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Group A—Continued Group B—Continued
6. (a) How many horsepower is a 1,400 W blow dryer? (b) How 6. (a) How many horsepower is a 250 W lightbulb? (b) How many
many watts is a 3.5 hp lawnmower? watts is a 230 hp car?
7. What is the kinetic energy of a 2,000 kg car moving at 72 km/h? 7. What is the kinetic energy of a 30-gram bullet that is traveling at
200 m/s?
8. How much work is needed to stop a 1,000.0 kg car that is moving 8. How much work will be done by a 30-gram bullet traveling at
straight down the highway at 54.0 km/h? 200 m/s?

9. A horizontal force of 10.0 lb is needed to push a bookcase 5 ft 9. A force of 50.0 lb is used to push a box 10.0 ft across a level

across the floor. (a) How much work was done on the bookcase? floor. (a) How much work was done on the box? (b) What is the
(b) How much did the gravitational potential energy change as a change of potential energy as a result of this move?
result?
10. (a) How much work is done in moving a 2.0 kg book to a shelf 10. (a) How much work is done in raising a 50.0 kg crate a distance
2.00 m high? (b) What is the change of potential energy of the of 1.5 m above a storeroom floor? (b) What is the change of

book as a result? (c) How much kinetic energy will the book have potential energy as a result of this move? (c) How much kinetic
as it hits the ground when it falls? energy will the crate have as it falls and hits the fl oor?
11. A 150 g baseball has a velocity of 30.0 m/s. What is its kinetic 11. What is the kinetic energy in J of a 60.0 g tennis ball approaching
energy in J? a tennis racket at 20.0 m/s?
12. (a) What is the kinetic energy of a 1,000.0 kg car that is 12. (a) What is the kinetic energy of a 1,500.0 kg car with a velocity
traveling at 90.0 km/h? (b) How much work was done to give of 72.0 km/h? (b) How much work must be done on this car to
the car this kinetic energy? (c) How much work must be done bring it to a complete stop?
to stop the car?
13. A 60.0 kg jogger moving at 2.0 m/s decides to double the 13. The driver of an 800.0 kg car decides to double the speed from

jogging speed. How did this change in speed change the kinetic 20.0 m/s to 40.0 m/s. What effect would this have on the amount
energy? of work required to stop the car, that is, on the kinetic energy of
the car?
14. A bicycle and rider have a combined mass of 70.0 kg and are 14. Compare the kinetic energy of an 800.0 kg car moving at 20.0 m/s
moving at 6.00 m/s. A 70.0 kg person is now given a ride on the to the kinetic energy of a 1,600.0 kg car moving at an identical
bicycle. (Total mass is 140.0 kg.) How did the addition of the new speed.
rider change the kinetic energy at the same speed?
15. A 170.0 lb student runs up a stairway to a classroom 25.0 ft above 15. A 175.0 lb hiker is able to ascend a 1,980.0 ft high slope in 1 hour

ground level in 10.0 s. (a) How much work did the student do? 45 minutes. (a) How much work did the hiker do? (b) What was
(b) What was the average power output in hp? the average power output in hp?
16. (a) How many seconds will it take a 20.0 hp motor to lift a 16. (a) How many seconds will it take a 10.0 hp motor to lift a
2,000.0 lb elevator a distance of 20.0 ft? (b) What was the average 2,000.0 lb elevator a distance of 20.0 feet? (b) What was the

velocity of the elevator? average velocity of the elevator?

17. A ball is dropped from 9.8 ft above the ground. Using energy 17. A ball is dropped from 20.0 ft above the ground. (a) At what
considerations only, find the velocity of the ball just as it hits the height is one-half of its energy kinetic and one-half potential?

ground. (b) Using energy considerations only, what is the velocity of the
ball just as it hits the ground?
18. What is the velocity of a 1,000.0 kg car if its kinetic energy is 18. What is the velocity of a 60.0 kg jogger with a kinetic energy of
200 kJ? 1,080.0 J?
19. A Foucault pendulum swings to 3.0 in above the ground at the 19. A small sports car and a pickup truck start coasting down a 10.0 m
highest points and is practically touching the ground at the hill together, side by side. Assuming no friction, what is the
lowest point. What is the maximum velocity of the pendulum? velocity of each vehicle at the bottom of the hill?

20. An electric hoist is used to lift a 250.0 kg load to a height of 80.0 m 20. A 70.0 kg student runs up the stairs of a football stadium to
in 39.2 s. (a) What is the power of the hoist motor in kW? a height of 10.0 m above the ground in 10.0 s. (a) What is the
(b) In hp? power of the student in kW? (b) In hp?

84 CHAPTER 3 Energy 3-24