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AP Physics C: Mechanics Practice Quiz: Newton's Third Law

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 10 questions to check your progress.

Question 1 of 10

A student pushes a heavy box across a rough floor. According to Newton's third law, the force exerted by the box on the student is:

All Questions (10)

A student pushes a heavy box across a rough floor. According to Newton's third law, the force exerted by the box on the student is:

A) equal in magnitude and in the same direction as the force the student exerts on the box.

B) equal in magnitude and in the opposite direction to the force the student exerts on the box.

C) less in magnitude than the force the student exerts on the box, because the student is causing the motion.

D) greater in magnitude than the force the student exerts on the box, due to friction.

Correct Answer: B

Newton's third law, represented by the equation $\vec{F}_{A \text{ on } B}=-\vec{F}_{B \text{ on } A}$, states that for every action, there is an equal and opposite reaction. The 'action' is the student pushing on the box ($\vec{F}_{\text{student on box}}$). The 'reaction' is the box pushing on the student ($\vec{F}_{\text{box on student}}$). These two forces form an interaction pair and must be equal in magnitude and opposite in direction.

A book is at rest on a horizontal tabletop. The gravitational force exerted by the Earth on the book is $\vec{F}_{g}$. What is the reaction force to $\vec{F}_{g}$ according to Newton's third law?

A) The normal force exerted by the tabletop on the book.

B) The gravitational force exerted by the book on the Earth.

C) The force of the book pushing down on the tabletop.

D) There is no reaction force since the book is not accelerating.

Correct Answer: B

Newton's third law describes paired forces between two interacting objects. The force $\vec{F}_{g}$ is the force of 'Earth on Book'. The corresponding third-law pair must be the force of 'Book on Earth'. The normal force is exerted by the table on the book, so its reaction pair is the book pushing on the table. While the normal force and gravity are equal and opposite in this specific case, they are not a Newton's third law pair because they both act on the same object (the book).

Two skaters, a 50 kg skater and an 80 kg skater, are initially at rest on frictionless ice. They push off each other. During the push, how does the magnitude of the force exerted on the 50 kg skater by the 80 kg skater compare to the force exerted on the 80 kg skater by the 50 kg skater?

A) The force on the 50 kg skater is greater.

B) The force on the 80 kg skater is greater.

C) The forces are equal in magnitude.

D) The forces are zero because they are internal to the system.

Correct Answer: C

According to Newton's third law, the forces two objects exert on each other are always equal in magnitude and opposite in direction. The interaction is between the two skaters. Therefore, the force the 80 kg skater exerts on the 50 kg skater is exactly equal in magnitude to the force the 50 kg skater exerts on the 80 kg skater. The difference in their masses will result in different accelerations (F=ma), but the forces themselves are equal.

A satellite orbits the Earth. Let $\vec{F}_{E \text{ on } S}$ represent the gravitational force exerted by the Earth on the satellite. According to the equation $\vec{F}_{A \text{ on } B}=-\vec{F}_{B \text{ on } A}$, what does $-\vec{F}_{E \text{ on } S}$ represent?

A) The net force on the satellite.

B) The gravitational force exerted by the satellite on the Earth.

C) The centripetal force required to keep the satellite in orbit.

D) The velocity vector of the satellite.

Correct Answer: B

The equation $\vec{F}_{A \text{ on } B}=-\vec{F}_{B \text{ on } A}$ defines a Newton's third law force pair. If A is the Earth (E) and B is the satellite (S), then $\vec{F}_{E \text{ on } S}$ is the force the Earth exerts on the satellite. The paired force, $\vec{F}_{S \text{ on } E}$, is the force the satellite exerts on the Earth. The equation shows that $\vec{F}_{S \text{ on } E} = -\vec{F}_{E \text{ on } S}$. Therefore, $-\vec{F}_{E \text{ on } S}$ represents the gravitational force exerted by the satellite on the Earth.

A system consists of two carts on a frictionless track connected by a compressed spring. When the spring is released, the carts move apart. Which of the following statements is true about the system's center of mass?

A) The center of mass accelerates in the direction of the more massive cart.

B) The center of mass accelerates in the direction of the less massive cart.

C) The velocity of the center of mass does not change.

D) The center of mass moves to a point midway between the two carts.

Correct Answer: C

The force exerted by the spring on the carts is an internal force to the system of the two carts and spring. The provided content states that 'Interactions between objects within a system (internal forces) do not influence the motion of a system's center of mass.' Since there are no external horizontal forces (the track is frictionless), the velocity of the system's center of mass remains constant. If it was initially at rest, it remains at rest.

A rope is stretched horizontally between two people. Person A pulls on the left end and Person B pulls on the right end, keeping the rope stationary. The force exerted by Person A on the rope is 100 N to the left. What is the tension in the middle of the rope?

A) 0 N

B) 50 N

C) 100 N

D) 200 N

Correct Answer: C

Tension is the result of forces that infinitesimal segments of the rope exert on each other. If you consider any point in the rope, the segment to the left of that point is pulling on the segment to the right, and vice versa. Since the rope is in equilibrium, the net force on any segment is zero. The tension throughout this ideal, stationary rope is uniform and equal to the magnitude of the external force applied at the ends, which is 100 N.

A large truck collides head-on with a small car. During the collision, which of the following is true?

A) The force exerted by the truck on the car is greater than the force exerted by the car on the truck.

B) The force exerted by the car on the truck is greater than the force exerted by the truck on the car.

C) The force exerted by the truck on the car is equal in magnitude to the force exerted by the car on the truck.

D) The truck exerts a force on the car, but the car does not exert a force on the truck.

Correct Answer: C

Newton's third law states that the forces between two interacting objects are always a paired force of equal magnitude and opposite direction. The truck and the car are the two interacting objects. Therefore, the force the truck exerts on the car is exactly equal in magnitude to the force the car exerts on the truck. The car experiences a much larger acceleration because it has a much smaller mass (F=ma).

Which of the following best describes tension in a cable that is pulling a block?

A) An external force that acts only at the end of the cable.

B) The net result of paired internal forces between adjacent segments of the cable.

C) A force that is always equal to the weight of the object being pulled.

D) The sum of all external forces acting on the cable-block system.

Correct Answer: B

The provided content defines tension as 'the macroscopic net result of forces that infinitesimal segments of a string, cable, chain, or similar system exert on each other in response to an external force.' This describes tension as a series of internal, paired forces consistent with Newton's third law acting all along the length of the cable.

A horse pulls a cart, causing it to accelerate. A student argues, 'According to Newton's third law, the cart pulls back on the horse with a force equal and opposite to the force the horse exerts on the cart. Since these forces are equal and opposite, they should cancel out, and no acceleration should be possible.' What is the flaw in this argument?

A) The horse's force on the cart is actually larger than the cart's force on the horse.

B) The action-reaction pair forces act on different objects and therefore cannot cancel each other out.

C) Friction from the ground is an internal force and does not affect the system's motion.

D) Newton's third law does not apply to objects that are accelerating.

Correct Answer: B

The student's argument is flawed because it attempts to cancel forces that act on different objects. The force of the horse on the cart acts *on the cart*. The force of the cart on the horse acts *on the horse*. To determine the acceleration of the cart, one must consider the net force *on the cart* (horse's pull vs. friction). To determine the acceleration of the horse, one must consider the net force *on the horse* (cart's pull vs. ground's push). The action-reaction pair can never cancel because they are not applied to the same object.

A bomb, initially at rest, explodes into many fragments. If the bomb and its fragments are considered a single system, what can be said about the motion of the system's center of mass immediately after the explosion?

A) The center of mass remains at its original position.

B) The center of mass accelerates in the direction of the largest fragment.

C) The center of mass moves with a constant velocity that is greater than zero.

D) The motion of the center of mass cannot be determined without knowing the mass of each fragment.

Correct Answer: A

The forces of the explosion are internal to the system of the bomb and its fragments. The content states that 'Interactions between objects within a system (internal forces) do not influence the motion of a system's center of mass.' Since the bomb was initially at rest, the velocity of its center of mass was zero. Because only internal forces are involved in the explosion, the velocity of the center of mass must remain zero. Therefore, the center of mass stays at its original position.