AP Physics 1:2.5 Newton’s Third Law and Free-Body  Diagrams- Exam Style questions with Answer- MCQ

Question

 

A student is asked to move a box from ground level to the top of a loading dock platform, as shown in the figures above. In Figure 1, the student pushes the box up an incline with negligible friction. In Figure 2, the student lifts the box straight up from ground level to the loading dock platform. In which case does the student do more work on the box, and why?

(A) Lifting the box straight up, because it requires a larger applied force to lift it straight up
(B) Pushing the box up the incline, because the force is applied for a longer distance
(C) Lifting the box straight up, because the incline acts as a simple machine and reduces the force required
(D) Neither method, because the work is the same in both cases, since using the ramp decreases the force by the same factor that it increases the distance

▶️Answer/Explanation

Ans:D

Question

A 1500kg car traveling along a road is hit by a 0.1kg rock that creates a small crack in the car’s windshield. Which of the following describes the interaction between the windshield and the rock?

A The car exerts a force on the rock, but the rock does not exert a force on the car.

B The rock exerts a force on the car, but the car does not exert a force on the rock.

C The car exerts a force on the rock, and the rock exerts a force on the car. The two forces are not equal in magnitude.

D The car exerts a force on the rock, and the rock exerts a force on the car. The two forces are equal in magnitude.

▶️Answer/Explanation

Ans:D

According to Newton’s third law, forces between two objects always occur in pairs (action-reaction pairs). The forces are equal in magnitude and opposite in direction.

Question

Two objects, X and Y, move toward one another and eventually collide. Object X has a mass of 2M and is moving at a speed of 2v0 to the right before the collision. Object Y has a mass of M and is moving at a speed of v0 to the left before the collision. Which of the following describes the magnitude of the forces F the objects exert on each other when they collide?

A The force exerted by X on Y is 4F to the right, and the force exerted by Y on X is F to the left.

B The force exerted by X on Y is 2F to the right, and the force exerted by Y on X is F to the left.

C The force exerted by X on Y is F to the right, and the force exerted by Y on X is F to the left.

D The force exerted by X on Y is F to the left, and the force exerted by Y on X is F to the right.

▶️Answer/Explanation

Ans:C

When objects exert forces on each other, the forces are equal in magnitude and opposite in direction.

Question

A block is placed on an inclined plane and remains stationary, as shown in the figure above. A student claims, “The block remains stationary because as gravity tries to pull the block down the ramp, the block exerts an equal and opposite force on itself up the ramp.” Is the student’s claim correct? Justify your answer.

A Yes. Newton’s first law states the block must remain stationary unless pushed.

B Yes. Newton’s third law states the block must exert a force equal and opposite to gravity.

C No. Newton’s first law states the block’s inertia must cause it to move down the ramp.

D No. Newton’s third law states the block cannot exert a force on itself.

▶️Answer/Explanation

Ans:D

The claim incorrectly asserts that the net force on the block is zero due to the block exerting a force on itself. This is a violation of Newton’s third law.

Question

Students connect a spring scale to a block on a rough horizontal surface. The students use the spring scale to measure the magnitude of the horizontal force needed to pull the block at a constant speed. Which of the following statements explains why two forces exerted between objects are equal in magnitude?

A The gravitational and normal forces exerted on the block, because they are a Newton’s third-law pair.

B The frictional force and force exerted by the spring scale on the block, because they are a Newton’s third-law pair.

C The normal force and the frictional force exerted on the block, because objects always exert forces of equal magnitude on each other.

D The frictional forces that the block and the surface exert on each other, because objects always exert forces of equal magnitude on each other.

▶️Answer/Explanation

Ans:D

 The block and the surface exert a frictional force on each other, and these forces are a Newton’s third-law pair. Claim: The frictional forces that the block and the surface exert on each other are equal in magnitude. Evidence: By Newton’s third law, objects always exert forces of equal magnitude on each other.

Question

A spring-loaded launcher has a mass of 0.60  kg and is placed on a platform 1.2m above the ground. The force of friction is negligible between the platform and the launcher. The launcher fires a 0.30kg ball that lands a distance D to the right of the platform, as shown in the diagram above. Which of the following explanations is true?

A The launcher will not fall off the platform because only the ball receives an applied force.

B The launcher will fall off the platform and land \(D_3\) to the left of the platform because the mass of the system is three times the mass of the ball.

C The launcher will fall off the platform and land \(D_2\) to the left of the platform because the launcher is twice the mass of the ball.

D The launcher will fall off the stand and land D to the left of the stand because the force is the same on both objects.

▶️Answer/Explanation

Ans:C

The launcher will have half the speed of the ball after the ball is launched. Evidence: Newton’s third law indicates that the force on the ball and the force on the launcher will be equal in magnitude. Reasoning Statement: The acceleration of the launcher will be half the acceleration of the ball, so the launcher will travel half the distance the ball travels.

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