Page 100 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 7] ENERGY 85

7.6. A boy pulls a wagon with a force of 45 N by means of a rope that makes an angle of 40 with the ground. How much
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work does he do in moving the wagon 50 m?
7.7. A horse exerts a force of 200 lb while pulling a sled for 3 mi. (a) How much work does the horse do? (b) If the trip
takes 30 min, what is the power output of the horse in horsepower?

7.8. A road slopes upward so that it climbs 1 m for each 12 m of distance covered. A car whose weight is 10 kN moves
up the road at a constant velocity of 24 m/s. Find the minimum power the car’s engine is delivering.
7.9. A certain 80-kg mountain climber has an average power output of 0.1 hp. (a) How much work does she perform
in climbing a mountain 2000 m high? (b) How long does she take to climb the mountain? (c) What is her potential
energy at the top?

7.10. A man uses a rope and a system of pulleys to raise a 200-lb box to a height of 10 ft. He exerts a force of 60 lb on the
rope and pulls a total of 40 ft of rope through the pulleys. (a) How much work does he perform? (b) By how much is
the potential energy of the box increased? (c) If these answers are different, what do you think the reason is?

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7.11. A total of 10 kg of water per second flows over a waterfall 25 m high. If 50 percent of the power this flow represents
could be converted into electricity, how many 100-W light bulbs could be supplied?
7.12. The four engines of a DC-8 airplane develops a total of 22 MW when its velocity is 240 m/s. How much force do the
engines exert?
7.13. Neglecting friction and air resistance, is more work needed to accelerate a car from 10 to 20 km/h or from 20 to
30 km/h?

7.14. A 3000-lb car has an engine which can deliver 80 hp to the driving wheels. What is the maximum velocity at which
the car can climb a 15 hill?
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7.15. Find the kinetic energy of a 2-g (0.002-kg) insect when it is flying at 0.4 m/s.
7.16. The electrons in a television picture tube whose impacts on the screen produce the flashes of light that make up the
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image have masses of 9.1 × 10 −31 kg and typical velocities of 3 × 10 m/s. What is the kinetic energy of such an
electron?
7.17. A 15-kg object initially at rest is raised to a height of8mbya force of 200 N. What is the velocity of the object at
this height?

7.18. A 7-kg iron shot is thrown 18 m. What was its minimum initial kinetic energy?

7.19. An 800-kg car moving at 70 km/h is carrying two 75-kg people. If the power output of the car’s engine is 30 kW, how
much time is needed for the car to reach a velocity of 110 km/h? Neglect friction and air resistance.

7.20. (a) What velocity does a 1-slug object have when its kinetic energy is 1 ft·lb? (b) What velocity does a 1-lb object
have when its kinetic energy is 1 ft·lb?

7.21. A stone is dropped from a height of 100 m. At what height is half of its energy potential and half kinetic?

7.22. A 10-g bullet has a velocity of 600 m/s when it leaves the barrel of a rifle. If the barrel is 60 cm long, find the average
force on the bullet while it is in the barrel.

7.23. A 16-lb shell has a velocity of 2000 ft/s when it leaves the barrel of a cannon. If the barrel is 10 ft long, find the
average force on the shell while it is in the barrel.

7.24. (a) An 8-N force pushes a 0.5-kg ball on a horizontal table for 3 m, starting from rest. If there is no friction, what is
the final KE of the ball? (b) The same force is used to raise the ball a height of 3 m, starting from rest. What is its
final KE now?