Chapter 3: Energy and Conservation Laws
TRUE/FALSE
1. Conservation laws can be used even when the details of what is occurring inside a system
aren’t known.
2. The momentum of an object never changes.
3. Kinetic energy is conserved in elastic collisions.
4. Linear momentum is conserved in all collisions.
5. Momentum is conserved only when there is no friction.
6. Work done within an isolated system can increase the energy of the system.
7. Centripetal force changes linear momentum but not angular momentum.
8. Kinetic energy is definitely conserved.
9. Linear momentum is definitely conserved.
10. Kinetic energy is definitely not conserved.
11. Linear momentum is definitely conserved.
12. The energy stored in a pile driver is an example of gravitational potential energy.
13. The energy stored in a stretched spring is an example of gravitational potential energy.
14. The energy of a body moving across a horizontal surface is an example of kinetic energy.
15. The energy in the water at the top of a waterfall is an example of gravitational potential
energy.
16. The energy of a ball rolling up an inclined plane is an example of both kinetic and
gravitational potential energy.
17. For a body oscillating at the end of a spring, the energy is only elastic potential energy.
18. Angular momentum is conserved.
19. Linear momentum is not conserved.
20. Kinetic energy is conserved.
21. The kinetic energy of the crate is 1/2 mv2.
22. The potential energy of the crate is mgd.
23. The linear momentum of the crate is Fv.
24. The work you do on the crate is Fv.
25. The power you supply to the crate is Fd/t.
26. The measure of a body’s resistance to being rotated is its angular momentum.
27. A watt is equal to 746 horsepower.
28. You carry a 20 N box 5 m horizontally across a room. The work you do is 100 N-m.
29. The work you do is 4 J.
30. The increase of potential energy of the box is 100 J.
31. You drop the box. As it falls to its original level, its kinetic energy is 100 J.
32. When the box hits the floor its kinetic energy is converted into internal energy.
33. A 2 kg car moving towards the right at 4 m/s collides head on with an 8 kg car moving
towards the left at 2 m/s, and they stick together. After the collision, the velocity of the
combined bodies is 0.8 m/s pointing towards the left.
34. A person runs up a long flight of stairs in 10 seconds. If the person’s weight is 600 N and
the vertical height of the stairs is 20 meters, the person’s power output is 1,200 W.
35. A rocket can never go faster than the speed of its exhaust.
36. When more people ride upward in an elevator, more power is required.
37. A spacecraft can gain energy by passing near a planet.
38. When a car comes to a stop its kinetic energy is converted to internal energy in its brakes,
heating them up.
39. The collision between two cars in an accident is an inelastic collision.
40. If the velocity of an aircraft is doubled, the kinetic energy of the aircraft is also doubled.
41. In a closed system, the kinetic energy plus the potential energy is a constant.
42. A barrel rolling down an inclined ramp has only kinetic energy.
MULTIPLE CHOICE
1. What does it mean for a physical quantity in a system to be conserved?
a.
It is used in the most efficient manner possible.
b.
It stays in its original state forever.
c.
The total amount of it in the system stays constant.
d.
none of the above.
2. Kinetic energy is conserved in
a.
an inelastic collision
b.
an elastic collision
c.
any collision where two objects rebound off each other
d.
none of the above
3. The linear momentum of a truck will be greater than that of a bus if
a.
the truck’s mass is larger than the bus’s but its speed is the same
b.
the truck’s speed is larger than the bus’s but its mass is the same
c.
the truck’s mass and speed are both larger than the bus’s
d.
any of the above
4. What are the SI units of momentum?
a.
kg m/s
c.
joules
b.
pascals
d.
watts
5. Work is done on an object when
a.
it moves in a circle at constant speed
b.
it is accelerated in a straight line
c.
it is carried horizontally at constant speed
d.
all of the above
6. If a force on an object is aimed opposite the direction of the object’s velocity, the force does
a.
no work
c.
negative work
b.
positive work
d.
any of the above
7. If a force on an object is aimed in the direction of the object’s velocity, the force does
a.
no work
c.
negative work
b.
positive work
d.
any of the above
8. If a force on an object is aimed perpendicular to the direction of the object’s velocity, the
force does
a.
no work
c.
negative work
b.
positive work
d.
any of the above
9. Kinetic energy
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved, depending on the nature of the collision
d.
none of the above
10. Linear momentum
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved, depending on the nature of the collision
d.
none of the above
11. Kinetic energy
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved, depending on the nature of the collision
d.
none of the above
12. Linear momentum
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved, depending on the nature of the collision
d.
none of the above
13. Internal energy
a.
remains constant
b.
decreases
c.
increases
d.
varies but in a way that cannot be predicted
14. Which of the following is not an example of potential energy?
a.
The energy stored in a pile driver.
b.
The energy stored in a stretched spring.
c.
The energy of a body moving across a horizontal surface.
d.
The energy in the water at the top of a waterfall.
15. The energy of a ball rolling up an inclined plane
a.
is entirely kinetic
b.
is entirely potential
c.
is both kinetic and potential
d.
none of the above
16. For a body oscillating at the end of a spring, the energy is
a.
kinetic energy
b.
elastic potential energy
c.
gravitational potential energy
d.
all of the above
e.
none of the above
17. Angular momentum
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved
d.
none of the above
18. Linear momentum
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved
d.
none of the above
19. Kinetic energy
a.
is definitely conserved
b.
is definitely not conserved
c.
may or may not be conserved
d.
none of the above
20. The quantity Fd is
a.
the kinetic energy of the crate
b.
the potential energy of the crate
c.
the linear momentum of the crate
d.
the work you do on the crate
e.
the power you supply to the crate
21. The quantity 1/2 mv2 is
a.
the kinetic energy of the crate
b.
the potential energy of the crate
c.
the linear momentum of the crate
d.
the work you do on the crate
e.
the power you supply to the crate
22. The quantity Fd/t is
a.
the kinetic energy of the crate
b.
the potential energy of the crate
c.
the linear momentum of the crate
d.
the work you do on the crate
e.
the power you supply to the crate
23. The quantity mv is
a.
the kinetic energy of the crate
b.
the potential energy of the crate
c.
the linear momentum of the crate
d.
the work you do on the crate
e.
the power you supply to the crate
24. You carry a 20 N box 5 m horizontally across a room. The work you do is
a.
0
c.
100 J
b.
4 J
d.
none of the above
25. The work you do is
a.
0
c.
100 J
b.
4 J
d.
none of the above
26. The increase of potential energy of the box is
a.
0
c.
100 J
b.
4 J
d.
none of the above
27. You drop the box. As it falls to its original level, its kinetic energy is
a.
0
c.
100 J
b.
4 J
d.
none of the above
28. What is the impulse provided by the force?
a.
150 Ns
b.
50 Ns
c.
75 Ns
d.
undetermined—not enough information given
e.
none of the above
29. What is the work done by the force?
a.
150 J
b.
50 J
c.
75 J
d.
undetermined—not enough information given
e.
none of the above
30. What is the magnitude of the change in the object’s momentum?
a.
150 kg m/s
b.
50 kg m/s
c.
75 kg m/s
d.
undetermined—not enough information given
e.
none of the above
31. When we say work is done by a force, we mean the force does
a.
all the work
c.
negative work
b.
positive work
d.
zero work
32. When we say work is done against a force, we mean the force does
a.
none of the work
c.
negative work
b.
positive work
d.
zero work
33. Using a lever to move a rock reduces the force a person must exert. What does it do to the
work the person must do?
a.
it reduces the work also
c.
the work increases
b.
the work stays the same
d.
none of the above
34. Which of the following quantities are conserved in an isolated system?
a.
mass
c.
momentum
b.
energy
d.
all of the above
35. A 2 kg car moving towards the right at 4 m/s collides head on with an 8 kg car moving
towards the left at 2 m/s, and they stick together. After the collision, the velocity of the
combined bodies is
a.
0
b.
0.8 m/s towards the right
c.
0.8 m/s towards the left
d.
2.4 m/s towards the right
e.
2.4 m/s towards the left
36. A person runs up a long flight of stairs in 10 seconds. If the person’s weight is 600 N and
the vertical height of the stairs is 20 meters, the person’s power output is
a.
0
b.
3 W
c.
30 W
d.
1,200 W
e.
12,000 W
37. After an elastic collision of two balls, the kinetic energies of the two balls are measured to
be 12J and 24J. What can be said about the kinetic energy of the two balls before the
collision?
a.
nothing—not enough information is given
b.
the balls had the same energies, 12J and 24J respectively
c.
the total kinetic energy of the two balls was 36J
d.
none of the above
38. About how many 100W light bulbs would you have to light up to use energy at the rate of
one horsepower?
a.
one or two
b.
seven or eight
c.
the question doesn’t make sense—hp and W are unrelated
d.
dozens
e.
none of the above
39. About how many 100W light bulbs would you have to light up to use energy at the rate of
one horsepower?
a.
one or two
b.
seven or eight
c.
the question doesn’t make sense—hp and W are unrelated
d.
dozens
e.
none of the above
40. A watt is equal to
a.
746 horsepower
b.
one calorie
c.
one joule
d.
1 J/s