Deck 5: Force and Motion

Question 1

A dog is standing in the bed of a pickup truck. The bed is coated with ice, causing the force of friction between the dog and the truck to be zero. The truck is initially at rest, and then accelerates to the right, moving along a flat road. As seen from a stationary observer (watching the truck move to the right), the dog

A) does not move left or right, but the back of the truck moves towards the dog.
B) moves to the right, but not as quickly as the truck is moving to the right, causing it to slide towards the back of the truck.
C) moves to the right at the same rate as the truck, so it doesn't slide.
D) moves to the left, as the truck moves to the right, causing the dog to slide towards the back of the truck.

Answer 1

does not move left or right, but the back of the truck moves towards the dog.

Question 2

If a 5.0 kg box is pulled simultaneously by a 10.0 N force and a 5.0 N force, then its acceleration must be

A) 3.0 m/s².
B) 2.2 m/s².
C) 1.0 m/s².
D) We cannot tell from the information given.

Answer 2

We cannot tell from the information given.

Question 3

You are seated in a bus and notice that a hand strap that is hanging from the ceiling hangs away from the vertical in the backward direction. From this observation, you can conclude that

A) the velocity of the bus is forward.
B) the velocity of the bus is backward.
C) You cannot conclude anything about the direction of the velocity of the bus.

Answer 3

You cannot conclude anything about the direction of the velocity of the bus.

Question 4

A ball is tossed vertically upward. When it reaches its highest point (before falling back downward)

A) the velocity is zero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward.
B) the velocity is zero, the acceleration is zero, and the force of gravity acting on the ball is zero.
C) the velocity is zero, the acceleration is zero, and the force of gravity acting on the ball is directed downward.
D) the velocity and acceleration reverse direction, but the force of gravity on the ball remains downward.
E) the velocity, acceleration, and the force of gravity on the ball all reverse direction.

Question 5

Which one of the following free-body diagrams best represents the free-body diagram, with correct relative force magnitudes, of a person in an elevator that is traveling upward with an unchanging velocity? _f is the force of the floor on the person and _g is the force of gravity on the person.

A)
B)
C)
D)
E)

Question 6

A stalled car is being pushed up a hill at constant velocity by three people. The net force on the car is

A) zero.
B) up the hill and equal to the weight of the car.
C) down the hill and equal to the weight of the car.
D) up the hill and greater than the weight of the car.
E) down the hill and greater than the weight of the car.

Question 7

A 615 N student standing on a scale in an elevator notices that the scale reads 645 N. From this information, the student knows that the elevator must be moving

A) downward.
B) upward.
C) You cannot tell if it is moving upward or downward.

Question 8

An 1100-kg car traveling at 27.0 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

Question 9

Point P in the figure indicates the position of an object traveling at constant speed clockwise around the circle. Which arrow best represent the direction the object would travel if the net external force on it were suddenly reduced to zero?

A)
B)
C)
D)
E)

Question 10

A 7.0-kg object is acted on by two forces. One of the forces is 10.0 N acting toward the east. Which of the following forces is the other force if the acceleration of the object is 1.0 m/s² toward the east?

A) 6.0 N east
B) 3.0 N west
C) 12 N east
D) 9.0 N west
E) 7.0 N west

Question 11

Suppose you are playing hockey on a new-age ice surface for which there is no friction between the ice and the hockey puck. You wind up and hit the puck as hard as you can. After the puck loses contact with your stick, the puck will

A) start to slow down.
B) not slow down or speed up.
C) speed up a little, and then slow down.
D) speed up a little, and then move at a constant speed.

Question 12

An object is moving forward with a constant velocity. Which statement about this object MUST be true?

A) The net force on the object is zero.
B) The net force on the object is in the forward direction.
C) No forces are acting on the object.
D) The acceleration of the object is in the forward direction.

Question 13

Which one of the following free-body diagrams best represents the free-body diagram, with correct relative force magnitudes, of a person in an elevator that is traveling upward but is gradually slowing down at a rate of 9 m/s²? _f is the force of the floor on the person and _g is the force of gravity on the person.

A)
B)
C)
D)
E)

Question 14

A car is being towed at constant velocity on a horizontal road using a horizontal chain. The tension in the chain must be equal to the weight of the car in order to maintain constant velocity.

A) True
B) False

Question 15

You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. The force acting on you that causes you to fall forward is

A) the force of gravity.
B) the normal force due to your contact with the floor of the bus.
C) the force due to static friction between you and the floor of the bus.
D) the force due to kinetic friction between you and the floor of the bus.
E) No forces were acting on you to cause you to fall.

Question 16

An object is moving to the right, and experiencing a net force that is directed to the right. The magnitude of the force is decreasing with time. The speed of the object is

A) increasing.
B) decreasing.
C) constant in time.

Question 17

A woman is straining to lift a large crate, without success because it is too heavy. We denote the forces on the crate as follows: P is the upward force the woman exerts on the crate, C is the vertical contact force exerted on the crate by the floor, and W is the weight of the crate. How are the magnitudes of these forces related while the woman is trying unsuccessfully to lift the crate?

A) P + C = W
B) P + C < W
C) P + C > W
D) P = C

Question 18

The figure shows an acceleration-versus-force graph for three objects pulled by rubber bands. The mass of object 2 is 36 kg. What are the masses of objects 1 and 3?

A) 14 kg and 90 kg
B) 72 kg and 18 kg
C) 90 kg and 18 kg
D) 14 kg and 72 kg

Question 19

Suppose the force of the air drag on an object is proportional to the speed of the object and in the direction opposite the object's velocity. If you throw an object upward, the magnitude of its acceleration is greatest

A) right after the object is released.
B) at the top of its trajectory.
C) The acceleration of the object is the same throughout the entire trajectory.

Question 20

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 down the ramp.

A) True
B) False

Question 21

The graph in the figure shows the x component of the acceleration of a 2.4-kg object as a function of time (in ms). (a) At what time(s) does the x component of the net force on the object reach its maximum magnitude, and what is that maximum magnitude?
(b) What is the x component of the net force on the object at time t = 0.0 ms and at t = 4.0 ms?

Question 22

The figure shows two forces acting on an object, with magnitudes and What third force will cause the object to be in equilibrium (acceleration equals zero)?

A) 52 N pointing down
B) 52 N pointing up
C) 82 N pointing down
D) 82 N pointing up

Question 23

The figure shows two forces acting at right angles on an object. They have magnitudes = 6.3 N and = 2.1 N. What third force will cause the object to be in equilibrium (acceleration equals zero)?

A) 6.6 N at 162° counterclockwise from ₁
B) 6.6 N at 108° counterclockwise from ₁
C) 4.2 N at 162° counterclockwise from ₁
D) 4.2 N at 108° counterclockwise from ₁

Question 24

The graph in the figure shows the net force acting on a 3.0-kg object as a function of time. (a) What is the acceleration of this object at time t = 2.0 s?
(b) Draw, to scale, a graph of the acceleration of this object as a function of time over the range t = 0.00 s to t = 7.0 s.

Question 25

The figure shows a graph of the acceleration of an object as a function of the net force acting on it. The mass of this object, in grams, is closest to

A) 130.
B) 11.
C) 89.
D) 8000.

Question 26

The figure shows a graph of the acceleration of a 125-g object as a function of the net force acting on it. What is the acceleration at points A and B?

Question 27

The figure shows two forces, each of magnitude 4.6 N, acting on an object. The angle between these forces is 40°, and they make equal angles above and below the horizontal. What third force will cause the object to be in equilibrium (acceleration equals zero)?

A) 8.6 N pointing to the right
B) 7.0 N pointing to the right
C) 4.3 N pointing to the right
D) 3.5 N pointing to the right

Question 28

Two forces act on a object. One force has magnitude 65 N directed 59° clockwise from the positive x-axis, and the other has a magnitude 35 N at 32° clockwise from the positive y-axis. What is the magnitude of this object's acceleration?

A) 1.1 m/s²
B) 1.3 m/s²
C) 1.5 m/s²
D) 1.7 m/s²

Question 29

On its own, a certain tow-truck has a maximum acceleration of 3.0 m/s². What would be the maximum acceleration when this truck was towing 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

Question 30

A child on a sled starts from rest at the top of a 15° slope. If the trip to the bottom takes how long is the slope? Assume that frictional forces may be neglected.

A) 293 m
B) 586 m
C) 1130 m
D) 147 m