Willis High School
Physics Homework
Unit D
Energy
Name:
Due Date:
Instructor:
Period
Physics Textbook Reference
CHAPTER 7: WORK, ENERGY, AND ENERGY RESOURCES
http://openstaxcollege.org/textbooks/college-physics
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E NERGY
Homework
1. A horizontal force of 300 N is used to push a 50 kg dog for 0.5 m across a level hardwood floor. How much work
is done? (150 J)
2. In raising a 150 kg bag of coffee, 5,000 J of work is performed. How high is it raised? (3.3 m)
3. From the following data, find the work done on the object in the first 5 m. (11.5 J)
Displacement (m)
0
1
2
3
4
5
6
7
8
Force (N )
0
1
2
3
4
3
2
1
0
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4. A girl drags a 7 kg wagon behind her.
(a) If the coefficient of friction is 0.4, what force must be applied in order to travel at constant velocity? (28 N )
(b) If she drags the wagon at this constant velocity for 30 m, how much work has she done? (840 J)
5. On his way off to college, Russel drags his suitcase 15.0 m from the door of his house to the car at a constant
speed with a horizontal force of 95.0 N .
(a) How much work does Russell do to overcome the force of friction? (1425 J)
(b) If the floor has just been waxed, does he have to do more or less work to move the suitcase?
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6. The force-displacement graph of Carrie’s broken-down automobile (that was pushed off the highway) is shown
below. Refer to the graph to answer the questions.
(a) How far did the car move horizontally? (15 m)
(b) What was the magnitude of the force that acted on the car between 8 and 15 m? (130 N )
(c) What does the area under the graph represent?
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(d) How much work was done in moving the car the entire 15 m? (1530 J)
7. A 70 kg alpaca runs up a hill 2 m high in 0.8 s. Find its power. (1750 W )
8.
(a) Calculate the work done by a chimpanzee who carries a 50 N bag of bananas vertically 15 m up a tree. (750
J)
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(b) If the chimp in part (a) took 25 seconds to climb the tree, calculate its power. (30 W )
9. The force-displacement graph of an object that was pushed horizontally is shown below. Refer to the graph to
answer the questions.
(a) What force acts on the object when it has been moved 4 m? (15 N )
(b) How far has the object been moved when the force on it is 40 N ? (13 m)
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(c) How much work is done in moving the object from 1 m to 6 m? (62.5 J)
(d) How much work is done in moving the object from 1 m to the 15 m distance shown on the graph? (347.5 J)
(e) If it takes 4.5 s to travel the 15 m, how much power is needed? (77 N )
10.
(a) A 2.7 kg soccer ball is rolling at 0.3 m/s. What is its kinetic energy? (0.12 J)
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(b) If the velocity of the soccer ball doubles, what is its new kinetic energy? (0.49 J))
11. A 3 kg cat is held out an eleventh story window, 100 m above the ground. Find the potential energy of the cat:
(a) with respect to the fifth floor, 45 m above the ground. (1,650 J)
(b) With respect to the ground. (3,000 J)
12. An alien abducts a little dog (100 kg) into its spaceship 500 m above the earth.
(a) How much work did the alien do on the dog? (500,000 J)
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(b) How much potential energy does the dog now have with respect to the earth below? (500,000 J)
13. The 2000 Belmont Stakes winner, Commendable, ran the horse race at an average speed of 15.98 m/s. If Commendable and jockey Pat Day has a combined mass of 550.0 kg, what was their KE as they crossed the finish
line? (70,224 J)
14. While on the moon, the Apollo astronauts enjoyed the effects of a gravity much smaller than on Earth. If Neil
Armstrong jumped up on the moon with an initial speed of 1.51 m/s to a height of 0.700 m, what amount of
gravitational acceleration did he experience? (1.6 m/s2 )
15. A 40 kg package is sliding across a conveyor belt. The UPS worker exerts a constant 3 N force over a distance of
2.8 m.
(a) How much work does the worker do on the package? (8.4 J)
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(b) What is the change in the package’s energy? (8.4 J)
(c) What is the final velocity of the package? (0.65 m/s)
16. A 70 kg crate, starting from rest, is accelerated by a 22 N force for a distance of 8 m along a horizontal frictionless
surface. What is the final velocity of the mass? (2.24 m/s)
17. A cell phone that weighs 0.3 kg is dropped from a desk 1.5 m above the ground.
(a) What is the kinetic energy of the phone when it reaches the ground? (4.5 J)
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(b) What is the velocity of the phone as it contacts the ground? (5.5 m/s)
18. Your physics teacher gets angry with one of their students (80 kg). They lifts the student 1.4 m above the ground,
releasing them into the air at 2 m/s. How much work is done by your teacher? (1,280 J)
19. At its highest point, a pole-vaulter jumps 5 m above the ground (assume v = 0 m/s), and at her lowest point she
lands on a mat 2.5 m above the ground. What is her maximum velocity? (7.1 m/s)
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20. A 225 kg roller coaster car starts from the top of the track at 52 m of height (Point A). Find the speed of the car at
points B (0 m), C (31 m), and D (18 m). (32 m/s, 20 m/s, 26 m/s)
21. What is the potential energy of a 100 N box when it is raised 2 m? (200 J)
22. Calculate the change in energy when the speed of a 1.3 kg remote-control car increases from 2.3 m/s to 7.6 m/s.
(34 J)
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23. A tiger weighing 150 N jumps from the top of its caged fence, 4 m high.
(a) What is the potential energy of the tiger before it jumps? (600 J)
(b) What is the kinetic energy of the tiger when it is 2.4 meters from the ground? (240 J)
(c) What is the speed of the tiger when it reaches the ground? (8.9 m/s)
24. A 3 kg ball rolls along a 4 m high table at 2.1 m/s.
(a) What is the ball’s kinetic energy when it reaches the floor? (127 J)
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(b) What is the ball’s final velocity? (9.2 m/s)
25. A skate-boarder (78 kg) is traveling on a horizontal track with a velocity of 5 m/s. If he decides to skate up a
ramp, how high will he go up the incline? (1.25 m)
26. A 200 kg roller coaster car is pulled to the top of a hill 90 m high. Its velocity is 0 m/s.
(a) How much work is done by the chain that pulls the car to the top? (180,000 J)
(b) How fast will the car be going when it gets back to ground level, 90 m below? (42.4 m/s)
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(c) How fast will the car go at a point 54 m above ground level? (26.8 m/s)
27. The same roller coaster car is pulled to the top of the 90 m high hill, but this time it arrives with a speed of 15
m/s. How fast is the car when at a point 22 m above ground? (39.8 m/s)
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