**32. A young girl of mass 40 kg slides down a metal slide that is 4 m long and angled 30 above the horizontal, as shown. She arrives at the bottom moving at a speed of 5 m/s. How much energy was converted by friction into thermal energy, warming the girl's jeans and the 30° metal of the slide? 1

**32. A young girl of mass 40 kg slides down a metal slide that is 4 m long and angled 30 above the horizontal, as shown. She arrives at the bottom moving at a speed of 5 m/s. How much energy was converted by friction into thermal energy, warming the girl's jeans and the 30° metal of the slide? 1

College Physics

1st Edition

ISBN:9781938168000

Author:Paul Peter Urone, Roger Hinrichs

Publisher:Paul Peter Urone, Roger Hinrichs

Chapter7: Work, Energy, And Energy Resources

Section: Chapter Questions

Problem 25PE: (a) How high a hill can a car coast up (engine disengaged) if work done by friction is negligible...

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(a) How long will the energy in a 1470kJ (350kcal) cup of yogurt last in a woman doing work at the rate of 150 W with an efficiency of 20.0% (such as in leisurely climbing stairs)? (b) Does the time found in part (a) imply that it is easy to consume more food energy than you can reasonably expect to work off with exercise?
If you run down some stairs and stop, what happens to your kinetic energy and your initial gravitational potential energy?
A 50.0-kg student evaluates a weight loss program by calculating the number of times she would need to climb a 12.0-m high flight of steps in order to lose one pound (0.45 kg) of fat. Metabolizing 1.00 kg of fat can release 3.77 107 J of chemical energy and the body can convert about 20.0% of this into mechanical energy. (The rest goes into internal energy.) (a) How much mechanical energy can the body produce from 0.450 kg of fat? (b) How many trips up the flight of steps are required for the student to lose 0.450 kg of fat? Ignore the relatively small amount of energy required to return down the stairs.
Using data from Table 7.5, calculate the daily energy needs of a person who sleeps for 7.00 h, walks for 2.00 h, attends classes for 4.00 h, cycles for 2.00 h, sits relaxed for 3.00 h, and studies for 6.00 h. (Studying consumes energy at the same rate as sitting in class.)
A block of mass m = 200 g is released from rest at point along the horizontal diameter on the inside of hemispherical bowl of radius R = 30.0 cm, and the surface of the bowl is rough (Fig. P8.23). The blocks speed at point is 1.50 m/s. Figure P8.23 (a) What is its kinetic energy at point ? (b) How much mechanical energy is transformed into internal energy as the block moves from point to point ? (c) Is it possible to determine the coefficient of friction from these results in any simple manner? (d) Explain your answer to part (c).
(a) How high a hill can a car coast up (engine disengaged) if work done by friction is negligible and its initial speed is 110 km/h? (b) If, in actuality, a 750-kg car with an initial speed of 110 km/h is observed to coast up a hill to a height 22.0 m above its starting point, how much thermal energy was generated by friction? (c) What is the average force of friction if the hill has a slope 2.5° above the horizontal?
A ball of clay falls freely to the hard floor. It does not bounce noticeably, and it very quickly comes to rest. What, then, has happened to the energy the ball had while it was falling? (a) It has been used up in producing the downward motion. (b) It has been transformed back into potential energy. (c) It has been transferred into the ball by heat. (d) It is in the ball and floor (and walls) as energy of invisible molecular motion. (e) Most of it went into sound.
When jogging at 13 km/h on a level surface, a 70-kg man uses energy at a rate of approximately 850 W. Using the facts that the “human engine” is approximately 25 efficient, determine the rate at which this man uses energy when jogging up a 5.0 slope at this same speed. Assume that the frictional retarding force is the same in both cases.
A hydroelectric power facility (see Figure 7.38) converts the gravitational potential energy of water behind a dam to electric energy. (a) What is the gravitational potential energy relative to the generators of a lake of volume 50.0 km3(mass=5.001013Kg), given that the lake has an average height of 40.0 m above the generators? (b) Compare this with the energy stored in a 9-megaton fusion bomb. Figure 7.38 Hydroelectric facility (credit: Denis Belevich, Wikimedia Commons)
A 75-kg sprinter accelerates from rest to a speed of 11.0 m/s in 5.0 s. (a) Calculate the mechanical work done by the sprinter during this time. (b) Calculate the average power the sprinter must generate. (c) If the sprinter converts food energy to mechanical energy with an efficiency of 25%, at what average rate is he burning Calories? (d) What happens to the other 75% of the food energy being used?
Kanellos Kanellopoulos flew 119 km from Crete to Santorini, Greece, on April 23, 1988, in the Daedalus 88, an aircraft powered by a bicycle-type drive mechanism (see Figure 7.43). His useful power output for the 234-min trip was about 350 W. Using the efficiency for cycling from Table 7.2, calculate the food energy in kilojoules he metabolized during the flight. Figure 7.43 The Daedalus 88 in flight. (credit: NASA photo by Beasley)
What is the efficiency of a subject on a treadmill who puts out work at the rate of 100 W while consuming oxygen at the rate of 2.00 L/min? (Hint: See Table 7.5.)