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Chapter 7: Q.22CQ (page 259)
Do you do work on the outside world when you rub your hands together to warm them? What is the efficiency of this activity?
A very small amount of work is done in the outside world and the efficiency of this process the very high.
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Consider a person climbing and descending stairs. Construct a problem in which you calculate the long-term rate at which stairs can be climbed considering the mass of the person, his ability to generate power with his legs, and the height of a single stair step. Also consider why the same person can descend stairs at a faster rate for a nearly unlimited time in spite of the fact that very similar forces are exerted going down as going up. (This points to a fundamentally different process for descending versus climbing stairs.)
A 75.0-kg cross-country skier is climbing a 3.0潞 slope at a constant speed of 2.00 m/s and encounters air resistance of 25.0 N. Find his power output for work done against the gravitational force and air resistance.
(b) What average force does he exert backward on the snow to accomplish this?
(c) If he continues to exert this force and to experience the same air resistance when he reaches a level area, how long will it take him to reach a velocity of 10.0 m/s?
A subway train is brought to a stop from a speed ofinby a large spring bumper at the end of its track. What is the force constant k of the spring?
a) Calculate the work done on a 1500-kg elevator car by its cable to lift it 40.0 m at constant speed, assuming friction averages 100 N. b) What is the work done on the lift by the gravitational force in this process? c) What is the total work done on the lift?
A shopper pushes a grocery cart 20.0 m at constant speed on level ground, against a 35.0 N frictional force. He pushes in a direction 25.0潞 below the horizontal.
(a) What is the work done on the cart by friction?
(b) What is the work done on the cart by the gravitational force?
(c) What is the work done on the cart by the shopper?
(d) Find the force the shopper exerts, using energy considerations.
(e) What is the total work done on the cart?
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