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 exerts a smaller force because it initially was not moving.
2. The ball exerts a larger force because the ball accelerates more than the foot.
3. The ball exerts an equal-magnitude force on the foot in the opposite direction. (correct answer)
4. The ball exerts no force on the foot after the foot starts moving forward.

Explanation: This question tests understanding of Newton's third law during a collision. When the foot exerts a force on the ball, the ball simultaneously exerts an equal magnitude force back on the foot in the opposite direction. These interaction forces exist during the entire contact time and are always equal, regardless of which object was initially moving or which accelerates more. The forces act on different objects: the foot's force acts on the ball, while the ball's force acts on the foot. Choice B incorrectly relates the forces to the accelerations, when Newton's third law concerns only the interaction forces themselves. Remember that Newton's third law applies at every instant during an interaction, not just before or after contact.

Question 12

1. The chair’s force on the person is larger because it supports the person’s weight.
2. The person’s force on the chair is larger because gravity acts on the person.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. They are equal only if the person’s weight is 0 N.

Explanation: This question tests understanding of Newton's third law for normal forces. The person pushes down on the chair, and the chair pushes up on the person with exactly the same magnitude force. These interaction forces are always equal in magnitude and opposite in direction, regardless of the person's weight or any other forces acting. The forces act on different objects: the person's force acts on the chair, while the chair's force acts on the person. Choice A incorrectly suggests the chair's force is larger because it "supports" the person, confusing the interaction force with the equilibrium of forces on the person. When applying Newton's third law, remember it concerns only the forces between two interacting objects, not the balance of all forces on one object.

Question 13

1. Student A exerts a larger force on Student B because Student A is less massive.
2. Student B exerts a larger force on Student A because Student B is more massive.
3. They exert equal-magnitude forces on each other in opposite directions. (correct answer)
4. The forces are equal only if both students move away at the same speed.

Explanation: This question tests understanding of Newton's third law during a push interaction. When Student A pushes Student B, Student B simultaneously pushes back on Student A with exactly the same magnitude force but in the opposite direction. These interaction forces are always equal regardless of the different masses of the students. The forces act on different objects: Student A's force acts on Student B, while Student B's force acts on Student A. Choice A incorrectly suggests that the less massive student exerts a larger force, confusing the effect of force (acceleration) with the force itself. To correctly apply Newton's third law, focus on the interaction forces themselves, not on the resulting accelerations or velocities.

Question 14

1. The table on the book is greater because it supports the book.
2. They are equal in magnitude and opposite in direction. (correct answer)
3. The book on the table is greater because the book has weight.
4. The forces are equal in direction because neither object moves.

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 of the table on the book and the force of the book on the table are an action-reaction pair. These forces are equal in magnitude but opposite in direction, and they act on different objects—the table and the book. A common distractor is choice A, which mistakenly suggests the table's force is greater because it supports the book, overlooking that support forces are third-law pairs and thus equal. 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.

Question 15

1. It is 120 N in the opposite direction. (correct answer)
2. It is less than 120 N because the wall does not move.
3. It is greater than 120 N because the wall is massive.
4. It is 0 N because the student is the only object pushing.

Explanation: This question tests Newton's third law of motion. When the student pushes on the wall with 120 N, the wall simultaneously pushes back on the student—these forces form an interaction pair. According to Newton's third law, interaction forces are always equal in magnitude and opposite in direction, acting on different objects (the push on the wall and the push on the student). The wall pushes back with exactly 120 N in the opposite direction to the student's push. Choice B incorrectly suggests that motion affects force magnitude, but Newton's third law applies regardless of whether objects move. The key strategy is to identify interaction pairs: when object A pushes on object B with force F, object B always pushes back on object A with force F in the opposite direction.

Question 16

1. The road’s force is larger because the car accelerates forward.
2. The tires’ force is larger because the engine provides power.
3. They are equal in magnitude and opposite in direction. (correct answer)
4. They are equal in magnitude and in the same direction.

Explanation: This question tests Newton's third law of motion. The tires and road exert forces on each other that form an interaction pair. According to Newton's third law, these interaction forces are always equal in magnitude and opposite in direction—as the tires push backward on the road, the road simultaneously pushes forward on the tires with equal force. These forces act on different objects (one on the tires/car, one on the road) and represent the friction interaction between them. Choice A incorrectly suggests that the car's acceleration means the road exerts more force, but Newton's third law requires equal forces (the car accelerates because the forward force from the road is unbalanced by other forces on the car). The key strategy is to identify force pairs: the force of A on B always equals the force of B on A in magnitude, regardless of motion.

Question 17

1. The 2.0 kg cart exerts a larger force on the 0.5 kg cart.
2. The 0.5 kg cart exerts a larger force on the 2.0 kg cart.
3. Each cart exerts the same magnitude force on the other. (correct answer)
4. The cart that is moving faster exerts the larger force.

Explanation: This question tests Newton's third law of motion. When the 2.0 kg cart collides with the 0.5 kg cart through the spring, the carts form an interaction pair—the 2.0 kg cart pushes on the 0.5 kg cart, and simultaneously the 0.5 kg cart pushes back on the 2.0 kg cart. According to Newton's third law, these interaction forces are always equal in magnitude and opposite in direction, regardless of the masses of the objects. The forces act on different objects (one force on each cart) and exist only during the contact time. Choice A incorrectly assumes the more massive object exerts a larger force, confusing force with the resulting acceleration (which will be different for each cart due to F=ma). The key strategy is to remember that Newton's third law forces are always equal in magnitude—what differs between objects of different masses is their acceleration, not the force.

Question 18

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 19

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 20

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.