AP Physics 1 Quiz: Newtons Third Law

What this quiz covers

This quiz focuses on Newtons Third Law, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Physics 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Puck 1 exerts a greater force because it was moving initially.
2. Puck 2 exerts a greater force because it has greater mass.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The forces are unequal because only puck 2 experiences a net force at first.

Explanation: This question tests Newton's third law of motion. During the collision, the pucks exert forces on each other that are equal in magnitude and opposite in direction—puck 1 pushes on puck 2, and puck 2 pushes back on puck 1 with exactly the same magnitude of force. These interaction forces act on different objects and remain equal throughout the contact time. The fact that puck 1 was initially moving while puck 2 was stationary, or that they have different masses, doesn't affect this fundamental relationship. Choice D incorrectly suggests that initial motion states affect the interaction forces, confusing net force with interaction forces. Remember that Newton's third law applies to all interactions—the forces between any two objects are always equal in magnitude and opposite in direction.

Question 2

1. The rope's force on the sled is greater because the sled is moving forward.
2. The sled's force on the rope is greater because the sled is more massive.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. They are equal only if the sled's acceleration is zero.

Explanation: This question tests Newton's third law of motion. When the rope and sled interact, they exert forces on each other that are equal in magnitude and opposite in direction. The rope pulls forward on the sled, and the sled pulls backward on the rope with exactly the same magnitude of force. These interaction forces act on different objects and remain equal whether the sled is accelerating, moving at constant speed, or at rest. Choice D incorrectly suggests that the forces are only equal when acceleration is zero, confusing interaction forces with net force. Remember that Newton's third law applies to all interactions at all times—interaction forces are always equal in magnitude and opposite in direction.

Question 3

1. The rope pulls on the car with the same magnitude force that the car pulls on the rope, in opposite directions. (correct answer)
2. The rope pulls harder on the car than the car pulls on the rope because the car has larger mass.
3. The car pulls harder on the rope because it resists motion due to inertia.
4. The forces are equal only if the car's net force is zero; otherwise the rope's force must be larger.

Explanation: This question examines Newton's third law in tension forces during towing. Per Newton's third law, the force one object exerts on another is matched by an equal and opposite force from the second object. The rope pulls on the car with the same magnitude as the car pulls back on the rope, but in the opposite direction. These forces are on different objects—the rope and the car—and remain equal even at constant speed. Choice B incorrectly links force magnitude to mass, but third-law pairs are independent of mass differences. To generalize, always pair forces between two objects and verify equality before considering overall system dynamics like acceleration.

Question 4

1. The scale's force on the person is greater when the elevator is moving upward.
2. The forces are equal in magnitude and opposite in direction. (correct answer)
3. The person's force on the scale is always greater because of gravity.
4. The larger force is the one acting in the direction of the elevator's motion.

Explanation: This question tests Newton's third law of motion. The person's feet and the scale form an interaction pair—the feet push down on the scale, and the scale pushes up on the feet with a force equal in magnitude and opposite in direction. These forces act on different objects (one on the scale, one on the person) and are always equal at any given instant. Whether the elevator is accelerating upward, downward, or moving at constant velocity doesn't change this fundamental relationship between the interaction forces. Choice A incorrectly confuses the magnitude of these interaction forces with the scale reading, which can vary with acceleration. Remember that Newton's third law applies to all interactions—the forces between two objects are always equal in magnitude.

Question 5

1. The magnet's force on the nail is greater because the magnet is the source.
2. The nail's force on the magnet is greater because iron is attracted to magnets.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The larger force is on whichever object is moving toward the other.

Explanation: This question tests Newton's third law of motion. The magnet and nail form an interaction pair where the magnet pulls on the nail and the nail pulls back on the magnet with forces that are equal in magnitude and opposite in direction. These magnetic forces act on different objects and obey Newton's third law just like contact forces do. Even though the magnet might seem like the "source" of the attraction, both objects participate equally in the interaction. Choice A incorrectly suggests that being the source makes the magnet's force greater, but interaction forces are always mutual and equal. To apply Newton's third law correctly, remember it applies to all types of forces—gravitational, electromagnetic, and contact forces all produce equal and opposite interaction pairs.

Question 6

1. The magnet's force on the nail is greater because the magnet is the source.
2. The nail's force on the magnet is greater because iron is attracted to magnets.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The larger force is on whichever object is moving toward the other.

Explanation: This question tests Newton's third law of motion. The magnet and nail form an interaction pair where the magnet pulls on the nail and the nail pulls back on the magnet with forces that are equal in magnitude and opposite in direction. These magnetic forces act on different objects and obey Newton's third law just like contact forces do. Even though the magnet might seem like the "source" of the attraction, both objects participate equally in the interaction. Choice A incorrectly suggests that being the source makes the magnet's force greater, but interaction forces are always mutual and equal. To apply Newton's third law correctly, remember it applies to all types of forces—gravitational, electromagnetic, and contact forces all produce equal and opposite interaction pairs.

Question 7

1. A exerts a greater force because A has smaller mass.
2. B exerts a greater force because B has larger mass.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The larger force is on the cart that ends up with the larger speed.

Explanation: This question tests Newton's third law of motion. When the students push off each other, they form an interaction pair—Student A pushes on Student B, and Student B pushes back on Student A with a force equal in magnitude and opposite in direction. These forces act on different objects and are equal despite the 30 kg mass difference between the students. The different masses will result in different accelerations (the lighter student will accelerate more), but the forces remain equal. Choice B incorrectly suggests that greater mass creates a greater force, confusing the effect of mass on acceleration with the interaction forces themselves. Remember that Newton's third law applies universally—interaction forces are always equal in magnitude regardless of mass differences.

Question 8

1. The racket exerts a greater force because it is connected to the player's arm.
2. The ball exerts a greater force because it is moving faster.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. They are equal only if the racket is not accelerating.

Explanation: This question tests Newton's third law of motion. During the 0.005 s contact, the ball and racket exert forces on each other that are equal in magnitude and opposite in direction. The ball pushes on the racket strings, and the strings push back on the ball with exactly the same magnitude of force. These interaction forces act on different objects—one on the ball, one on the racket. Choice B incorrectly suggests that the ball's speed affects the relative magnitudes of these forces, but Newton's third law states that interaction forces are always equal regardless of the objects' velocities or accelerations. To correctly identify interaction pairs, remember that the forces always have equal magnitudes and act on different objects in the pair.

Question 9

1. A exerts a greater force because A has smaller mass.
2. B exerts a greater force because B has larger mass.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The larger force is on the cart that ends up with the larger speed.

Explanation: This question tests Newton's third law of motion. When the students push off each other, they form an interaction pair—Student A pushes on Student B, and Student B pushes back on Student A with a force equal in magnitude and opposite in direction. These forces act on different objects and are equal despite the 30 kg mass difference between the students. The different masses will result in different accelerations (the lighter student will accelerate more), but the forces remain equal. Choice B incorrectly suggests that greater mass creates a greater force, confusing the effect of mass on acceleration with the interaction forces themselves. Remember that Newton's third law applies universally—interaction forces are always equal in magnitude regardless of mass differences.

Question 10

1. The magnet exerts a larger force because magnets are the source of magnetic fields.
2. The paperclip exerts a larger force because it is made of steel.
3. The forces are equal in magnitude and opposite in direction. (correct answer)
4. Both forces are zero because the magnet and paperclip are not touching.

Explanation: This question tests understanding of Newton's third law for non-contact forces. The magnet exerts an attractive magnetic force on the paperclip, and simultaneously the paperclip exerts an equal magnitude attractive force back on the magnet. These interaction forces are always equal in magnitude and opposite in direction, even though they act at a distance without physical contact. The forces act on different objects: the magnet's force acts on the paperclip, while the paperclip's force acts on the magnet. Choice A incorrectly assumes the magnet exerts a larger force because it's the "source" of magnetism, but Newton's third law applies equally to all interactions. Remember that Newton's third law applies to all forces, whether contact or non-contact.

Question 11

1. The ball's force on the glove is greater because the ball changes direction.
2. The glove's force on the ball is greater because the glove has greater mass.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The larger force is on whichever object has the larger net force.

Explanation: This question tests Newton's third law of motion. When two objects interact, they exert forces on each other that are equal in magnitude and opposite in direction. These interaction forces act on different objects—the ball exerts a force on the glove, and the glove exerts an equal and opposite force on the ball. The fact that the ball changes direction or that the glove has greater mass doesn't affect this fundamental relationship between interaction forces. Choice A incorrectly suggests that a change in direction creates unequal forces. To apply Newton's third law, identify the interaction pair and remember that these forces are always equal in magnitude, regardless of the objects' masses, velocities, or accelerations.

Question 12

1. The floor's friction force on the crate is greater than the crate's friction force on the floor because the crate is moving.
2. The crate's friction force on the floor is greater because the person is pulling the crate forward.
3. The floor exerts a friction force on the crate equal in magnitude and opposite in direction to the crate's friction force on the floor. (correct answer)
4. The friction forces are equal only if the crate is at rest; if it moves, the floor's friction is larger.

Explanation: This question evaluates Newton's third law for friction in constant-speed motion. Newton's third law dictates that action-reaction forces are equal in magnitude and opposite in direction. The friction force the floor exerts on the crate is equal and opposite to the friction force the crate exerts on the floor. These forces are on distinct objects and remain equal even during motion at constant speed. Choice A errs by linking inequality to motion, but constant speed indicates balanced net forces, not unequal pairs. A general strategy is to isolate friction (or any) pairs via third law, then use first or second law for the object's overall motion.

Question 13

1. The wall's force on the swimmer is greater because the swimmer speeds up forward.
2. The swimmer's force on the wall is greater because the swimmer is the one exerting effort.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The forces are unequal because the wall does not move.

Explanation: This question tests Newton's third law of motion. During the turn, the swimmer's feet and the wall form an interaction pair—the feet push backward on the wall, and the wall pushes forward on the feet with a force equal in magnitude and opposite in direction. These forces act on different objects and remain equal throughout the contact time. The fact that the swimmer accelerates forward while the wall remains stationary doesn't violate this principle because the forces act on different objects with different masses. Choice D incorrectly suggests that the wall's lack of motion makes the forces unequal, confusing the effect of a force with the force itself. Remember that Newton's third law always applies—interaction forces are equal regardless of the resulting motion.

Question 14

1. The magnet exerts a greater force on the paper clip because the paper clip moves toward the magnet.
2. The paper clip exerts a greater force on the magnet because the magnet is doing the attracting.
3. The magnet exerts force on the paper clip, but the paper clip exerts no force on the magnet.
4. The magnet and paper clip exert forces on each other equal in magnitude and opposite in direction. (correct answer)

Explanation: This problem explores Newton's third law in non-contact forces like magnetism. The third law applies to all interactions, stating forces are equal in magnitude and opposite in direction. The magnet exerts a magnetic force on the paper clip equal and opposite to the force the paper clip exerts on the magnet. These forces act on different objects and are equal regardless of which moves or attracts. Distractor A mistakenly ties force inequality to observed motion, but motion results from net forces, not the pair. Broadly, extend third law to all force types by identifying pairs and confirming equality before considering causes of motion.

Question 15

1. The wall's force on the swimmer is greater because the swimmer speeds up forward.
2. The swimmer's force on the wall is greater because the swimmer is the one exerting effort.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The forces are unequal because the wall does not move.

Explanation: This question tests Newton's third law of motion. During the turn, the swimmer's feet and the wall form an interaction pair—the feet push backward on the wall, and the wall pushes forward on the feet with a force equal in magnitude and opposite in direction. These forces act on different objects and remain equal throughout the contact time. The fact that the swimmer accelerates forward while the wall remains stationary doesn't violate this principle because the forces act on different objects with different masses. Choice D incorrectly suggests that the wall's lack of motion makes the forces unequal, confusing the effect of a force with the force itself. Remember that Newton's third law always applies—interaction forces are equal regardless of the resulting motion.

Question 16

1. Puck 1 exerts a greater force because it was moving initially.
2. Puck 2 exerts a greater force because it has greater mass.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The forces are unequal because only puck 2 experiences a net force at first.

Explanation: This question tests Newton's third law of motion. During the collision, the pucks exert forces on each other that are equal in magnitude and opposite in direction—puck 1 pushes on puck 2, and puck 2 pushes back on puck 1 with exactly the same magnitude of force. These interaction forces act on different objects and remain equal throughout the contact time. The fact that puck 1 was initially moving while puck 2 was stationary, or that they have different masses, doesn't affect this fundamental relationship. Choice D incorrectly suggests that initial motion states affect the interaction forces, confusing net force with interaction forces. Remember that Newton's third law applies to all interactions—the forces between any two objects are always equal in magnitude and opposite in direction.

Question 17

1. The racket exerts a greater force because it is connected to the player's arm.
2. The ball exerts a greater force because it is moving faster.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. They are equal only if the racket is not accelerating.

Explanation: This question tests Newton's third law of motion. During the 0.005 s contact, the ball and racket exert forces on each other that are equal in magnitude and opposite in direction. The ball pushes on the racket strings, and the strings push back on the ball with exactly the same magnitude of force. These interaction forces act on different objects—one on the ball, one on the racket. Choice B incorrectly suggests that the ball's speed affects the relative magnitudes of these forces, but Newton's third law states that interaction forces are always equal regardless of the objects' velocities or accelerations. To correctly identify interaction pairs, remember that the forces always have equal magnitudes and act on different objects in the pair.

Question 18

1. The truck exerts a greater force because it has greater mass.
2. The car exerts a greater force because it undergoes a larger change in velocity.
3. The forces are equal in magnitude and opposite in direction. (correct answer)
4. The larger force is on the object that ends with the smaller speed.

Explanation: This question tests Newton's third law of motion. During the collision, the car and truck exert forces on each other that are equal in magnitude and opposite in direction. The car pushes on the truck, and the truck pushes back on the car with exactly the same magnitude of force throughout the 0.15 s contact time. These interaction forces act on different objects—one on the car, one on the truck. Choice B incorrectly suggests that a larger change in velocity creates a larger force, confusing the effect (acceleration) with the interaction forces themselves. To correctly apply Newton's third law, focus on the interaction pair and remember that these forces are always equal, regardless of mass differences or velocity changes.

Question 19

1. The cart pulls on the student with a force greater than 3.0 N because the student is more massive.
2. The student pulls on the cart with 3.0 N, and the cart pulls on the student with 3.0 N in the opposite direction. (correct answer)
3. The cart pulls on the student with 0 N because the cart's speed is constant.
4. The cart pulls on the student with less than 3.0 N because the cart is moving rightward.

Explanation: This question tests understanding of Newton's third law. When the student pulls on the cart with 3.0 N through the string, the cart simultaneously pulls back on the student with 3.0 N in the opposite direction. These interaction forces are always equal in magnitude and opposite in direction, regardless of the masses of the objects or their motion. The forces act on different objects: the student's force acts on the cart, while the cart's force acts on the student. Choice A incorrectly suggests that mass differences affect the interaction forces, when Newton's third law applies equally regardless of mass. To solve Newton's third law problems, identify the two interacting objects and remember that they always exert equal and opposite forces on each other.

Question 20

1. The 80 kg skater exerts a greater force because they have more mass.
2. The 50 kg skater exerts a greater force because they move away faster.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. The forces are unequal because each skater experiences a different net force.

Explanation: This question assesses understanding of Newton's third law of motion. Newton's third law states that for every action force, there is an equal and opposite reaction force. The force exerted by the 50 kg skater on the 80 kg skater and vice versa form an action-reaction pair. These forces are equal in magnitude but opposite in direction, and they act on different objects—the two skaters. A common distractor is choice A, which incorrectly claims the heavier skater exerts a greater force due to mass, but third-law pairs are equal regardless of mass. To apply Newton's third law effectively, identify the interacting objects and remember that their mutual forces are always equal and opposite, independent of other system details.