College Physics: A Strategic Approach, 3e (Knight)
Chapter 4 Forces and Newton’s Laws of Motion
4.1 Conceptual Questions
1) In a collision between a huge SUV and a small hybrid car, the SUV exerts a larger force on
the hybrid than the hybrid exerts on the SUV.
A) True
B) False
C) It depends on whether the collision is a head-on collision or a rear-end collision.
2) A box is placed on a table which rests on the floor. The box pushes on the table; the reaction
force to the box’s push on the table is the table’s push on the floor.
A) True
B) False
3) In order to get an object moving, you must push harder on it than it pushes back on you.
A) True
B) False
4) A girl attaches a rock to a string, which she then swings counter-clockwise in a horizontal
circle. The string breaks at point P in the figure, which shows a bird’s-eye view (as seen from
above). Which path (A-E) will the rock follow?
A) Path A
B) Path B
C) Path C
D) Path D
E) Path E
5) If you pound a feather with a hammer, which one feels a greater force?
A) always the feather
B) always the hammer
C) The size of the force is always exactly the same on both of them.
D) If the feather moves, then it felt the greater force. Otherwise the force was the same on both.
6) A satellite is in orbit around the earth. Which one feels the greater force?
A) the satellite because the earth is so much more massive
B) the earth because the satellite has so little mass
C) Earth and the satellite feel exactly the same force.
D) It depends on the distance of the satellite from Earth.
7) You pull on a crate with a rope. If the crate moves, the rope’s pull on the crate must have been
larger than the crate’s pull on the rope, but if the crate does not move, both of these pulls must
have been equal.
A) True
B) False
8) In order to lift a bucket of concrete, you must pull up harder on the bucket than the bucket
pulls down on you.
A) True
B) False
9) If the rockets of a spaceship in outer space (far from all gravity) suddenly lose power and go
off, the spaceship will gradually slow to a stop.
A) True
B) False
10) You are in a train traveling on a horizontal track and notice that a piece of luggage starts to
slide directly toward the front of the train. From this observation, you can conclude that this train
is
A) speeding up.
B) slowing down.
C) changing direction.
D) speeding up and changing direction.
E) slowing down and changing direction.
11) While flying horizontally in an airplane, you notice that a string dangling from the overhead
luggage compartment hangs at rest at 15° away from the vertical toward the front of the plane.
Using this observation, you can conclude that the airplane is
A) moving forward.
B) moving backward.
C) accelerating forward.
D) accelerating backward.
E) not accelerating because the string is at rest.
12) An elevator suspended by a vertical cable is moving downward but slowing down. The
tension in the cable must be
A) greater than the weight of the elevator.
B) less than than the weight of the elevator.
C) equal to the weight of the elevator.
13) A crate is sliding down an inclined ramp at a constant speed of 0.55 m/s. The vector sum of
all the forces acting on this crate must point
A) down the ramp.
B) up the ramp.
C) perpendicular to the ramp.
D) vertically downward.
E) None of the above choices is correct.
14) A bucket is being lowered by a very light rope with a constant downward velocity. The
tension in the rope must be
A) equal to the weight of the bucket.
B) greater than the weight of the bucket.
C) less than the weight of the bucket.
15) Bill and his daughter Susan are both standing on identical skateboards (with really good
frictionless ball bearings), initially at rest. Bill weighs three times as much as Susan. Bill pushes
horizontally on Susan’s back, causing Susan to start moving away from Bill. Just after Bill stops
pushing,
A) Susan and Bill are moving away from each other, and Susan’s speed is three times that of Bill.
B) Susan is moving away from Bill, and Bill is stationary.
C) Susan and Bill are moving away from each other, with equal speeds.
D) Susan and Bill are moving away from each other, and Susan’s speed is one-third that of Bill.
16) You push on box G that is next to box H, causing both boxes to slide along the floor, as
shown in the figure. The reaction force to your push is
A) the push of box G on box H.
B) the push of box H on box G.
C) the push of box G against you.
D) the upward force of the floor n box G.
E) the acceleration of box G.
17) A small car and a large SUV are at a stoplight. The car has a mass equal to half that of the
SUV, and the SUV can produce a maximum accelerating force equal to twice that of the car.
When the light turns green, both drivers push their accelerators to the floor at the same time.
Which vehicle pulls ahead of the other vehicle after a few seconds?
A) The car pulls ahead.
B) The SUV pulls ahead.
C) It is a tie.
18) An object is moving with constant non-zero velocity. Which of the following statements
about it must be true?
A) A constant force is being applied to it in the direction of motion.
B) A constant force is being applied to it in the direction opposite of motion.
C) A constant force is being applied to it perpendicular to the direction of motion.
D) The net force on the object is zero.
E) Its acceleration is in the same direction as it velocity.
19) A 20-ton truck collides with a 1500-lb car. Which of the following statements must be true?
A) During the collision, the force on the truck is greater then the force on the car.
B) During the collision, the force on the truck is equal to the force on the car.
C) During the collision, the force on the truck is smaller than the force on the car.
D) The truck did not slow down during the collision, but the car did.
E) The car did not slow down during the collision, but the truck did.
20) A truck is using a hook to tow a car whose mass is one quarter that of the truck. If the force
exerted by the truck on the car is 6000 N, then the force exerted by the car on the truck is
A) 1500 N.
B) 24000 N.
C) 6000 N.
D) 12000 N.
E) We need to know if the car is accelerating to answer this question.
21) A golf club hits a golf ball with a force of 2400 N, sending the ball into the air. The force
exerted on the club by the ball must be less than 2400 N or else the ball would not have moved
forward.
A) True
B) False
C) The answer depends on whether the golfer followed through with the swing.
22) The figure shows two boxes, with m1 > m2, that are on a level frictionless surface. We can
apply a horizontal force either toward the right on m1 or toward the left on m2. The magnitude
of the force that the boxes exert on each other is
A) zero newtons in either case.
B) larger if is applied toward the right.
C) larger if is applied toward the left.
D) the same in either case.
23) A horse pulls a cart with force . As a result of this force the cart accelerates with constant
acceleration. The magnitude of the force that the cart exerts on the horse
A) is zero newtons.
B) equal to the magnitude of .
C) less than the magnitude of .
D) greater than the magnitude of .
E) cannot be determined without knowing the mass of the horse.
24) An object of weight W is in freefall close to the surface of Earth. The magnitude of the force
that the object exerts on Earth is
A) greater than W.
B) less than W.
C) equal to W.
D) zero.
E) cannot be determined without knowing the relative masses of the object and the earth.
25) An object of mass m rests on a flat table. The earth pulls on this object with a force of
magnitude mg. What is the reaction force to this pull?
A) The table pushing up on the object with force mg.
B) The object pushing down on the table with force mg.
C) The table pushing down on the floor with force mg.
D) The object pulling upward on the Earth with force mg.
E) The table pulling upward on the Earth with force mg.
26) You are standing in a moving bus, facing forward, when you suddenly slide forward as the
bus comes to an immediate stop. What force caused you to slide forward?
A) gravity
B) the normal force due to your contact with the floor of the bus
C) the force due to friction between you and the floor of the bus
D) There is not a force causing you to slide forward.
27) A fireman is sliding down a fire pole. As he speeds up, he tightens his grip on the pole, thus
increasing the vertical frictional force that the pole exerts on the fireman. When the force on his
hands equals his weight, what happens to the fireman?
A) The fireman comes to a stop.
B) The fireman descends with slower and slower speed.
C) The fireman descends with a smaller but non-zero acceleration.
D) The fireman continues to descend, but with constant speed.
E) The acceleration of the fireman is now upward.
28) A person is using a rope to lower a 5.0-N bucket into a well with a constant speed of 2.0 m/s.
What is the magnitude of the force exerted by the rope on the bucket?
A) 0.00 N
B) 2.0 N
C) 5.0 N
D) 10 N
E) 49 N
29) A person who normally weighs 700 N is riding in a rather swift elevator that is moving at a
constant speed of 9.8 m/s. If this person is standing on a bathroom scale inside the elevator, what
would the scale read?
A) more than 700 N
B) less than 700 N
C) 700 N
D) It could be more or less than 700 N, depending on whether the elevator is going up or down.
30) A person who normally weighs 700 N is riding in an elevator that is moving upward but
slowing down at a steady rate. If this person is standing on a bathroom scale inside the elevator,
what would the scale read?
A) more than 700 N
B) less than 700 N
C) 700 N
D) It could be more or less than 700 N, depending on whether the magnitude of the acceleration
is greater than or less than 9.8 m/s2.
31) A 75-N box rests on a perfectly smooth horizontal surface. The minimum force need to start
the box moving is
A) 75 N.
B) 7.5 N.
C) 750 N.
D) any horizontal force greater than zero.
32) The acceleration due to gravity is lower on the Moon than on Earth. Which one of the
following statements is true about the mass and weight of an astronaut on the Moon’s surface,
compared to Earth?
A) Mass is less, weight is the same.
B) Mass is the same, weight is less.
C) Both mass and weight are less.
D) Both mass and weight are the same.
33) When a 45-kg person steps on a scale in an elevator, the scale reads a steady 480 N. Which
of the following statements must be true? (There could be more than one correct choice.)
A) The elevator is accelerating upward at a constant rate.
B) The elevator is accelerating downward at a constant rate.
C) The elevator is moving upward at a constant rate.
D) From the given information, we cannot tell if the elevator is moving up or down.
34) When a 45-kg person steps on a scale in an elevator, the scale reads a steady 410 N. Which
of the following statements must be true? (There could be more than one correct choice.)
A) The elevator is accelerating upward at a constant rate.
B) The elevator is accelerating downward at a constant rate.
C) The elevator is moving upward at a constant rate.
D) From the given information, we cannot tell if the elevator is moving up or down.
35) Inside of a train a ball of weight W is hanging by a light wire at rest from the ceiling. The
wire makes an angle θ with the ceiling, as shown in the figure. Which one of the following
conditions must be true about the tension T in the wire?
A) T sin θ = W
B) T cos θ = W
C) T tan θ = W
D) T = W
E) T = ma
4.2 Problems
1) What is the mass of an object that experiences a gravitational force of 685 N near Earth’s
surface where g = 9.80 m/s2?
A) 69.9 kg
B) 68.5 kg
C) 71.3 kg
D) 72.7 kg
2) If I weigh 741 N on Earth at a place where g = 9.80 m/s2 and 5320 N on the surface of another
planet, what is the acceleration due to gravity on that planet?
A) 70.4 m/s2
B) 51.4 m/s2
C) 61.2 m/s2
D) 81.0 m/s2
3) An astronaut weighs 99 N on the Moon, where the acceleration of gravity is 1.62 m/s2. How
much does she weigh on Earth?
A) 16 N
B) 61 N
C) 99 N
D) 600 N
E) 440 N
4) An object has a mass of 60 kg on Earth. What are the mass and weight of this object on the
surface of the Moon where the acceleration due to gravity is only 1/6 of what it is on Earth?
5) A net force of 125 N is applied to a certain object. As a result, the object accelerates with an
acceleration of 24.0 m/s2. The mass of the object is
A) 3000 kg.
B) 2880 kg.
C) 144 kg.
D) 0.200 kg.
E) 5.21 kg.
6) If a net force accelerates a 4.5-kg tool at 40 m/s2, what acceleration would that same net force
give to an 18-kg tool?
A) 180 m/s2
B) 10 m/s2
C) 160 m/s2
D) 9.8 m/s2
E) 32 m/s2
7) A block is on a frictionless table, on earth. The block accelerates at 3.0 m/ when a
horizontal force is applied to it. The block and table are set up on the Moon where the
acceleration due to gravity is 1.62 m/ . The weight of the block on the Moon is closest to
A) 11 N.
B) 9.5 N.
C) 8.1 N.
D) 6.8 N.
E) 5.5 N.
8) A block is on a frictionless table, on earth. The block accelerates at 5.3 m/ when a
horizontal force is applied to it. The block and table are set up on the Moon where the
acceleration due to gravity is 1.62 m/ . A horizontal force of 5 N is now applied to the block
when it is on the Moon. The acceleration imparted to the block by this force is closest to
A) 2.7 m/ .
B) 2.4 m/ .
C) 2.9 m/ .
D) 3.2 m/ .
E) 3.4 m/ .
9) A 40-kg crate is being lowered with a downward acceleration is 2.0 m/s2 by means of a rope.
(a) What is the magnitude of the force exerted by the rope on the crate?
(b) What would be the magnitude of the force exerted by the rope if the crate were being raised
with an acceleration of 2.0 m/s2?
10) A 450-kg sports car accelerates from rest to 100 km/h in 4.80 s. What magnitude force does a
68.0 kg passenger experience during the acceleration?
A) 394 N
B) 82.0 N
C) 342 N
D) 311 N
11) On its own, a tow truck has a maximum acceleration of 3.0 m/s2. What will be its maximum
acceleration when the truck is using a light horizontal chain to tow a bus of twice its own mass?
A) 2.5 m/s2
B) 2.0 m/s2
C) 1.5 m/s2
D) 1.0 m/s2
12) A car of mass 1100 kg that is traveling at 27 m/s starts to slow down and comes to a
complete stop in 578 m. What is the magnitude of the average braking force acting on the car?
A) 690 N
B) 550 N
C) 410 N
D) 340 N
13) A certain aircraft has a mass of 300,000 kg. At a certain instant during its landing, its speed
is 27.0 m/s. If the braking force is a constant 445,000 N, what is the speed of the airplane 10.0 s
later?
A) 10.0 m/s
B) 12.2 m/s
C) 14.0 m/s
D) 18.0 m/s
E) 20.0 m/s
14) What magnitude net force is required to accelerate a 1200-kg car uniformly from 0 m/s to
27.0 m/s in 10.0 s?
A) 444 N
B) 1620 N
C) 3240 N
D) 4360 N
E) 11800 N
15) A catcher stops a 0.15-kg ball traveling at 40 m/s in a distance of 20 cm. What is the
magnitude of the average force that the ball exerts against his glove?
16) An object that weighs 75 N is pulled on a horizontal surface by a horizontal pull of 50 N to
the right. The friction force on this object is 30 N to the left. What is the acceleration of the
object?
A) 0.27 m/s2
B) 1.1 m/s2
C) 2.6 m/s2
D) 11 m/s2
17) A 1200-kg car is pulling a 500-kg trailer along level ground. Friction of the road on the
trailer is negligible. The car accelerates with an acceleration of 1.3 m/s2. What is the force
exerted by the car on the trailer?
A) 550 N
B) 600 N
C) 650 N
D) 700 N
E) 750 N
18) In a certain particle accelerator, a proton reaches an acceleration of 9.0 × 1013 m/s2. The
mass of a proton is 1.67 × 10-27 kg. What is the force on the proton?
19) During a hard stop, a car and its passengers slow down with an acceleration of 8.0 m/s2.
What magnitude force does a 50-kg passenger exert on the seat belt in such a stop?
20) A flatbed truck is carrying an 800-kg load of timber that is not tied down. The maximum
friction force between the truck bed and the load is 2400 N. What is the greatest acceleration that
the truck can have without losing its load?
21) A box of mass 72 kg is at rest on a horizontal frictionless surface. A constant horizontal force
of magnitude F then acts on the box, accelerating it to the right. You observe that it takes the box
3.4 seconds to travel 13 meters. What is the magnitude of the force F?
22) A 590-kg rocket is at rest on the launch pad. What upward thrust force is needed to
accelerate the rocket uniformly to an upward speed of 28 m/s in 3.3 s?
23) A 958-N rocket is coming in for a vertical landing. It starts with a downward speed of 25 m/s
and must reduce its speed to 0 m/s in 8.0 s for the final landing.
(a) During this landing maneuver, what must be the thrust due to the rocket’s engines?
(b) What must be the direction of the engine thrust force?
24) Calculate the average force a bumper would have to exert to bring a 1200-kg car to rest in 15
cm when the car had an initial speed of 2.0 m/s (about 4.5 mph).
A) 1.8 × 104 N
B) 1.6 × 104 N
C) 5.4 × 104 N
D) 6.5 × 105 N
E) 3.2 × 104 N
25) A 10-kg object is hanging by a very light wire in an elevator that is traveling upward. The
tension in the rope is measured to be 75 N. What are the magnitude and direction of the
acceleration of the elevator?
26) A 45.0-kg person steps on a scale in an elevator. The scale reads 460 N. What is the
magnitude of the acceleration of the elevator?
A) 4.91 m/s2
B) 9.81 m/s2
C) 46.9 m/s2
D) 0.206 m/s2
E) 0.422 m/s2
27) The figure shows an acceleration-versus-force graph for a 125-g object. What should be the
value of the first tick-mark on the vertical scale, as indicated by the arrow in the figure?
A) 4
B) 8
C) 0.00400
D) 0.00800
28) The figure shows an object’s acceleration-versus-force graph. What is the mass of this
object?
A) 2.5 g
B) 1.6 g
C) 630 g
D) 400,000 g
29) The figure shows an acceleration-versus-force graph for three objects pulled by wires. If the
mass of object 2 is 36 kg, what are the masses of objects 1 and 3?
A) 12 kg and 90 kg
B) 72 kg and 18 kg
C) 90 kg and 12 kg
D) 12 kg and 72 kg