AP Physics 1 Unit 2 Forces MCQ Set A (Conceptual)AP Physics 1 Unit 2 Forces MCQ Set A (Conceptual)

APPHYSICS1 · U N I T 2 F O R C E S T E S T · MCQSETA

Conceptual MCQ (Hard)

15 questions · no calculator needed · about 25 minutes. These are AP-style reasoning questions on third- law pairs, FBDs, apparent weight, static friction, Atwood tension, functional dependence, springs and center of mass. Most wrong answers are real misconceptions. The answer key with explanations is on a new page. 1. 2.3 · THIRD LAW A large truck pushes a small car whose engine is off. They speed up together to the right on a level road. During this time, how does the force exerted by the truck on the car compare to the force exerted by the car on the truck?

(A) The truck's force on the car is larger, because the pair is accelerating forward. (B) The truck's force on the car is larger, because the truck has more mass.

(C) The forces have equal magnitudes.

(D) The car's force on the truck is larger, because the car resists the motion.

2. 2.2 · FBD

30°30°

vv

sliding up, slowing downsliding up, slowing down

A block is launched up a rough incline. It slides upward and slows down. No one is touching it. Which free- body diagram correctly shows the forces on the block while it moves up?

(A)(A)

FFgg

FFNN

ffkk

(B)(B)

FFgg

FFNN

ffkk

FFmotionmotion

(C)(C)

FFgg

FFNN

ffkk

(D)(D)

FFgg

FFNN

ffkk

(A) (B)

(C) (D)

3. 2.6 · APPARENT WEIGHT A student stands on a scale in an elevator that is moving downward and slowing down. How does the scale reading compare to the student's weight mg?

(A) Less than mg, because the elevator is moving downward.

(B) Equal to mg, because the student's mass hasn't changed.

(C) Less than mg, because the elevator is slowing down.

(D) Greater than mg, because the acceleration is upward.

4. 2.4 · FORCES AT ANGLES

m

FFTT

30°30°

constant velocity, rough floorconstant velocity, rough floor

A rope pulls a box with tension FT at 30° above horizontal. The box moves at constant velocity across a rough floor. Which of the following is true?

(A) The normal force equals mg, and friction equals FT.

(B) Friction equals FT cos 30°, and the normal force is less than mg.

(C) FT cos 30° is greater than friction, because the box is moving forward.

(D) Friction equals FT cos 30°, and the normal force is greater than mg.

5. 2.5 · MODIFIED ATWOOD

M

m

frictionlessfrictionless

Block M slides on a frictionless table, pulled by block m hanging from an ideal string over an ideal pulley. Both blocks accelerate. How does the tension FT compare to the weight of the hanging block, mg? (A) FT < mg

(B) FT = mg

(C) FT > mg

(D) FT = Mg

6. 2.5 · FUNCTIONAL DEPENDENCE For the frictionless setup in question 5, both M and m are doubled. What happens to the magnitude of the acceleration?

(A) It doubles. (B) It stays the same.

(C) It is halved. (D) It quadruples.

7. 2.7 · STATIC FRICTION A 10 kg box sits on a floor with μs = 0.50 and μk = 0.40. A student pushes it horizontally with 20 N, and the box does not move. What is the magnitude of the friction force on the box?

(A) 49 N (B) 39 N

(C) 20 N (D) 0 N

8. 2.7 · INCLINES A block stays at rest on an incline. The incline is slowly made steeper, and the block remains at rest. As the angle increases, what happens to the normal force and the static friction force on the block? (A) Both increase.

(B) Both decrease.

(C) The normal force increases, and static friction decreases.

(D) The normal force decreases, and static friction increases.

9. 2.1 · CENTER OF MASS A firework is launched as a projectile. At the top of its path it explodes into several pieces that fly in different directions. Air resistance is negligible. Which statement about the center of mass of all the pieces is true until the first piece hits the ground?

(A) It continues along the same parabolic path the firework would have followed.

(B) It stops and falls straight down, because the explosion cancels its horizontal velocity. (C) It moves in the direction of the heaviest piece.

(D) It speeds up, because the explosion adds energy to the system.

10. 1.X/2.5 · GRAPH

123456789

−1

1

2

3

4

5

6

7

t (s)

v (m/s)

The velocity of a cart moving along a straight track is shown. Which describes the net force on the cart during the three intervals 0–3 s, 3–6 s and 6–9 s?

(A) Increasing, constant, decreasing

(B) Constant and positive, zero, constant and negative

(C) Positive, positive, negative

(D) Constant and positive, constant and positive, zero

11. 2.3 · ATWOOD

3 kg

5 kg

A 3 kg block and a 5 kg block hang from an ideal string over an ideal pulley and are released from rest. Which is true about the tension FT in the string?

(A) FT = (5 kg)g, because the string has to hold the heavier block.

(B) FT = (8 kg)g, because the string supports both blocks.

(C) FT = (2 kg)g, the difference in weights.

(D) (3 kg)g < FT < (5 kg)g

12. 2.8 · SPRINGS

0.1 0.2 0.3

8

16

24

x (m)

F (N)

PP

QQ

The graph shows force vs. stretch for two springs, P and Q. The same block is hung at rest from each spring in turn. Which is true?

(A) P stretches more, because its graph is higher.

(B) Both stretch the same amount, because the block's weight is the same.

(C) Q stretches twice as far as P.

(D) P stretches twice as far as Q.

13. 2.3 · TENSION WITH MASS A heavy chain hangs straight down from a hook and is at rest. Where is the tension in the chain greatest?

(A) At the top (B) In the middle

(C) At the bottom (D) It's the same everywhere.

14. 2.3 · IDENTIFYING PAIRS A book rests on a table. Earth's gravitational pull on the book is one force of a Newton's-third-law pair. What is the other force in the pair?

(A) The table's upward normal force on the book

(B) The book's upward gravitational pull on Earth

(C) The book's downward push on the table

(D) Earth's upward push on the table

15. 2.5 · SYSTEMS

3 kg 1 kg

FF

frictionlessfrictionless

A horizontal force F pushes a 3 kg block, which pushes a 1 kg block ahead of it on a frictionless surface. In a second trial, the same F is applied from the other side, pushing the 1 kg block into the 3 kg block. How do the contact forces between the blocks compare?