Cutnell/Johnson Physics 7th edition

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Cutnell/JohnsonPhysics 7th edition

• Classroom Response System Questions

Chapter 6 Work and Energy
Interactive Lecture Questions
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6.1.1. A block is in contact with a rough surface
as shown in the drawing. The block has a rope
attached to one side. Someone pulls the rope
with a force , which is represented by the vector
in the drawing. The force is directed at an
angle ? with respect to the horizontal direction.
The magnitude of is equal to two times the
magnitude of the frictional force, which is
designated f. For what value of ? is the net
work on the block equal to zero joules? a)
0? b) 30? c) 45? d) 60? e) Net work will
be done in the object for all values of ?.
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6.1.1. A block is in contact with a rough surface
as shown in the drawing. The block has a rope
attached to one side. Someone pulls the rope
with a force , which is represented by the vector
in the drawing. The force is directed at an
angle ? with respect to the horizontal direction.
The magnitude of is equal to two times the
magnitude of the frictional force, which is
designated f. For what value of ? is the net
work on the block equal to zero joules? a)
0? b) 30? c) 45? d) 60? e) Net work will
be done in the object for all values of ?.
4
6.1.2. Some children are practicing catching
baseballs. The coach tosses the ball into the
air and hits the ball with his bat. The ball
travels nearly horizontally, directly at the
short stop who manages to catch the line drive.
Did the coach, via the bat, do any work on the
ball as it was hit? a) No, it travels nearly
horizontally, and no work is done. b) Yes, work
was done on the ball because during the time the
force acted on the ball, the bat and ball moved
through some distance. c) No, there was a
force acting on the ball, but there was no
displacement while the force was acting. d)
Yes, work was done on the ball because the force
of gravity was acting on the ball while it was
being hit. e) No work was done on the ball
because the ball flew even though the force was
no longer acting on it.
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6.1.2. Some children are practicing catching
baseballs. The coach tosses the ball into the
air and hits the ball with his bat. The ball
travels nearly horizontally, directly at the
short stop who manages to catch the line drive.
Did the coach, via the bat, do any work on the
ball as it was hit? a) No, it travels nearly
horizontally, and no work is done. b) Yes, work
was done on the ball because during the time the
force acted on the ball, the bat and ball moved
through some distance. c) No, there was a
force acting on the ball, but there was no
displacement while the force was acting. d)
Yes, work was done on the ball because the force
of gravity was acting on the ball while it was
being hit. e) No work was done on the ball
because the ball flew even though the force was
no longer acting on it.
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6.1.3. Kevin is refinishing his rusty
wheelbarrow. He moves his sandpaper back and
forth 45 times over a rusty area, each time
moving a total distance of 0.15 m. Kevin pushes
the sandpaper against the surface with a normal
force of 1.8 N. The coefficient of friction for
the metal/sandpaper interface is 0.92. How much
work is done by the kinetic frictional force
during the sanding process? a) 12 J b) ? 12
J c) 24 J d) ? 24 J e) zero J
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6.1.3. Kevin is refinishing his rusty
wheelbarrow. He moves his sandpaper back and
forth 45 times over a rusty area, each time
moving a total distance of 0.15 m. Kevin pushes
the sandpaper against the surface with a normal
force of 1.8 N. The coefficient of friction for
the metal/sandpaper interface is 0.92. How much
work is done by the kinetic frictional force
during the sanding process? a) 12 J b) ? 12
J c) 24 J d) ? 24 J e) zero J
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6.2.1. Which one of the following choices
represents the largest kinetic energy? a) Mars
is moving in its orbit around the Sun. b) A
cyclist is racing in the annual Tour de France
bicycle race. c) A leaf falls from a tree. d)
A cheetah runs at its maximum speed to catch a
fleeing zebra. e) An oil tanker sails through
the Panama Canal.
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6.2.1. Which one of the following choices
represents the largest kinetic energy? a) Mars
is moving in its orbit around the Sun. b) A
cyclist is racing in the annual Tour de France
bicycle race. c) A leaf falls from a tree. d)
A cheetah runs at its maximum speed to catch a
fleeing zebra. e) An oil tanker sails through
the Panama Canal.
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6.2.2. A high school baseball pitcher can
typically throw a ball at 22 m/s. Professional
baseball pitchers can throw the ball with twice
that speed, but few others can. To see why this
is the case, determine the difference in the
kinetic energy of a baseball thrown at v m/s and
one thrown at 2v m/s and express the difference
as a percentage. a) 50 b) 100 c) 200
d) 300 e) 400
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6.2.2. A high school baseball pitcher can
typically throw a ball at 22 m/s. Professional
baseball pitchers can throw the ball with twice
that speed, but few others can. To see why this
is the case, determine the difference in the
kinetic energy of a baseball thrown at v m/s and
one thrown at 2v m/s and express the difference
as a percentage. a) 50 b) 100 c) 200
d) 300 e) 400
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6.3.1. An elevator supported by a single cable
descends a shaft at a constant speed. The only
forces acting on the elevator are the tension in
the cable and the gravitational force. Which one
of the following statements is true? a) The
work done by the tension force is zero
joules. b) The net work done by the two forces
is zero joules. c) The work done by the
gravitational force is zero joules. d) The
magnitude of the work done by the gravitational
force is larger than that done by the tension
force. e) The magnitude of the work done by the
tension force is larger than that done by the
gravitational force.
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6.3.1. An elevator supported by a single cable
descends a shaft at a constant speed. The only
forces acting on the elevator are the tension in
the cable and the gravitational force. Which one
of the following statements is true? a) The
work done by the tension force is zero
joules. b) The net work done by the two forces
is zero joules. c) The work done by the
gravitational force is zero joules. d) The
magnitude of the work done by the gravitational
force is larger than that done by the tension
force. e) The magnitude of the work done by the
tension force is larger than that done by the
gravitational force.
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6.3.2. Larrys gravitational potential energy is
1870 J as he sits 2.20 m above the ground in a
sky diving airplane. What is his gravitational
potential energy when be begins to jump from the
airplane at an altitude of 923 m? a) 3.29 ? 104
J b) 9.36 ? 102 J c) 4.22 ? 106 J d) 1.87 ?
103 J e) 7.85 ? 105 J
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6.3.2. Larrys gravitational potential energy is
1870 J as he sits 2.20 m above the ground in a
sky diving airplane. What is his gravitational
potential energy when be begins to jump from the
airplane at an altitude of 923 m? a) 3.29 ? 104
J b) 9.36 ? 102 J c) 4.22 ? 106 J d) 1.87 ?
103 J e) 7.85 ? 105 J
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6.3.3. A mountain climber pulls a supply pack up
the side of a mountain at constant speed. Which
one of the following statements concerning this
situation is false? a) The net work done by all
the forces acting on the pack is zero joules. b)
The work done on the pack by the normal force of
the mountain is zero joules. c) The work done
on the pack by gravity is zero joules. d) The
gravitational potential energy of the pack is
increasing. e) The climber does "positive" work
in pulling the pack up the mountain.
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6.3.3. A mountain climber pulls a supply pack up
the side of a mountain at constant speed. Which
one of the following statements concerning this
situation is false? a) The net work done by all
the forces acting on the pack is zero joules. b)
The work done on the pack by the normal force of
the mountain is zero joules. c) The work done
on the pack by gravity is zero joules. d) The
gravitational potential energy of the pack is
increasing. e) The climber does "positive" work
in pulling the pack up the mountain.
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6.5.1. Two balls of equal size are dropped from
the same height from the roof of a building. One
ball has twice the mass of the other. When the
balls reach the ground, how do the kinetic
energies of the two balls compare? a) The
lighter one has one fourth as much kinetic energy
as the other does. b) The lighter one has one
half as much kinetic energy as the other
does. c) The lighter one has the same kinetic
energy as the other does. d) The lighter one
has twice as much kinetic energy as the other
does. e) The lighter one has four times as much
kinetic energy as the other does.
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6.5.1. Two balls of equal size are dropped from
the same height from the roof of a building. One
ball has twice the mass of the other. When the
balls reach the ground, how do the kinetic
energies of the two balls compare? a) The
lighter one has one fourth as much kinetic energy
as the other does. b) The lighter one has one
half as much kinetic energy as the other
does. c) The lighter one has the same kinetic
energy as the other does. d) The lighter one
has twice as much kinetic energy as the other
does. e) The lighter one has four times as much
kinetic energy as the other does.
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6.5.2. After an ice storm, ice falls from one of
the top floors of a 65-story building. The ice
falls freely under the influence of gravity.
Which one of the following statements concerning
this situation is true? a) The kinetic energy
of the ice increases by equal amounts for equal
distances. b) The kinetic energy of the ice
increases by equal amounts for equal times. c)
The potential energy of the ices decreases by
equal amounts for equal times. d) The total
energy of the block increases by equal amounts
over equal distances. e) As the block falls,
the net work done by all of the forces acting on
the ice is zero joules.
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6.5.2. After an ice storm, ice falls from one of
the top floors of a 65-story building. The ice
falls freely under the influence of gravity.
Which one of the following statements concerning
this situation is true? a) The kinetic energy
of the ice increases by equal amounts for equal
distances. b) The kinetic energy of the ice
increases by equal amounts for equal times. c)
The potential energy of the ices decreases by
equal amounts for equal times. d) The total
energy of the block increases by equal amounts
over equal distances. e) As the block falls,
the net work done by all of the forces acting on
the ice is zero joules.
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6.5.3. Determine the amount of work done in
firing a 2.0-kg projectile with an initial speed
of 50 m/s. Neglect any effects due to air
resistance. a) 900 J b) 1600 J c) 2500
J d) 4900 J e) This cannot be determined
without knowing the launch angle.
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6.5.3. Determine the amount of work done in
firing a 2.0-kg projectile with an initial speed
of 50 m/s. Neglect any effects due to air
resistance. a) 900 J b) 1600 J c) 2500
J d) 4900 J e) This cannot be determined
without knowing the launch angle.
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6.5.4. A roller coaster car travels down a hill
and is moving at 18 m/s as it passes through a
section of straight, horizontal track. The car
then travels up another hill that has a maximum
height of 15 m. If frictional effects are
ignored, determine whether the car can reach the
top of the hill. If it does reach the top, what
is the speed of the car at the top? a) No, the
car doesnt make it up the hill. It is going too
slow. b) Yes, the car just barely makes it to
the top and stops. The final speed is zero
m/s. c) Yes, the car not only makes it to the
top, but it is moving at 5.4 m/s. d) Yes, the
car not only makes it to the top, but it is
moving at 9.0 m/s. e) Yes, the car not only
makes it to the top, but it is moving at 18 m/s.
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6.5.4. A roller coaster car travels down a hill
and is moving at 18 m/s as it passes through a
section of straight, horizontal track. The car
then travels up another hill that has a maximum
height of 15 m. If frictional effects are
ignored, determine whether the car can reach the
top of the hill. If it does reach the top, what
is the speed of the car at the top? a) No, the
car doesnt make it up the hill. It is going too
slow. b) Yes, the car just barely makes it to
the top and stops. The final speed is zero
m/s. c) Yes, the car not only makes it to the
top, but it is moving at 5.4 m/s. d) Yes, the
car not only makes it to the top, but it is
moving at 9.0 m/s. e) Yes, the car not only
makes it to the top, but it is moving at 18 m/s.
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6.5.5. You are investigating the safety of a
playground slide. You are interested in finding
out what the maximum speed will be of children
sliding on it when the conditions make it very
slippery (assume frictionless). The height of
the slide is 2.5 m. What is that maximum speed
of a child if she starts from rest at the
top? a) 1.9 m/s b) 2.5 m/s c) 4.9 m/s d)
7.0 m/s e) 9.8 m/s
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6.5.5. You are investigating the safety of a
playground slide. You are interested in finding
out what the maximum speed will be of children
sliding on it when the conditions make it very
slippery (assume frictionless). The height of
the slide is 2.5 m. What is that maximum speed
of a child if she starts from rest at the
top? a) 1.9 m/s b) 2.5 m/s c) 4.9 m/s d)
7.0 m/s e) 9.8 m/s
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6.5.6. A quarter is dropped from rest from the
fifth floor of a very tall building. The speed
of the quarter is v just before striking the
ground. From what floor would the quarter have
to be dropped from rest for the speed just before
striking the ground to be approximately 2v?
Ignore all air resistance effects to determine
your answer. a) 14 b) 25 c) 20 d) 7 e)
10
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6.5.6. A quarter is dropped from rest from the
fifth floor of a very tall building. The speed
of the quarter is v just before striking the
ground. From what floor would the quarter have
to be dropped from rest for the speed just before
striking the ground to be approximately 2v?
Ignore all air resistance effects to determine
your answer. a) 14 b) 25 c) 20 d) 7 e)
10
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6.5.7. Two identical balls are thrown from the
same height from the roof of a building. One
ball is thrown upward with an initial speed v.
The second ball is thrown downward with the same
initial speed v. When the balls reach the
ground, how do the kinetic energies of the two
balls compare? Ignore any air resistance
effects. a) The kinetic energies of the two
balls will be the same. b) The first ball will
have twice the kinetic energy as the second
ball. c) The first ball will have one half the
kinetic energy as the second ball. d) The first
ball will have four times the kinetic energy as
the second ball. e) The first ball will have
three times the kinetic energy as the second ball.
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6.5.7. Two identical balls are thrown from the
same height from the roof of a building. One
ball is thrown upward with an initial speed v.
The second ball is thrown downward with the same
initial speed v. When the balls reach the
ground, how do the kinetic energies of the two
balls compare? Ignore any air resistance
effects. a) The kinetic energies of the two
balls will be the same. b) The first ball will
have twice the kinetic energy as the second
ball. c) The first ball will have one half the
kinetic energy as the second ball. d) The first
ball will have four times the kinetic energy as
the second ball. e) The first ball will have
three times the kinetic energy as the second ball.
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6.6.1. A car is being driven along a country road
on a dark and rainy night at a speed of 20 m/s.
The section of road is horizontal and straight.
The driver sees that a tree has fallen and
covered the road ahead. Panicking, the driver
locks the brakes at a distance of 20 m from the
tree. If the coefficient of friction between the
wheels and road is 0.8, determine the
outcome. a) The car stops 5.5 m before the
tree. b) The car stops just before reaching the
tree. c) As the car crashes into the tree, its
speed is 18 m/s. d) As the car crashes into the
tree, its speed is 9.3 m/s. e) This problem
cannot be solved without knowing the mass of the
car.
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6.6.1. A car is being driven along a country road
on a dark and rainy night at a speed of 20 m/s.
The section of road is horizontal and straight.
The driver sees that a tree has fallen and
covered the road ahead. Panicking, the driver
locks the brakes at a distance of 20 m from the
tree. If the coefficient of friction between the
wheels and road is 0.8, determine the
outcome. a) The car stops 5.5 m before the
tree. b) The car stops just before reaching the
tree. c) As the car crashes into the tree, its
speed is 18 m/s. d) As the car crashes into the
tree, its speed is 9.3 m/s. e) This problem
cannot be solved without knowing the mass of the
car.
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6.6.2. A rubber ball is dropped from rest from a
height h. The ball bounces off the floor and
reaches a height of 2h/3. How can we use the
principle of the conservation of mechanical
energy to interpret this observation? a) During
the collision with the floor, the floor did not
push hard enough on the ball for it to
reach its original height. b) Some of the
balls potential energy was lost in accelerating
it toward the floor. c) The force of the
earths gravity on the ball prevented it from
returning to its original height. d) Work was
done on the ball by the gravitational force that
reduced the balls kinetic energy. e) Work was
done on the ball by non-conservative forces that
resulted in the ball having less total mechanical
energy after the bounce.
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6.6.2. A rubber ball is dropped from rest from a
height h. The ball bounces off the floor and
reaches a height of 2h/3. How can we use the
principle of the conservation of mechanical
energy to interpret this observation? a) During
the collision with the floor, the floor did not
push hard enough on the ball for it to
reach its original height. b) Some of the
balls potential energy was lost in accelerating
it toward the floor. c) The force of the
earths gravity on the ball prevented it from
returning to its original height. d) Work was
done on the ball by the gravitational force that
reduced the balls kinetic energy. e) Work was
done on the ball by non-conservative forces that
resulted in the ball having less total mechanical
energy after the bounce.
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6.6.3. The Jensens decided to spend their family
vacation white water rafting. During one segment
of their trip down a horizontal section of the
river, the raft (total mass 544 kg) has an
initial speed of 6.75 m/s. The raft then drops a
vertical distance of 14.0 m, ending with a final
speed of 15.2 m/s. How much work was done on the
raft by non-conservative forces? a) ?12 100
J b) ?18 200 J c) ?24 200 J d) ?36 300
J e) ?48 400 J
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6.6.3. The Jensens decided to spend their family
vacation white water rafting. During one segment
of their trip down a horizontal section of the
river, the raft (total mass 544 kg) has an
initial speed of 6.75 m/s. The raft then drops a
vertical distance of 14.0 m, ending with a final
speed of 15.2 m/s. How much work was done on the
raft by non-conservative forces? a) ?12 100
J b) ?18 200 J c) ?24 200 J d) ?36 300
J e) ?48 400 J
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6.7.1. A dam blocks the passage of a river and
generates electricity. Approximately, 57 000 kg
of water fall each second through a height of 19
m. If one half of the gravitational potential
energy of the water were converted to electrical
energy, how much power would be generated? a)
2.7 106 W b) 5.3 106 W c) 1.1 107 W d)
1.3 108 W e) 2.7 108 W
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6.7.1. A dam blocks the passage of a river and
generates electricity. Approximately, 57 000 kg
of water fall each second through a height of 19
m. If one half of the gravitational potential
energy of the water were converted to electrical
energy, how much power would be generated? a)
2.7 106 W b) 5.3 106 W c) 1.1 107 W d)
1.3 108 W e) 2.7 108 W
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6.7.2. If the amount of energy needed to operate
a 100 W light bulb for one minute were used to
launch a 2-kg projectile, what maximum height
could the projectile reach, ignoring any
resistive effects? a) 20 m b) 50 m c) 100
m d) 200 m e) 300 m
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6.7.2. If the amount of energy needed to operate
a 100 W light bulb for one minute were used to
launch a 2-kg projectile, what maximum height
could the projectile reach, ignoring any
resistive effects? a) 20 m b) 50 m c) 100
m d) 200 m e) 300 m
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6.7.3. An SUV is accelerated from rest to a speed
v in a time interval t. Neglecting air
resistance effects and assuming the engine is
operating at its maximum power rating when
accelerating, determine the time interval for the
SUV to accelerate from rest to a speed 2v. a)
2t b) 4t c) 2.5t d) 3t e) 3.5t
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6.7.3. An SUV is accelerated from rest to a speed
v in a time interval t. Neglecting air
resistance effects and assuming the engine is
operating at its maximum power rating when
accelerating, determine the time interval for the
SUV to accelerate from rest to a speed 2v. a)
2t b) 4t c) 2.5t d) 3t e) 3.5t
44
6.8.1. A 12 500-kg truck is accelerated from rest
by a net force that decreases linearly with
distance traveled. The graph shows this force.
Using the information provided and work-energy
methods, determine the approximate speed of the
truck when the force is removed. a) 8.41
m/s b) 12.5 m/s c) 17.7 m/s d) 25.0 m/s e)
35.4 m/s
45
6.8.1. A 12 500-kg truck is accelerated from rest
by a net force that decreases linearly with
distance traveled. The graph shows this force.
Using the information provided and work-energy
methods, determine the approximate speed of the
truck when the force is removed. a) 8.41
m/s b) 12.5 m/s c) 17.7 m/s d) 25.0 m/s e)
35.4 m/s