high platform with two elastic cords tied to the ankles. The jumper falls freely for a while, with the cords slack. Then the jumper falls an additional distance with the cords increasingly tense. You have cords that are 10 mlong, and these cords stretch in the jump an additional 24 mfor a jumper whose mass is 80 kg, the heaviest adult you will allow to use your bungee jump (heavier customers would hit the ground). You can neglect air resistance. (a) Make a series of five simple diagrams, like a comic strip, showing the platform, the jumper, and the two cords at various times in the fall and the rebound. On each diagram, draw and label vectors representing the forces acting on the jumper, and the jumper’s velocity. Make the relative lengths of the vectors reflect their relative magnitudes. (b) At what instant is there the greatest tension in the cords? How do you know? (c) What is the jumper’s speed at this instant? (d) Is the jumper’s momentum changing at this instant or not? (That is, isnonzero or zero?) Explain briefly. (e) Focus on this instant, and use the principles of this chapter to determine the spring stiffnessfor each cord. Explain your analysis. (f) What is the maximum tension that each cord must support without breaking? (g) What is the maximum acceleration (in’s) that the jumper experiences? What is the direction of this maximum acceleration? (h) State clearly what approximations and estimates you have made in your design.
An electric hot plate raises its own internal energy and the internal energy of a cup of water by 8000J, and there is at the same time 1000Jtransferred to the cooler air (that is,Q=1000J ). How much energy was transferred to the hot plate in the form of electricity?
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