Download 1. No real machine is 100 percent efficient. Hypothesize some of the

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Chapter 10 Test
1. No real machine is 100 percent efficient. Hypothesize some of the energy losses experienced by any real
machine.
2. Could a mechanical advantage that is less than one ever be useful? Give an example.
3. The six simple machines described in the chapter, the lever, the pulley, the wheel and axle, the inclined plane,
the wedge, and the screw, can be classified into two families. Organize these families, and describe their
properties.
4. The output work of a machine is always less than the input work of a machine. Are machines valuable? Why or
why not?
Indicate the answer choice that best completes the statement or answers the question.
5. Which is not a part of the walking mechanism of the human body?
a. a rigid bar
b. a fulcrum
c. a resistance
d. a gear
6. What type of simple machine is a wheelbarrow?
a. lever
b. inclined plane
c. wedge
d. screw
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Chapter 10 Test
7. What type of simple machine is a flight of stairs?
a. pulley
b. wedge
c. inclined plane
d. lever
8. Machines make tasks easier by changing either the magnitude or the direction of the _____ needed.
a. force
b. work
c. energy
d. velocity
9. What type of simple machine is most often used to cut things?
a. wedge
b. pulley
c. wheel and axle
d. screw
10. Which is not a simple machine?
a. pulley
b. ramp
c. bicycle
d. wood screw
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Chapter 10 Test
11. Predict whether the box shown in the diagram will gain kinetic energy.
12. In the previous problem, suppose you lift the water to a height of 3 m in 1.35 seconds. Now how much power
have you generated?
13. What are the two equivalent expressions of the efficiency of a machine?
14. According to the work-energy theorem, what happens to the energy of a system when work is done by that
system?
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Chapter 10 Test
15. What are the units for work and energy?
16. A spring is compressed over a distance of 0.25 meters. The force required to hold the compressed spring is 64
newtons. Explain why the work done on the spring is not 64 N 0.25 m = 16 joules.
17. Finally, what if you lifted the water 1 m in 1.35 seconds. How much power have you generated?
18. Two identical 2200 kg cars, traveling at 22 m/s, collide head on and stop.
a) What is the change in momentum for each car?
b) What is the change in kinetic energy for each car?
19. According to the work-energy theorem, what happens to the energy of a system when work is done on that
system?
20. If an elevator and I both lift 100 kg up 3 flights (10 meters) but I reach the top before the elevator, which has
produced more power?
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Chapter 10 Test
21. You pull your dog on a sled through the snow. You pull up on the rope at an angle of 27° from the horizontal
and exert a constant force of 75 N. The sled and the dog move only horizontally, a distance of 950 meters.
What work do you do on the sled and the dog?
22. Carol and Bruno move a box of mass 58.0 kg along a frictionless floor. Carol pushes the box with a force of
11.4 N at an angle of 40.0° downward from the horizontal. Bruno pulls the box from the other side with a force
of 11.0 N at an angle of 40.0° above the horizontal. What is the net work done on the box if the displacement of
the box is 14.5 m?
23. Derive the work-kinetic energy theorem by combining W=Fd, F=ma, and the constant acceleration formula 2ad
= vf2 - vi2.
24. State the work-energy theorem.
25. What is the mechanical advantage of any machine?
Indicate whether the statement is true or false.
26. To obtain the same resistance force, a greater force must be exerted in a machine of lower efficiency than in a
machine of higher efficiency.
a. True
b. False
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27. Power equals work multiplied by time.
a. True
b. False
28. If several forces are exerted on a system, calculate the work done by each force, then multiply the results.
a. True
b. False
29. Ideal mechanical advantage is equal to the displacement of the effort force divided by the displacement of the
load.
a. True
b. False
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Chapter 10 Test
Answer Key
1. Answers will vary. These may include heat produced by the machine, sound produced by the machine, and the work
needed to get the parts of the machine moving or stopped.
2. Yes, a mechanical advantage that is less than one could be useful. Sometimes it is possible to supply a great deal of
force over a short distance to create a larger movement. One classic example is a foot-triggered trash can. The
pedal moves a short distance with a large effort from the person’s foot, and the trash can lid moves a greater
distance than the foot pedal.
3. The lever, pulley, and wheel and axle form one family. The lever makes use of forces and lever arms. The pulley is a
sort of lever with the fulcrum in the center, the load on one side, and the effort on the other. The two lever arms of
the pulley are generally equal. The wheel and axle is similar to the pulley, but the load and effort can be at different
distances, reflecting different lever arms.
The inclined plane, wedge, and screw form the other family. The inclined plane trades less force for more distance.
The wedge is like two inclined planes together. While the inclined plane is generally still, the wedge generally moves.
Like the inclined plane, the wedge redirects force. The screw is like a wedge and inclined plane together; the shaft
of the screw acting as the wedge and the threads acting as the inclined plane.
4. Machines are valuable because they make work easier. For instance, the work needed to lift a 1500 N block a distance
of 10 meters is 15,000 J, a huge number. A machine such as a pulley or inclined plane might increase that number to
20,000 J, but might reduce the force needed to just a fraction of the original 1500 N. What was perhaps an
impossible job is now something that can be done, even though the machine adds to the total work that must be done.
5. d
6. a
7. c
8. a
9. a
10. c
11. No, the box will not gain kinetic energy. The applied force is perpendicular to the surface.
12. 490 watts
13. Efficiency, e, equals either (work output)/(work input) times 100 or (mechanical advantage)/(ideal mechanical
advantage) times 100.
14. When work is done by a system, the system’s energy decreases.
15. Since work = force x distance, the units are N · m, or kg m2/s2. The units for energy are joules. 1 N · m equals 1
joule.
16. The force needed to compress the spring is not constant over the entire distance. For a non-compressed spring, the
force needed starts at zero and increases as the spring is compressed.
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17. 160 watts
18. a) For each car, loss of momentum is 4.8
reference frame.
104 kg m/s, one positive and the other negative depending on the
b) For each car, loss of kinetic energy is 5.3
105 J. Kinetic energy is always positive.
19. When work is done on a system, the energy of that system increases.
20. Power is work over time. Since I took less time to do the same work, I produced more power.
21. 6.3 104 J
22.
J
23. 2ad = vf2
vi2
a = F/m, so 2Fd/m = vf2
W = Fd = 1/2 (mvf2)
vi2
1/2 (mvi2) = δKE
24. The work-energy theorem says that the work done equals the change in kinetic energy.
25. Mechanical advantage is equal to the resistance force divided by the effort force.
26. True
27. False
28. False
29. True
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