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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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During three hours one winter afternoon, when the outside temperature was , a house heated by electricity was kept at with the expenditure of(kilowatt hours) of electric energy. What was the average energy leakage in joules per second through the walls of the house to the environment (the outside air and ground)?
The rate at which energy is transferred between two systems is often proportional to their temperature difference. Assuming this to hold in this case, if the house temperature had been kept at, how manyof electricity would have been consumed?
You place into an insulated container a 1.5 kg block of aluminium at a temperature of in contact with a 2.1 kg block of copper at a temperature of . The specific heat of aluminium is 0.91 J/g and the specific heat of copper is 0.39 J/g. What is the final temperature of the two blocks?
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鈥檚 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鈥檚 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?
You drop a single coffee filter of mass 1.7 g from a very tall building, and it takes 52 s to reach the ground. In a small fraction of that time the coffee filter reached terminal speed. (a) What was the upward force of the air resistance while the coffee filter was falling at terminal speed? (b) Next you drop a stack of five of these coffee filters. What was the upward force of the air resistance while this stack of coffee filters was falling at terminal speed? (c) Again assuming that the stack reaches terminal speed very quickly, about how long will the stack of coffee filters take to hit the ground?
Question: Consider the process of a woman lifting a barbell discussed in Section. Analyse the energy changes in this process,choosing the woman alone as the system. What quantities can be calculated with this choice of system?
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