22T
Teacher Guide and Answers
9
Name
Date
Chapter Assessment
8. A sphere of mass 5.00 kg moving at 4.00 m/s collides with an identical sphere that is at rest. The
first sphere moves off at an angle of 60.08 to the left of its original path, and the second sphere
moves off in a direction 90.08 to the right of the first sphere’s final path. What are the speeds of
the two spheres as they separate? Assume frictionless conditions.
pB1 5 mBvB1
5 (5.00 kg)(4.00 m/s)
5 (5.00 kg) (0.00 m/s)
5 20.0 kg?m/s
5 0.0 kg?m/s
Chapter Assessment
Write the letter of the choice that best completes the statement or answers the question.
b
1. Any object that has energy has the ability to
.
a. burn
pB2 5 p 2 sin 60.08
b. produce a change
2. If the environment does work on a system,
5 (20.0 kg?m/s)(0.866)
b. the energy of the system increases
5 10.0 kg?m/s
5 17.3 kg?m/s
c. the energy of the system decreases
pA2 5 mAvA2
pB2 5 mBvB2
pA2
vA2 5 }
mA
p B2
vB2 5 }
mB
5 2.00 m/s
c. fall
.
a. the environment warms
5 (20.0 kg? m/s)(0.500)
10.0 kg?m/s
5 }}
5.00 kg
Use with Chapter 10.
Energy, Work, and Simple Machines
b
p 1 5pA1 1 p B1 5 20.0 kg?m/s 5 p 2
pA2 5 p 2cos 60.08
10
Name
Understanding Concepts Part A
p 1 5 pA1 1 p B1 5 pA2 1 p B2 5 p 2
p A1 5 mAvA1
Period
d. the quantity of work done on the system has a negative value
c
17.3 kg?m/s
5 }}
5.00 kg
5 3.46 m/s
b
3. When a force is exerted on an object, work is done only if the object
.
a. is heavy
c. moves
b. remains stationary
d. has no momentum
4. In which of the following situations is no work done on a football?
a. picking up the football
c. dropping the football
b. carrying the football down the field
a
5. In which of the following situations is work done on the football by a person?
a. picking up the football
c. dropping the football
b. carrying the football down the field
c
6. In which of the following situations is work done on the football by gravity?
a. picking up the football
c. dropping the football
b. carrying the football down the field
Physics: Principles and Problems
Write the term that correctly completes each statement.
power
7. One definition of
is “work done per unit time.”
effort force
8. A machine with a mechanical advantage greater than 1 increases
9. For an ideal machine, efficiency is always
100
10. If the mechanical advantage of a machine is less than 1, effort force is
than resistance force.
simple
11. A wedge used to split wood is a
machine.
12. A pair of gears make up a
44 Chapter Assessment
Physics: Principles and Problems
Physics: Principles and Problems
compound
.
percent.
greater
machine.
Chapter Assessment 45
Physics: Principles and Problems
10
Name
10
Chapter Assessment
Name
Chapter Assessment
Understanding Concepts Part B
Applying Concepts
Answer the following questions, showing your calculations.
Answer the following questions, using complete sentences.
1. How much work is done if you raise a 6.0-N weight 1.5 m above the ground?
W 5 Fd 5 (6.0 N)(1.5 m) 5 9.0 N?m 5 9.0 J
1. Two students are moving 80-N cartons of books from the floor onto a platform. One student
moves 12 cartons in 7 minutes. The other student moves the same number of cartons in 5 minutes. Which student does more work? Explain your answer.
Each student does the same amount of work. Each exerts the same amount of force over the
same distance.
2. Using an ideal machine, a worker exerts an effort force of 5.0 N to lift a 12.0-N weight a distance
of 3.0 m. How far does the effort force move?
Fede 5 Frd r
2. What type of simple machine is the handle of a pencil sharpener? How does it make work easier?
The handle is a wheel and axle. Force applied at the perimeter of the wheel is transferred to
(12.0 N) (3.0 m)
Frd r
de 5 } 5 }} 5 7.2 m
(5.0 N)
Fe
the axle at the center of the system, where it is multiplied and used to do work.
3. An effort force of 200.0 N is applied to an ideal machine to move a 750.0-N resistance a distance
of 300.0 cm. What is the mechanical advantage of the machine?
750.0 N
Fr
MA 5 } 5 }} 5 3.750
Fe
200.0 N
3. If a machine is used to increase force, what factor is sacrificed? Cite an example.
Effort distance is sacrificed. Examples may vary, but should be machines in which a small
effort force acts over a large distance to exert a large resistance force over a small distance.
4. How much power is developed by an electric motor that moves a 500-N load a distance of 20 m in
10 s?
Fd
W
(500 N) (20 m)
P 5 } 5 } 5 }} 5 1000 (N?m)/s 5 1000 W
t
t
(10 s)
4. Distinguish between work and power.
Work is the transfer of energy by mechanical means. Power is the rate at which that energy is
transferred. Power involves time; work does not.
5. What is the efficiency of a machine that requires a work input of 190 J to achieve a work output of
160 J?
Teacher Guide and Answers 23T
160 J
Wo
efficiency 5 } 3 100% 5 } 3 100% 5 84%
Wi
190 J
5. Explain the following: When a screwdriver is used to drive a screw, the diameter of its handle is
more important than its length, but when used to pry open a stuck window, its length is more
important than the diameter of its handle.
When used to drive a screw, a screwdriver is a wheel and axle; the larger the ratio of the diameter of the handle to the shaft, the greater the IMA of the machine. When used to pry open a
window, the screwdriver is a lever, in which the edge of the windowsill is the fulcrum. The
longer the effort arm of the lever, the greater its IMA.
6. How much power is generated by a machine in lifting 250 kg a distance of 150 m in 30.0 s?
W
Fd
mgd
(250 kg) (9.80 m/s2) (150 m)
P 5 } 5 } 5 } 5 }}}} 5 1.2 3 104 (N?m)/s 5 1.2 3 104 J
t
t
t
(30.0 s)
6. Use mechanical advantage to explain what happens when you shift the gears of a multispeed bike.
The gears in a bike are a type of wheel-and-axle machine in which the MA is the ratio of the
effort gear attached to the pedals and the resistance gear attached to the rear wheel. Shifting
gears changes this gear ratio, thus changing the MA of the wheel and axle.
46 Chapter Assessment
Physics: Principles and Problems
Physics: Principles and Problems
Chapter Assessment 47
24T
Teacher Guide and Answers
10
Name
Date
Period
Chapter Assessment
11
Answer the following questions, showing your calculations.
7. An adult and a child exert a total force of 930 N in pushing a car 15 m down their driveway. The
adult exerts twice as much force as the child. How much work does each person do?
Total force, F 5 930 N
Force exerted by adult,
2
F 5 (930 N)(}3 ) 5 620 N
Work done by adult,
W 5 Fd
5 (620 N)(15 m) 5 9300 N ? m
Force exerted by child,
F 5 930 N 2 620 N 5 310 N
Work done by child,
W 5 Fd
5 (310 N)(15 m) 5 4700 N ? m
5 9300 J
5 4700 J
8. An adult exerts a force of 26 N to pull a child across the snow on a sled. The rope that joins the
sled at an angle of 30.0° above horizontal. Ignoring friction, how much work is done in pulling
the sled 390 m?
W 5 (Fd )(cos 30.0°)
W50J
5 (0 N)(390 m)(0.866)
9. A grocer lifts an 8.0-kg carton from the floor to a height of 0.80 m, carries it 24 m across the store,
and places it on a shelf 1.8 m above the floor. How much work does the grocer accomplish?
W 5 Fd 5 (8.0 kg)(9.8 m/s2)(1.80 m) 5 140 N?m 5 140 J
Note: No work is done on the carton while it is carried horizontally.
Physics: Principles and Problems
5 (680 N)(7.0 m)
5 4800 N?m 5 4800 J
For each of the statements below, write true or rewrite the italicized part to make the statement true.
1. When you throw a ball into the air, its mechanical energy at any point is the difference in its kinetic
energy and gravitational potential energy.
sum of
2. As an object falls towards Earth, the gravitational potential energy of the object increases.
decreases
3. The total amount of energy in an isolated, closed system remains constant.
true
4. If a tabletop is used as a reference point in a mechanical-energy problem, an object lying on the
tabletop has a gravitational potential energy greater than zero.
equal to zero
5. Of the four sets of bar graphs below, an inelastic collision is represented by set A.
set B
A
C
B
E thermal
Kafter
Kafter
c. What is the efficiency of the machine?
Wo
efficiency 5 }
3 100%
Wi
4500 J
5 } 3 100% 5 94%
4800 J
b. What work is done on the piano by the
machine?
Wo 5 Frdr 5 m r gd r
Kbefore
Physics: Principles and Problems
Kbefore
c
6. K
a. Kafter = Kbefore
b
7. Ug
b. 5 mgh
d
8. 1 joule
c. 5
e
9. work-energy theorem
d. 1 kg?m2/s2
a
5 4500 N?m 5 4500 J
Kbefore
Kafter
For each term on the left, write the letter of the matching item.
5 (260 kg)(9.8 m/s2)(1.75 m)
48 Chapter Assessment
Use with Chapter 11.
Understanding Concepts Part A
11. A mover pushes a 260-kg piano on wheels up a ramp 7.0 m long onto a stage 1.75 m above the
auditorium floor. The mover pushes the piano with a force of 680 N.
Wi 5 Fede
Chapter Assessment
Energy
10. At what speed can a 150-W motor lift a 2500-N load?
W
Fd
d
P 5 } 5 } 5 F } 5 Fv
t
t
t
150 W
P
(N?m)/s
v 5 } 5 } 5 0.060 } 5 0.060 m/s
F
2500 N
N
a. How much work does the mover do?
Name
10. result of an elastic collision
Physics: Principles and Problems
1
}
2
mv2
e. Fd 5 DK
Chapter Assessment 49