Halliday/Resnick/Walker
Fundamentals of Physics
Classroom Response System Questions
Chapter 6 Forces and Motion II
Interactive Lecture Questions
6.3.1. Jennifer is pushing a heavy box up a rough inclined surface at a
constant speed by applying a horizontal force F as shown in the
drawing. The coefficient of kinetic friction for the box on the
inclined surface is k. Which one of the following expressions
correctly determines the normal force on the box?
a) FN 
F  mg tan 
k
b) FN  F  k mg tan 
c) FN 
F cos   mg sin 
k
d) FN  F cos   k mg sin 
e) FN 
F sin   mg cos 
k
6.3.1. Jennifer is pushing a heavy box up a rough inclined surface at a
constant speed by applying a horizontal force F as shown in the
drawing. The coefficient of kinetic friction for the box on the
inclined surface is k. Which one of the following expressions
correctly determines the normal force on the box?
a) FN 
F  mg tan 
k
b) FN  F  k mg tan 
c) FN 
F cos   mg sin 
k
d) FN  F cos   k mg sin 
e) FN 
F sin   mg cos 
k
6.3.2. Three pine blocks, each with identical mass, are sitting on a rough surface as
shown. If the same horizontal force is applied to each block, which one of the
following statements is false?
a) The coefficient of kinetic friction is the same for all three blocks.
b) The magnitude of the force of kinetic friction is greater for block 3.
c) The normal force exerted by the surface is the same for all three blocks.
d) Block 3 has the greatest apparent area in contact with the surface.
e) If the horizontal force is the minimum to start block 1 moving, then that same
force could be used to start block 2 or block 3 moving.
6.3.2. Three pine blocks, each with identical mass, are sitting on a rough surface as
shown. If the same horizontal force is applied to each block, which one of the
following statements is false?
a) The coefficient of kinetic friction is the same for all three blocks.
b) The magnitude of the force of kinetic friction is greater for block 3.
c) The normal force exerted by the surface is the same for all three blocks.
d) Block 3 has the greatest apparent area in contact with the surface.
e) If the horizontal force is the minimum to start block 1 moving, then that same
force could be used to start block 2 or block 3 moving.
6.3.3. A crate of mass m is at rest on a horizontal frictionless surface.
Another identical crate is placed on top of it. Assuming that there
is no slipping of the top crate as a horizontal force F is applied to
the bottom crate, determine an expression for the magnitude of the
static frictional force acting on the top crate.
a)
f F
b)
f 
c)
f F
d)
f 
mg
2
e)
f 
F
2
F
 mg
2
mg
2
6.3.3. A crate of mass m is at rest on a horizontal frictionless surface.
Another identical crate is placed on top of it. Assuming that there
is no slipping of the top crate as a horizontal force F is applied to
the bottom crate, determine an expression for the magnitude of the
static frictional force acting on the top crate.
a)
f F
b)
f 
c)
f F
d)
f 
mg
2
e)
f 
F
2
F
 mg
2
mg
2
6.3.4. A crate of mass m is at rest on a horizontal frictionless surface.
Another identical crate is placed on top of it. Assuming a
horizontal force F is applied to the bottom crate, determine an
expression for the minimum static coefficient of friction so that the
top crate does not slip.
F
2mg
F

mg
2mg

F
F  mg

F  mg
F  mg

2( F  mg )
a)  
b) S
c) S
d) S
e) S
6.3.4. A crate of mass m is at rest on a horizontal frictionless surface.
Another identical crate is placed on top of it. Assuming a
horizontal force F is applied to the bottom crate, determine an
expression for the minimum static coefficient of friction so that the
top crate does not slip.
F
2mg
F

mg
2mg

F
F  mg

F  mg
F  mg

2( F  mg )
a)  
b) S
c) S
d) S
e) S
6.3.5. On a rainy evening, a truck is driving along a straight, level road
at 25 m/s. The driver panics when a deer runs onto the road and
locks the wheels while braking. If the coefficient of friction for
the wheel/road interface is 0.68, how far does the truck slide
before it stops?
a) 55 m
b) 47 m
c) 41 m
d) 36 m
e) 32 m
6.3.5. On a rainy evening, a truck is driving along a straight, level road
at 25 m/s. The driver panics when a deer runs onto the road and
locks the wheels while braking. If the coefficient of friction for
the wheel/road interface is 0.68, how far does the truck slide
before it stops?
a) 55 m
b) 47 m
c) 41 m
d) 36 m
e) 32 m
6.3.6. Jake bought a new dog and is carrying a new dog house on the flatbed of his
brand new pickup truck. Jake isn’t sure if he should tie the house down, but he
doesn’t want it to scratch the paint if it should slide during braking. During the
trip home, Jake will travel along straight, level roads and have to stop from a
maximum speed of 21 m/s in a distance of 29 m. What is the minimum
coefficient of static friction between the dog house and the flatbed that is
required to prevent it from sliding? Compare your answer to the actual
coefficient of static friction of 0.35 to determine if the dog house should be tied
down.
a) 0.22, no need to tie the house down
b) 0.30, no need to tie the house down
c) 0.35, he may want to tie it down…just in case
d) 0.56, the house needs to be tied down
e) 0.78, the house needs to be tied down
6.3.6. Jake bought a new dog and is carrying a new dog house on the flatbed of his
brand new pickup truck. Jake isn’t sure if he should tie the house down, but he
doesn’t want it to scratch the paint if it should slide during braking. During the
trip home, Jake will travel along straight, level roads and have to stop from a
maximum speed of 21 m/s in a distance of 29 m. What is the minimum
coefficient of static friction between the dog house and the flatbed that is
required to prevent it from sliding? Compare your answer to the actual
coefficient of static friction of 0.35 to determine if the dog house should be tied
down.
a) 0.22, no need to tie the house down
b) 0.30, no need to tie the house down
c) 0.35, he may want to tie it down…just in case
d) 0.56, the house needs to be tied down
e) 0.78, the house needs to be tied down
6.3.7. A block of mass m is pressed against a wall with an initial force and the block
is at rest. The coefficient of static friction for the block against the wall is equal
to 0.5. The coefficient of kinetic friction is less than the coefficient of static
friction. If the force is equal to the weight of the block, which one of the
following statements is true?
a) The block will continue to remain at rest because the force of static friction is two
times the weight of the block.
b) The block will slide down the wall because the force of static friction is only
equal to one-half of the block’s weight.
c) The block will accelerate at 9.8 m/s2 down the wall.
d) The block will slide down the wall at constant speed.
e) The block will accelerate at less than 4.9 m/s2 down the wall.
6.3.7. A block of mass m is pressed against a wall with an initial force and the block
is at rest. The coefficient of static friction for the block against the wall is equal
to 0.5. The coefficient of kinetic friction is less than the coefficient of static
friction. If the force is equal to the weight of the block, which one of the
following statements is true?
a) The block will continue to remain at rest because the force of static friction is two
times the weight of the block.
b) The block will slide down the wall because the force of static friction is only
equal to one-half of the block’s weight.
c) The block will accelerate at 9.8 m/s2 down the wall.
d) The block will slide down the wall at constant speed.
e) The block will accelerate at less than 4.9 m/s2 down the wall.
6.3.8. A 1.0 kg block is placed against a wall and is held stationary by
a force of 8.0 N applied at a 45° angle as shown in the drawing.
What is the magnitude of the friction force?
a) 3.7 N
b) 4.1 N
c) 5.8 N
d) 7.0 N
e) 8.0 N
6.3.8. A 1.0 kg block is placed against a wall and is held stationary by
a force of 8.0 N applied at a 45° angle as shown in the drawing.
What is the magnitude of the friction force?
a) 3.7 N
b) 4.1 N
c) 5.8 N
d) 7.0 N
e) 8.0 N
6.3.9. A rancher puts a hay bail into the back of her SUV. Later, she drives
around an unbanked curve with a radius of 48 m at a speed of 16 m/s.
What is the minimum coefficient of static friction for the hay bail on the
floor of the SUV so that the hay bail does not slide while on the curve?
a) This cannot be determined without knowing the mass of the hay bail.
b) 0.17
c) 0.33
d) 0.42
e) 0.54
6.3.9. A rancher puts a hay bail into the back of her SUV. Later, she drives
around an unbanked curve with a radius of 48 m at a speed of 16 m/s.
What is the minimum coefficient of static friction for the hay bail on the
floor of the SUV so that the hay bail does not slide while on the curve?
a) This cannot be determined without knowing the mass of the hay bail.
b) 0.17
c) 0.33
d) 0.42
e) 0.54
6.4.1. Consider the following situations:
(i) A minivan is following a hairpin turn on a mountain road at a constant speed of twenty
miles per hour.
(ii) A parachutist is descending at a constant speed 10 m/s.
(iii) A heavy crate has been given a quick shove and is now sliding across the floor.
(iv) Jenny is swinging back and forth on a swing at the park.
(v) A football that was kicked is flying through the goal posts.
(vi) A plucked guitar string vibrates at a constant frequency.
In which one of these situations does the object or person experience zero acceleration?
a) i only
b) ii only
c) iii and iv only
d) iv, v, and vi only
e) all of the situations
6.4.1. Consider the following situations:
(i) A minivan is following a hairpin turn on a mountain road at a constant speed of twenty
miles per hour.
(ii) A parachutist is descending at a constant speed 10 m/s.
(iii) A heavy crate has been given a quick shove and is now sliding across the floor.
(iv) Jenny is swinging back and forth on a swing at the park.
(v) A football that was kicked is flying through the goal posts.
(vi) A plucked guitar string vibrates at a constant frequency.
In which one of these situations does the object or person experience zero acceleration?
a) i only
b) ii only
c) iii and iv only
d) iv, v, and vi only
e) all of the situations
6.4.2. A sky diver jumps from a
flying airplane and falls for
several seconds before she
reaches terminal velocity.
She then opens her
parachute, reaches a new
terminal velocity, and
continues her descent to the
ground. Which one of the
following graphs of the drag
force versus time best
represents this situation?
6.4.2. A sky diver jumps from a
flying airplane and falls for
several seconds before she
reaches terminal velocity.
She then opens her
parachute, reaches a new
terminal velocity, and
continues her descent to the
ground. Which one of the
following graphs of the drag
force versus time best
represents this situation?
6.4.3. A feather and a minivan are dropped vertically downward from a height of
twenty meters and both are subject to air drag as they fall. The minivan reaches
the ground much faster than the feather. Which one of the following statements
concerning this situation is true, if any?
a) The minivan has a larger terminal velocity than the feather because it experiences
less air resistance than the feather.
b) The minivan encounters a smaller force of air resistance than the feather and falls
faster.
c) Each object experiences the same amount of air drag, but the minivan experiences
the greatest force of gravity.
d) The feather experiences more air drag than the minivan and has a smaller
terminal velocity.
e) None of the above statements are true.
6.4.3. A feather and a minivan are dropped vertically downward from a height of
twenty meters and both are subject to air drag as they fall. The minivan reaches
the ground much faster than the feather. Which one of the following statements
concerning this situation is true, if any?
a) The minivan has a larger terminal velocity than the feather because it experiences
less air resistance than the feather.
b) The minivan encounters a smaller force of air resistance than the feather and falls
faster.
c) Each object experiences the same amount of air drag, but the minivan experiences
the greatest force of gravity.
d) The feather experiences more air drag than the minivan and has a smaller
terminal velocity.
e) None of the above statements are true.
6.4.4. A light ping-pong ball and a heavy rubber ball of exactly the same size are
each launched at the same angle and initial velocity, but the rubber ball goes
much farther than the ping-pong ball. Which one of the following statements
best explains this result?
a) The ping-pong ball weighs less, so the acceleration due to gravity is smaller for it.
b) The drag force on the ping-pong ball is larger.
c) The ping-pong ball has less momentum.
d) The ping-pong ball has less mass, so the same drag force slows the ping-pong ball
down more.
e) The ping-pong ball has a smaller moment of inertia since it is hollow and weighs
less.
6.4.4. A light ping-pong ball and a heavy rubber ball of exactly the same size are
each launched at the same angle and initial velocity, but the rubber ball goes
much farther than the ping-pong ball. Which one of the following statements
best explains this result?
a) The ping-pong ball weighs less, so the acceleration due to gravity is smaller for it.
b) The drag force on the ping-pong ball is larger.
c) The ping-pong ball has less momentum.
d) The ping-pong ball has less mass, so the same drag force slows the ping-pong ball
down more.
e) The ping-pong ball has a smaller moment of inertia since it is hollow and weighs
less.
6.5.1. A ball is whirled on the end of a string in a horizontal circle of
radius R at constant speed v. By which one of the following means
can the centripetal acceleration of the ball be increased by a factor
of two?
a) Keep the radius fixed and increase the period by a factor of two.
b) Keep the radius fixed and decrease the period by a factor of two.
c) Keep the speed fixed and increase the radius by a factor of two.
d) Keep the speed fixed and decrease the radius by a factor of two.
e) Keep the radius fixed and increase the speed by a factor of two.
6.5.1. A ball is whirled on the end of a string in a horizontal circle of
radius R at constant speed v. By which one of the following means
can the centripetal acceleration of the ball be increased by a factor
of two?
a) Keep the radius fixed and increase the period by a factor of two.
b) Keep the radius fixed and decrease the period by a factor of two.
c) Keep the speed fixed and increase the radius by a factor of two.
d) Keep the speed fixed and decrease the radius by a factor of two.
e) Keep the radius fixed and increase the speed by a factor of two.
6.5.2. A steel ball is whirled on the end of a chain in a horizontal circle
of radius R with a constant period T. If the radius of the circle is
then reduced to 0.75R, while the period remains T, what happens
to the centripetal acceleration of the ball?
a) The centripetal acceleration increases to 1.33 times its initial value.
b) The centripetal acceleration increases to 1.78 times its initial value.
c) The centripetal acceleration decreases to 0.75 times its initial value.
d) The centripetal acceleration decreases to 0.56 times its initial value.
e) The centripetal acceleration does not change.
6.5.2. A steel ball is whirled on the end of a chain in a horizontal circle
of radius R with a constant period T. If the radius of the circle is
then reduced to 0.75R, while the period remains T, what happens
to the centripetal acceleration of the ball?
a) The centripetal acceleration increases to 1.33 times its initial value.
b) The centripetal acceleration increases to 1.78 times its initial value.
c) The centripetal acceleration decreases to 0.75 times its initial value.
d) The centripetal acceleration decreases to 0.56 times its initial value.
e) The centripetal acceleration does not change.
6.5.3. A boy is whirling a stone at the end of a string around his head. The
string makes one complete revolution every second, and the tension in
the string is FT. The boy increases the speed of the stone, keeping the
radius of the circle unchanged, so that the string makes two complete
revolutions per second. What happens to the tension in the sting?
a) The tension increases to four times its original value.
b) The tension increases to twice its original value.
c) The tension is unchanged.
d) The tension is reduced to one half of its original value.
e) The tension is reduced to one fourth of its original value.
6.5.3. A boy is whirling a stone at the end of a string around his head. The
string makes one complete revolution every second, and the tension in
the string is FT. The boy increases the speed of the stone, keeping the
radius of the circle unchanged, so that the string makes two complete
revolutions per second. What happens to the tension in the sting?
a) The tension increases to four times its original value.
b) The tension increases to twice its original value.
c) The tension is unchanged.
d) The tension is reduced to one half of its original value.
e) The tension is reduced to one fourth of its original value.
6.5.4. An aluminum rod is designed to break when it is under a tension of
600 N. One end of the rod is connected to a motor and a 12-kg spherical
object is attached to the other end. When the motor is turned on, the
object moves in a horizontal circle with a radius of 6.0 m. If the speed of
the motor is continuously increased, at what speed will the rod break?
Ignore the mass of the rod for this calculation.
a) 11 m/s
b) 17 m/s
c) 34 m/s
d) 88 m/s
e) 3.0 × 102 m/s
6.5.4. An aluminum rod is designed to break when it is under a tension of
600 N. One end of the rod is connected to a motor and a 12-kg spherical
object is attached to the other end. When the motor is turned on, the
object moves in a horizontal circle with a radius of 6.0 m. If the speed of
the motor is continuously increased, at what speed will the rod break?
Ignore the mass of the rod for this calculation.
a) 11 m/s
b) 17 m/s
c) 34 m/s
d) 88 m/s
e) 3.0 × 102 m/s
6.5.5. A ball is attached to a string and whirled in a horizontal circle. The ball is
moving in uniform circular motion when the string separates from the ball (the
knot wasn’t very tight). Which one of the following statements best describes
the subsequent motion of the ball?
a) The ball immediately flies in the direction radially outward from the center of the
circular path the ball had been following.
b) The ball continues to follow the circular path for a short time, but then it
gradually falls away.
c) The ball gradually curves away from the circular path it had been following.
d) The ball immediately follows a linear path away from, but not tangent to the
circular path it had been following.
e) The ball immediately follows a line that is tangent to the circular path the ball had
been following
6.5.5. A ball is attached to a string and whirled in a horizontal circle. The ball is
moving in uniform circular motion when the string separates from the ball (the
knot wasn’t very tight). Which one of the following statements best describes
the subsequent motion of the ball?
a) The ball immediately flies in the direction radially outward from the center of the
circular path the ball had been following.
b) The ball continues to follow the circular path for a short time, but then it
gradually falls away.
c) The ball gradually curves away from the circular path it had been following.
d) The ball immediately follows a linear path away from, but not tangent to the
circular path it had been following.
e) The ball immediately follows a line that is tangent to the circular path the ball had
been following
6.5.6. Complete the following statement: The maximum speed at
which a car can safely negotiate an unbanked curve depends on all
of the following factors except
a) the coefficient of kinetic friction between the road and the tires.
b) the coefficient of static friction between the road and the tires.
c) the acceleration due to gravity.
d) the diameter of the curve.
e) the ratio of the static frictional force between the road and the tires
and the normal force exerted on the car.
6.5.6. Complete the following statement: The maximum speed at
which a car can safely negotiate an unbanked curve depends on all
of the following factors except
a) the coefficient of kinetic friction between the road and the tires.
b) the coefficient of static friction between the road and the tires.
c) the acceleration due to gravity.
d) the diameter of the curve.
e) the ratio of the static frictional force between the road and the tires
and the normal force exerted on the car.
6.5.7. A 1000-kg car travels along a straight portion of highway at a constant
velocity of 10 m/s, due east. The car then encounters an unbanked curve
of radius 50 m. The car follows the curve traveling at a constant speed
of 10 m/s while the direction of the car changes from east to south.
What is the magnitude of the frictional force between the tires and the
road as the car negotiates the unbanked curve?
a) 500 N
b) 1000 N
c) 2000 N
d) 5000 N
e) 10 000 N
6.5.7. A 1000-kg car travels along a straight portion of highway at a constant
velocity of 10 m/s, due east. The car then encounters an unbanked curve
of radius 50 m. The car follows the curve traveling at a constant speed
of 10 m/s while the direction of the car changes from east to south.
What is the magnitude of the frictional force between the tires and the
road as the car negotiates the unbanked curve?
a) 500 N
b) 1000 N
c) 2000 N
d) 5000 N
e) 10 000 N
6.5.8. A space station is designed in the shape of a large, hollow donut
that is uniformly rotating. The outer radius of the station is 460 m.
With what period must the station rotate so that a person sitting on
the outer wall experiences “artificial gravity,” i.e. an acceleration
of 9.8 m/s2?
a) 43 s
b) 76 s
c) 88 s
d) 110 s
e) 230 s
6.5.8. A space station is designed in the shape of a large, hollow donut
that is uniformly rotating. The outer radius of the station is 460 m.
With what period must the station rotate so that a person sitting on
the outer wall experiences “artificial gravity,” i.e. an acceleration
of 9.8 m/s2?
a) 43 s
b) 76 s
c) 88 s
d) 110 s
e) 230 s
6.5.9. At a circus, a clown on a motorcycle with a mass M travels
along a horizontal track and enters a vertical circle of radius r.
Which one of the following expressions determines the minimum
speed that the motorcycle must have at the top of the track to
remain in contact with the track?
a) v  2 gr
b) v  gr
c) v = gR
d) v = 2gR
e) v = MgR
6.5.9. At a circus, a clown on a motorcycle with a mass M travels
along a horizontal track and enters a vertical circle of radius r.
Which one of the following expressions determines the minimum
speed that the motorcycle must have at the top of the track to
remain in contact with the track?
a) v  2 gr
b) v  gr
c) v = gR
d) v = 2gR
e) v = MgR
6.5.10. A ball on the end of a rope is moving in a vertical circle near
the surface of the earth. Point A is at the top of the circle; C is at
the bottom. Points B and D are exactly halfway between A and C.
Which one of the following statements concerning the tension in
the rope is true?
a) The tension is smallest at point A.
b) The tension is smallest at point C.
c) The tension is smallest at both points B and D.
d) The tension is the same at points A and C.
e) The tension is the same at all four points.
6.5.10. A ball on the end of a rope is moving in a vertical circle near
the surface of the earth. Point A is at the top of the circle; C is at
the bottom. Points B and D are exactly halfway between A and C.
Which one of the following statements concerning the tension in
the rope is true?
a) The tension is smallest at point A.
b) The tension is smallest at point C.
c) The tension is smallest at both points B and D.
d) The tension is the same at points A and C.
e) The tension is the same at all four points.
6.5.11. Imagine you are swinging a bucket by the handle around in a
circle that is nearly level with the ground (a horizontal circle).
What is the force, the physical force, holding the bucket in a
circular path?
a) the centripetal force
b) the centrifugal force
c) your hand on the handle
d) gravitational force
e) None of the above are correct.
6.5.11. Imagine you are swinging a bucket by the handle around in a
circle that is nearly level with the ground (a horizontal circle).
What is the force, the physical force, holding the bucket in a
circular path?
a) the centripetal force
b) the centrifugal force
c) your hand on the handle
d) gravitational force
e) None of the above are correct.
6.5.12. Imagine you are swinging a bucket by the handle around in a
circle that is nearly level with the ground (a horizontal circle).
Now imagine there's a ball in the bucket. What keeps the ball
moving in a circular path?
a) contact force of the bucket on the ball
b) contact force of the ball on the bucket
c) gravitational force on the ball
d) the centripetal force
e) the centrifugal force
6.5.12. Imagine you are swinging a bucket by the handle around in a
circle that is nearly level with the ground (a horizontal circle).
Now imagine there's a ball in the bucket. What keeps the ball
moving in a circular path?
a) contact force of the bucket on the ball
b) contact force of the ball on the bucket
c) gravitational force on the ball
d) the centripetal force
e) the centrifugal force
6.5.13. Which of the following parameters determine how fast you
need to swing a water bucket vertically so that water in the bucket
will not fall out?
a) radius of swing
b) mass of bucket
c) mass of water
d) a and b
e) a and c
6.5.13. Which of the following parameters determine how fast you
need to swing a water bucket vertically so that water in the bucket
will not fall out?
a) radius of swing
b) mass of bucket
c) mass of water
d) a and b
e) a and c
6.5.14. The moon, which is approximately 4 × 109 m from Earth, has a
mass of 7.4 × 1022 kg and a period of 27.3 days. What must is the
magnitude of the gravitational force between the Earth and the
moon?
a) 1.8 × 1018 N
b) 2.1 × 1022 N
c) 1.7 × 1013 N
d) 5.0 × 1022 N
e) 4.2 × 1020 N
6.5.14. The moon, which is approximately 4 × 109 m from Earth, has a
mass of 7.4 × 1022 kg and a period of 27.3 days. What must is the
magnitude of the gravitational force between the Earth and the
moon?
a) 1.8 × 1018 N
b) 2.1 × 1022 N
c) 1.7 × 1013 N
d) 5.0 × 1022 N
e) 4.2 × 1020 N
6.5.15. The Rapid Rotor is spinning fast enough that
the floor beneath the rider drops away and the
rider remains in place. If the Rotor speeds up until
it is going twice as fast as it was previously, what
is the effect on the frictional force on the rider?
a) The frictional force is reduced to one-fourth of its
previous value.
b) The frictional force is the same as its previous value.
c) The frictional force is reduced to one-half of its
previous value.
d) The frictional force is increased to twice its previous
value.
e) The frictional force is increased to four times its
previous value.
6.5.15. The Rapid Rotor is spinning fast enough that
the floor beneath the rider drops away and the
rider remains in place. If the Rotor speeds up until
it is going twice as fast as it was previously, what
is the effect on the frictional force on the rider?
a) The frictional force is reduced to one-fourth of its
previous value.
b) The frictional force is the same as its previous value.
c) The frictional force is reduced to one-half of its
previous value.
d) The frictional force is increased to twice its previous
value.
e) The frictional force is increased to four times its
previous value.