Deck 2: The Laws of Physics Are Universal Newtonian Mechanics

Question 1

Do the following statements about the physical world express

-If you push on something, it moves. If you push twice as hard, it moves twice as fast.

A)a primarily Aristotelian viewpoint .
B)aprimarily Newtonian viewpoint.
C)are they consistent with both viewpoints

Answer 1

A

Question 2

Do the following statements about the physical world express

-A heavy object falls faster than a light object.

A)a primarily Aristotelian viewpoint .
B)aprimarily Newtonian viewpoint.
C)are they consistent with both viewpoints

Answer 2

A

Question 3

Do the following statements about the physical world express

-The moon is held in its orbit around the earth by the force of the earth's gravity.

A)a primarily Aristotelian viewpoint .
B)aprimarily Newtonian viewpoint.
C)are they consistent with both viewpoints

Answer 3

B

Question 4

Do the following statements about the physical world express

-If you push on an object, it moves. After you release it, it gradually comes to rest because friction drains away the force of the initial push.

A)a primarily Aristotelian viewpoint .
B)aprimarily Newtonian viewpoint.
C)are they consistent with both viewpoints

Question 5

Do the following statements about the physical world express

-The speed of a falling object increases as it falls.

A)a primarily Aristotelian viewpoint .
B)aprimarily Newtonian viewpoint.
C)are they consistent with both viewpoints

Question 6

A spaceship with a mass of $20,000 \mathrm{~kg}$ is traveling in a straight line at a constant speed of $300 \mathrm{~km} / \mathrm{s}$ in deep space. Its rockets engines must be:

A) off
B) firing

Question 7

A $6-\mathrm{kg}$ bowling ball moving at $3 \mathrm{~m} / \mathrm{s}$ collides with a 1.4-kg bowling pin at rest. How do the magnitudes of the forces exerted by the collision on each object compare?

A) The force on the pin is larger than the force on the ball.
B)The force on the ball is larger than the force on the pin.
C) Both objects experience the same magnitude of force.
D) The collision exerts no force on the ball.
E) There is no way to tell.

Question 8

A large car drags a small trailer in such a way that their common speed increases rapidly. Which tugs harder on the other?

A) The car tugs harder on the trailer than vice versa.
B) The trailer tugs harder on the car than vice versa.
C) Both tug equally on each other.
D) The trailer exerts no force on the car at all.
E) There is no way to tell.

Question 9

A parent pushes a small child on a swing so that the child moves rapidly away while the parent remains at rest. How does the magnitude of the force that the child exerts on the parent compare to the magnitude of the force that the parent exerts on the child?

A) The force on the child is larger in magnitude.
B) The force on the parent is larger in magnitude.
C) These forces have equal magnitudes.
D) The child exerts zero force on the parent.
E)There is no way to tell.

Question 10

Two marbles are rolling along parallel tracks (which may or may not be inclined). A stroboscopic photograph showing a top view of the positions of the marbles at equally spaced instants of time looks like this:

At what instant(s) of time do the marbles have the same instantaneous velocity?

A) At time $t_{B}$
B) At time $t_{G}$
C) At both $t_{B}$ and $t_{G}$
D) Some instant between $t_{C}$ and $t_{D}$
E) Some instant between $t_{D}$ and $t_{E}$
F) Roughly time $t_{D}$

Question 11

Two marbles are rolling along parallel tracks (which may or may not be inclined). A stroboscopic photograph showing a top view of the positions of the marbles at equally spaced instants of time looks like this:

At what instant(s) of time do the marbles have the same instantaneous velocity?

A) At time $t_{B}$
B) At time $t_{G}$
C) At both $t_{B}$ and $t_{G}$
D) Some instant between $t_{C}$ and $t_{D}$
E) Some instant between $t_{D}$ and $t_{E}$
F) Roughly time $t_{D}$

Question 12

An object travels halfway around a circle at a constant instantaneous speed $|\vec{v}|$ . What is the magnitude $|\vec{\psi}|$ of its average velocity during this time interval?

A) $|\vec{v}|$
B) $1.41|\vec{v}|$
C) $1.57|\vec{v}|$
D) $|\vec{v}| / 1.41$
E) $|\vec{v}| / 1.57$
F) Some other multiple of $|\vec{v}|$ (specify)
G) We do not have enough information to answer.

Question 13

An object can have a constant speed and still be accelerating.

Question 14

An object's acceleration vector always points in the direction that it is moving.

Question 15

An object can have zero velocity (at an instant) and still be accelerating.

Question 16

An object's $x$ -velocity can be positive at the same time that its $x$ -acceleration is negative.

Question 17

An object falling vertically at a speed of $20 \mathrm{~m} / \mathrm{s}$ lands in a snowbank and comes to rest $0.5 \mathrm{~s}$ later. The object's average acceleration during this interval is

A) Upward
B) Downward
C) Zero

Question 18

Suppose you throw a ball vertically into the air. At the exact instant that the ball reaches its highest position, select from the choices below the most accurate description of

-The ball's instantaneous velocity.

A) Upward
B) Downward
C) Zero
D) Other (specify)

Question 19

Suppose you throw a ball vertically into the air. At the exact instant that the ball reaches its highest position, select from the choices below the most accurate description of

-The ball's instantaneous acceleration.

A) Upward
B) Downward
C) Zero
D) Other (specify)

Question 20

Suppose you throw a ball vertically into the air. At the exact instant that the ball reaches its highest position, select from the choices below the most accurate description of

-The gravitational force on the ball.

A) Upward
B) Downward
C) Zero
D) Other (specify)

Question 21

Suppose you throw a ball vertically into the air. At the exact instant that the ball reaches its highest position, select from the choices below the most accurate description of

-The net external force on the ball.

A) Upward
B) Downward
C) Zero
D) Other (specify)

Question 22

The diagram below shows a top view of the moon's nearly circular orbit around the earth. The moon orbits the earth at a nearly constant speed.

At the point shown, use the arrow choices shown to the right to indicate the direction of

-The moon's velocity.

Question 23

The diagram below shows a top view of the moon's nearly circular orbit around the earth. The moon orbits the earth at a nearly constant speed.

At the point shown, use the arrow choices shown to the right to indicate the direction of

-The moon's acceleration.

Question 24

The diagram below shows a top view of the moon's nearly circular orbit around the earth. The moon orbits the earth at a nearly constant speed.

At the point shown, use the arrow choices shown to the right to indicate the direction of

-The gravitational force on the moon due to the earth.

Question 25

The diagram below shows a top view of the moon's nearly circular orbit around the earth. The moon orbits the earth at a nearly constant speed.

At the point shown, use the arrow choices shown to the right to indicate the direction of

-The net external force on the moon.

Question 26

The diagram below shows a top view of the moon's nearly circular orbit around the earth. The moon orbits the earth at a nearly constant speed.

At the point shown, use the arrow choices shown to the right to indicate the direction of

-The gravitational force on the earth due to the moon. (To choose a double-letter answer using the letters on the back of the book, point with two fingers to the letter.)

Question 27

Figure N1.6 shows the acceleration for an object moving clockwise around the circle. If the object moves counterclockwise around the circle, its acceleration points outward instead of inward.

Question 28

An object travels at a constant speed $|\vec{v}|$ exactly once around a circle of radius $r$ . The magnitude of the object's average acceleration is:

A) $|\vec{v}|^{2} / r$
B) Zero
C) Something else (specify)

Question 29

When a baseball pitcher throws a curveball, he or she puts top-spin on the ball. This causes the ball to interact with the passing air in such a way as to make the air push the ball downward perpendicular to the ball's velocity. (This causes the ball to dive sharply as it approaches the plate, making it quite difficult to hit.) We would classify this force as a "lift" force on the ball.

Question 30

Suppose that a comet goes around the sun in an elliptical orbit such that the comet at the farthest point in its orbit is 3 times as far from the sun as it is when it is at its closest point. How many times stronger is the gravitational force on the comet due to the sun when it is at its closest point than when it is at its farthest?

A) $\left|\overrightarrow{F_{g}}\right|$ is the same at both positions.
B) $\sqrt{3}$ times stronger
C) 3 times stronger
D) 9 times stronger
E) 27 times stronger
F) The answer depends on the relative masses of the comet and the sun.

Question 31

Suppose an object is hanging from the end of a spring. Let the spring's length when the object is hanging at rest be L. Now suppose we pull the object downward until the spring has been stretched a length of $2 \mathrm{~L}$ . By what factor has the force that the spring is exerting on the object increased?

A) The spring force is the same in both cases.
B) $\sqrt{2}$ times stronger
C) 2 times stronger
D) 4 times stronger
E) The answer depends on the spring's relaxed length.

Question 32

A crate sits on the ground. You push as hard as you can on it, but you cannot move it. At any given time when you are pushing, what is the magnitude of the static friction force exerted on the crate by its contact interaction with the ground compared to the magnitude of your push (which is a normal force)? (Hints: What is the crate's acceleration? Draw a free-body diagram.)

A) $\left|\vec{F}_{S F}\right|<\left|\vec{F}_{N}\right|$
B) $\left|\vec{F}_{S F}\right|=\left|\vec{F}_{N}\right|$
C) $\left|\vec{F}_{S F}\right|>\left|\vec{F}_{N}\right|$
D) $\left|\vec{F}_{S F}\right|=0$ !
E) We do not have enough information to answer.}

Question 33

A box sits at rest on an inclined plank. How do the magnitudes of the normal force and the gravitational force exerted on the box compare? (Hints: What is the box's acceleration? Draw a free-body diagram!)

A) $\left|\vec{F}_{N}\right|<\left|\vec{F}_{g}\right|$
B) $\left|\vec{F}_{N}\right|=\left|\vec{F}_{g}\right|$
C) $\left|\vec{F}_{N}\right|>\left|\vec{F}_{g}\right|$
D) $\left|\vec{F}_{N}\right|=0$ !
E) We do not have enough information to answer.

Question 34

A boat hits a sandbar and slides some distance before coming to rest. Which of the arrows shown below best represents the direction of the boat's acceleration as it is sliding? (Hint: Draw a motion diagram.)

A) A
B) B
C) C
D) D
E) E
F) F
T) Zero

Question 35

A car moving at a constant speed travels past a valley in the road, as shown below. Which of the arrows shown most closely approximates the direction of the car's acceleration at the instant it is at the position shown? (Hint: Draw a motion diagram.)

A) A
B) B
C) C
D) D
E) E
F) F
T) T

Question 36

A bike (shown in a top view in the diagram) travels around a curve with its brakes on, so that it is constantly slowing down. Which of the arrows shown most closely approximates the direction of the bike's acceleration at the instant it is at the position shown? (Hint: Draw a motion diagram.)

A) A
B) B
C) C
D) D
E) E
F) F
T) T

Question 37

A car passes a dip in the road, going first down, then up. At the very bottom of the dip, when the car's instantaneous velocity is passing through horizontal, how does the magnitude of the normal force on the car compare to the magnitude of the car's weight?

A) $\left|\vec{F}_{N}\right|<\left|\vec{F}_{g}\right|$

B) $\left|\vec{F}_{N}\right|=\left|\vec{F}_{g}\right|$

C) $\left|\vec{F}_{N}\right|>\left|\vec{F}_{g}\right|$

D) $\left|\vec{F}_{N}\right|>\left|\vec{F}_{g}\right|$ .

E) We do not have enough information to answer.

Question 38

The drawing below is supposed to be a free-body diagram of a box that sits without slipping on the back of a truck that is moving to the right but is slowing down. Is the diagram correct?

A) Yes.
B) No, $\vec{F}_{S F}$ should point leftward.
C) No, the $\vec{F}_{S F}$ label should be $\vec{F}_{K F}$ .
D) No, there should be a leftward drag force.
E) No, $\left|\vec{F}_{N}\right|$ should not be equal to $\left|\vec{F}_{g}\right|$ .
F) No, there is some other problem (specify).

Question 39

The drawing shown is supposed to be a free-body diagram of a crate that is being lowered by a crane and is speeding up as it is being lowered. Is the diagram correct? (Ignore air resistance.)

A) Yes.
B) No, $\vec{F}_{T}$ should be labeled $\vec{F}_{N}$
C) No, $\left|\vec{F}_{T}\right|$ should be equal to $\left|\vec{F}_{g}\right|$ .
D) No, $\left|\vec{F}_{T}\right|$ should be greater than $\vec{F}_{K F}$
E) No, there should be an upward drag force $\left|\vec{F}_{D}\right|$
F) No, there is some other problem (specify).

Question 40

A crate is sliding down a $30^{\circ}$ incline, picking up speed as it slides. Ignore air friction.

-(a) Which of the forces listed below that acts on the crate has the greatest magnitude?

A) $\left|\vec{F}_{N}\right|$
B) $\left|\vec{F}_{g}\right|$
C) Both $\left|\vec{F}_{N}\right|$ and $\left|\vec{F}_{g}\right|$ (which are equal)
D) $\left|\vec{F}_{S F}\right|$
E) $\left|\vec{F}_{K F}\right|$

Question 41

A crate is sliding down a $30^{\circ}$ incline, picking up speed as it slides. Ignore air friction.

-(b) Which force has the smallest magnitude?

A) $\left|\vec{F}_{N}\right|$
B) $\left|\vec{F}_{g}\right|$
C) Both $\left|\vec{F}_{N}\right|$ and $\left|\vec{F}_{g}\right|$ (which are equal)
D) $\left|\vec{F}_{S F}\right|$
E) $\left|\vec{F}_{K F}\right|$

Question 42

A crate is sliding down a $30^{\circ}$ incline, picking up speed as it slides. Ignore air friction.

-(c) Which force listed does not actually act on the crate?

A) $\left|\vec{F}_{N}\right|$
B) $\left|\vec{F}_{g}\right|$
C) Both $\left|\vec{F}_{N}\right|$ and $\left|\vec{F}_{g}\right|$ (which are equal)
D) $\left|\vec{F}_{S F}\right|$
E) $\left|\vec{F}_{K F}\right|$

Question 43

An object's x-position is shown in the boxed graph of the following set of graphs. Which of the other graphs in the set most correctly describes its $x$ -velocity?

A) A
B) B
C) C
D) D
E) E

Question 44

Which graph best describes the x-acceleration of the object described in problem N2T.13?

A) A
B) B
C) C
D) D
E) E

Question 45

An object's $x$ -velocity $v_{x}(t)$ is shown in the boxed graph at the top left. Which of the other graphs in the set most correctly describes its $x$ -position?

A) A
B) B
C) C
D) D
E) E

Question 46

An object's $x$ -acceleration $a_{x}(t)$ is shown in the boxed graph at the top left. Which of the other graphs in the set most correctly describes its $x$ -velocity?

A) A
B) B
C) C
D) D
E) E

Question 47

If a car has an $x$ -acceleration of $a_{x}(t)=-b t+c$ , and its initial $x$ -velocity at time $t=0$ is $v_{x}(0)=v_{0}$ , which function below best describes $v_{x}(t)$ ?

A) $-\mathrm{b}$
B) $-b+v_{0}$
C) $1 / 2 b_{t}{ }^{2}+c_{t}+v_{0}$
D) $-\mathbf{1} / \mathbf{2} b_{t}{ }^{2}+c_{t}+v_{0}$
E) $-2 b_{t}{ }^{2}+v_{0}$
F) $-1 / 2 b_{t}{ }^{2}+v_{0}$

Question 48

If a car's $x$ -position at time $t=0$ is $x(0)=0$ and it has an $x$ -velocity of $v_{x}(t)=b(t-T)^{2}$ , where $b$ and $T$ are constants, which function below best describes $x(t)$ ?

A) $x(t)=2 b(t-T)$
B) $x(t)=3 b(t-T)^{3}$
C) $x(t)=\frac{1}{3} b(t-T)^{3}$
D) $x(t)=\frac{1}{2} b(t-T)$
E) $x(t)=\frac{1}{3} b\left[(t-T)^{3}+T^{3}\right]$
F) Other (specify)

Question 49

Suppose you are preparing an actual-size trajectory diagram of a freely falling object. The time interval between positions is $0.02 \mathrm{~s}$ . How long should you draw the acceleration arrows on your diagram?

A) $9.8 \mathrm{~m}$
B) $0.20 \mathrm{~m}$
C) $0.04 \mathrm{~m}$
D) $3.9 \mathrm{~cm}$
E) $0.39 \mathrm{~cm}$
F) Other (specify)

Question 50

At time $t=0$ a person is sliding due east on a flat, frictionless plane of ice. The net force on this person is due to a battery-powered fan the person holds that exerts a southward thrust force on the person. Assuming that drag is negligible, the eastward component of the person's velocity is unaffected by this force.

Question 51

Consider the person described in problem N3T.6. The person's trajectory will look most like which of the following? (The dot shows the person's position at $t=0$ and east is to the right and north to the top.)

A) A
B) B
C) C
D) D
E) E
F) F

Question 52

A weight hangs from a string but is pulled to one side by a horizontal string, as shown. The tension force exerted by the angled string is

A) Less than the hanging object's weight.
B) Equal to the hanging object's weight.
C) Greater than the hanging object's weight.

Question 53

A person would like to pull a car out of a ditch. This person ties one end of a chain to the car's bumper and wraps the other end around a tree so that the chain is taut. The person then pulls on the chain perpendicular to its length, as shown in the picture. The magnitude of the force that the chain exerts on the car in this situation is

A) Much smaller than the force the person exerts on the chain.
B) About equal to the force the person exerts on the chain.
C) Much bigger than the force the person exerts on the chain.

Question 54

Suppose an object is suspended from strings as shown in the diagram below.


-Which exerts the largest force on the connecting ring?

A) String A
B) String B
C) String C

Question 55

Suppose an object is suspended from strings as shown in the diagram below.


-Which exerts the smallest force on that ring?
(Hint: Draw a free-body diagram of the ring.)

A) String $A$
B) String B
C) String C

Question 56

The lid of a grand piano is propped open as shown. Which arrow most closely approximates the direction of the force that the hinge exerts on the lid? Assume that the prop exerts a force in parallel to the prop's length. (Hint: Draw a torque diagram of the lid.)

A) A
B) B
C) C
D) D
E) E
F) F
T) T

Question 57

If we choose our origin to be the hinge in the situation discussed in problem N4T.4, the torque exerted by the lid's weight around the hinge is away from the viewer.

Question 58

When viewed from above, suppose that a helicopter's rotor spins clockwise. The helicopter engine must continually exert a torque on the rotor to keep it spinning against the drag that the air exerts on the rotor. Note that a helicopter is usually designed so its center of mass is directly under the rotor. In order for the helicopter to hover motionless in the air, a small rotor at the helicopter's tail is necessary. As viewed by someone looking at the tail from the helicopter's front, the small rotor must blow air

A) To the left.
B) To the right.
C) Vertically upward.
D) Vertically downward.
E) In some combination of these directions.

Question 59

A board of mass $m$ lies on the ground. What is the magnitude of the force you would have to exert to lift one end of the board barely off the ground (assuming that the other end still touches the ground)?

A) $2 m|\vec{g}|$
B) $m|\vec{g}|$
C) $1 / 2 m|\vec{g}|$
D) The answer depends on the board's length.
E) Other (explain)

Question 60

Suppose you continue to lift the board described in problem N4T.7. Assume that the force you exert is always perpendicular to the board and that one end of the board always remains on the ground. What happens to the magnitude of the force you exert on the end as the angle between the board and the ground increases? It

A) Increases.
B) Decreases.
C) Remains the same.
D) The answer depends on information not given.

Question 61

The magnitude of the normal force on a box sitting on an incline is equal to that of its weight.

Question 62

A certain crate sits on a rough floor. You find that you have to apply a horizontal force of $200 \mathrm{~N}$ to get the crate moving. If you put some massive objects in the crate so that its mass is doubled, how much force does it take to get the crate moving now?

A) Still $200 \mathrm{~N}$
B) $400 \mathrm{~N}$
C) $800 \mathrm{~N}$
D) It depends (specify)

Question 63

Two boxes of the same mass sit on a rough floor. These boxes are made of the same kind of cardboard and are identical except that one is twice as large as the other. If it takes $200 \mathrm{~N}$ to start moving the smaller box, how much force does it take to start moving the larger one?

A) Still $200 \mathrm{~N}$
B) $400 \mathrm{~N}$
C) $800 \mathrm{~N}$
D) It depends (specify)

Question 64

The coefficient of static friction between Teflon and scrambled eggs is about 0.1. What is the smallest tilt angle from the horizontal that will cause the eggs to slide across the surface of a tilted Teflon-coated pan?

A) $0.002^{\circ}$
B) $5.7^{\circ}$
C) $15^{\circ}$
D) $33^{\circ}$
E) Other (specify)

Question 65

If you want to stop a car without anti-lock brakes as quickly as possible on an icy road, you should

A) Jam on the brakes as hard as you can.
B) Push on the brakes as hard as you can without locking the car's wheels and thus making the car skid.
C) Pump the brakes.
D) Do something else (specify).

Question 66

Putting wider tires on your car will clearly give you more traction.

Question 67

Assume that the coefficient of static friction between your car's tires and a certain road surface is about 0.75 . Your car can climb a $45^{\circ}$ slope.

Question 68

Imagine that an external force of $100 \mathrm{~N}$ must be applied to keep a bicycle and rider moving at a constant speed of 12 $\mathrm{mi} / \mathrm{h}$ against opposing air drag. To double the bike's speed to $24 \mathrm{mi} / \mathrm{h}$ , we must increase the magnitude of the force exerted on the bike to

A) $141 \mathrm{~N}$
B) $200 \mathrm{~N}$
C) $400 \mathrm{~N}$
D) It depends on the bike's shape and area
E) Other (specify)

Question 69

Imagine that a certain engine can cause the road to exert a certain maximum forward force $\left|\vec{F}_{S F}\right|$ on a certain car. If we change the car's design to reduce its drag coefficient by a factor of 2 , by what factor will the car's maximum speed increase (other things being equal)?

A) No increase
B) 1.41
C) 2
D) 4
E) Depends (specify)
F) Other (specify)

Question 70

A truck is traveling down a steady slope such that for each $1 \mathrm{~m}$ the truck goes forward along the slope it goes down $0.04 \mathrm{~m}$ (we call this a $4 \%$ grade). Imagine that the truck's brakes fail. What is the approximate increase in the truck's speed after $30 \mathrm{~s}$ , assuming that the engine is not used and there is little drag or other friction? $[\vec{g} \mid=$ $22(\mathrm{mi} / \mathrm{h}) / \mathrm{s}$ .]

A) $0.9 \mathrm{mi} / \mathrm{h}$
B) $11 \mathrm{mi} / \mathrm{h}$
C) $26 \mathrm{mi} / \mathrm{h}$
D) $53 \mathrm{mi} / \mathrm{h}$
E) $660 \mathrm{mi} / \mathrm{h}$
F) Other (specify)

Question 71

We can express the kinetic friction law in the form \(\vec{F}_{K F}=\mu_{k} \vec{F}_{N} .

Question 72

A car with four-wheel drive can climb a steeper slope than a car with two-wheel drive.

Question 73

A jet airplane flies at a constant velocity through the air. Its jet engines exert a constant force forward on the plane that exactly balances the force of air friction exerted backward on the plane. These forces are equal in magnitude and opposite in direction. Do we know this because of Newton's second law or Newton's third law?

A) Newton's second law
B) Newton's third law
C) Both laws
D) Neither (explain)

Question 74

Which of the following pairs of forces are third-law partners? Answer $\mathrm{T}$ if the two forces described are third-law partners, $\mathrm{F}$ if they are not.

-A thrust force from its propeller pulls a plane forward; a drag force pushes it backward.

Question 75

Which of the following pairs of forces are third-law partners? Answer $\mathrm{T}$ if the two forces described are third-law partners, $\mathrm{F}$ if they are not.

-A car exerts a forward force on a trailer; the trailer tugs backward on the car.

Question 76

Which of the following pairs of forces are third-law partners? Answer $\mathrm{T}$ if the two forces described are third-law partners, $\mathrm{F}$ if they are not.

-A motorboat propeller pushes backward on the water; the water pushes forward on the propeller.

Question 77

Which of the following pairs of forces are third-law partners? Answer $\mathrm{T}$ if the two forces described are third-law partners, $\mathrm{F}$ if they are not.

-Gravity pulls down on a person sitting in a chair; the chair pushes back up on the person.

Question 78

A box $B$ sits in the back of a truck $T$ as the truck slows down for a stop (the box remains motionless relative to the truck). What is the appropriate symbol for the horizontal force that the contact interaction between the box and the truck exerts on the truck?

A) $\vec{F}_{N}^{B(T)}$

B) $\vec{F}_{N}^{\mathrm{T}(\mathrm{B})}$

C) $\vec{F}_{S F}^{B(T)}$

D) $\vec{F}_{S F}^{\mathrm{T}(B)}$

E) $\vec{F}_{K F}^{\mathrm{B}(\mathrm{T})}$

F) Other (specify)

Question 79

A child $C$ pulls on a wagon $W$ , using a string $S$ ; the wagon moves forward at a constant speed as a result. The third-law partner to the forward force exerted on the wagon is which of the following forces?
( $\mathrm{R}=$ road.)

A) $\vec{F}_{T}^{S(W)}$

B) $\vec{F}_{T}^{\mathrm{W}(\mathrm{S})}$

C) $\vec{F}_{T}^{\mathrm{W}(\mathrm{C})}$

D) $\vec{F}_{T}^{\mathrm{C}(W)}$

E) $\vec{F}_{K F}^{\mathrm{W}(R)}$

F) Other (specify)

Question 80

A small car pushes on a disabled truck, accelerating it slowly forward. Each exerts a force on the other as a result of their contact interaction. Which vehicle exerts the greater force on the other?

A) The car
B) The truck
C) Both forces have the same magnitude.
D) The truck doesn't exert any force on the car.
E) One needs more information to answer.