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Work and Energy
Section Quiz: Conservation of Energy
Write the letter of the correct answer in the space provided.
______ 1. Which of the following is true of the conservation of energy in a closed
system?
a. Kinetic energy is always conserved.
b. Potential energy is always conserved.
c. Mechanical energy is always conserved.
d. Total energy is always conserved.
______ 2. The mechanical energy of a system of objects is
a. the sum of kinetic energy and gravitational potential energy.
b. the sum of kinetic energy and elastic potential energy.
c. the sum of kinetic energy and all relevant forms of potential energy.
d. the sum of all forms of energy.
______ 3. Mechanical energy is not conserved when
a. gravitational potential energy is converted to kinetic energy.
b. kinetic energy is converted to gravitational potential energy.
c. kinetic energy is converted to elastic potential energy.
d. friction is not negligible.
______ 4. In which of the following situations is mechanical energy most likely
to be conserved?
a. A football flies through the air.
b. A feather falls from the sky.
c. A skateboard rolls into the grass.
d. A hockey player digs his skates into the ice.
______ 5. If mechanical energy is conserved in a system, the energy at any point
in time can be in the form of
a. kinetic energy.
b. gravitational potential energy.
c. elastic potential energy.
d. all of the above
______ 6. Which of the following is not a form of mechanical energy?
a. kinetic energy
b. chemical potential energy
c. gravitational potential energy
d. elastic potential energy
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Holt Physics
33
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Work and Energy continued
______ 7. Which of the following is evidence that frictional forces are present in
a system?
a. Interactions in the system cause an increase in temperature.
b. Interactions in the system produce sound.
c. Mechanical energy is not conserved.
d. all of the above
______ 8. An egg suspended above the ground has 2.0 J of gravitational potential
energy. The egg is then dropped and falls to the ground. What is the
kinetic energy of the egg just as it reaches the ground?
a. 2.0 J
b. 0 J
c. 2.0 J
d. 4.0 J
9. A tennis ball is thrown up into the air starting from a height of 1.5 m. The ball
reaches a peak height, then falls down to the ground. Assuming air resistance
is negligible, describe the energy transfers that take place during the flight of
the ball. Is mechanical energy conserved in this situation?
10. The tennis ball in question 9 above has a mass of 5.7 102 kg and has an
initial speed of 2.0 m/s. Calculate the speed of the ball when it hits the ground.
Ignore air resistance.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Holt Physics
34
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TEACHER RESOURCE
5 Work and Energy
Solution
WORK
KEi 1.
2.
3.
4.
9.
d
5. a
c
6. b
b
7. b
c
8. d
While lifting the block, the worker
does positive work on the block while
gravity does negative work on the
block. The net work while lifting the
block is positive. When the worker is
holding the block, no forces do work
on the block and no net work is done
on the block. While lowering the
block, the worker does positive work
while gravity does negative work on
the block. The net work on the block
while it is lowered is negative. The
total net work on the block is zero
because the net displacement is zero.
10. 99 J
Given
d 3.0 m
Fchild 55 N
q 35°
Fk 12 N
Solution
Wnet Fnetd (Fchildcosq Fk)d [(55 N)(cos 35°) (12 N)](3.0 m)
99 J
5 Work and Energy
ENERGY
mv
1
2
2
i
(1.0 10
1
2
3
kg)
(15 m/s)2 1.1 105 J
Wnet KE KEf KEi
KEf KEi Wnet (1.1 105 J) (2.5 104 J) 8.5 104 J
5 Work and Energy
CONSERVATION OF ENERGY
1.
2.
3.
4.
9.
d
5. d
c
6. b
d
7. d
a
8. c
When the ball is first thrown, the ball
has some kinetic energy and some
gravitational potential energy. As the
ball rises, the kinetic energy is transferred to gravitational potential energy.
At the peak, all the energy is potential
energy. As the ball falls, the potential
energy is transferred to kinetic energy.
When the ball hits the ground, all the
energy is kinetic energy. Mechanical
energy is conserved throughout the
flight of the ball.
10. 5.8 m/s
Given
m 5.7 102 kg
vi 2.0 m/s
hi 1.5 m
hf 0 m
g 9.81 m/s2
Solution
MEi MEf
1.
2.
3.
4.
9.
a
5. b
c
6. b
d
7. d
c
8. c
The bocce ball has more kinetic
energy. Kinetic energy depends on
both mass and velocity. However,
kinetic energy is more strongly
dependent on velocity because the
velocity term is squared in the equation for kinetic energy: KE (1/2)mv2.
10. KEi 1.1 105 J; KEf 8.5 104 J
Given
m 1.0 103 kg
vi 15 m/s2
Wnet 25 kJ 2.5 104 J
1
mvi2
2
1
mghi 2mvf2 mghf
22mvi2 mghi mghf
2
vf m
1
vf 2 v gh
gh
1
2
2
i
i
f
vf 2 (2.0 m/s)
(9.81 m/s
1.5 m)
(9.81
m/s
)(0 m)
)(
1
2
2
2
2
vf 5.8 m/s
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Holt Physics
155
Answer Key