- Three blocks are in a line on a frictionless table: Block A touches Block B, and Block B touches Block C. A hand pushes Block A to the right, so A pushes on B, and B pushes on C. In this situation, which pair of forces is an action-reaction pair for the Block B–Block C contact interaction?
Answer: The rightward force of Block B on Block C and the leftward force of Block C on Block B
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, Block B and Block C are in contact, so Block B exerts a rightward push force on Block C, while simultaneously Block C exerts an equal push force on Block B in the leftward direction—these two forces form an action-reaction pair. Choice A is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice B identifies two forces that both act on the same object (Block B), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- A magnet is held near a paper clip without touching it, and the paper clip is pulled toward the magnet. Which pair of forces is an action-reaction pair for this magnetic interaction?
Answer: The magnetic force of the magnet on the paper clip and the magnetic force of the paper clip on the magnet
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the magnet exerts an attractive magnetic force on the paper clip, while simultaneously the paper clip exerts an equal attractive magnetic force on the magnet—these two forces form an action-reaction pair. Choice B is correct because it identifies forces that (1) act on different objects (magnet and paper clip), (2) involve the same interaction (magnetic attraction), (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A pairs a magnetic force with a gravitational force, but action-reaction pairs must be the same type of force resulting from the same interaction. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- A skater stands on a stationary skateboard and then pushes backward on the ground with one foot, causing the skater and skateboard to move forward. Which pair of forces is an action-reaction pair for the skater-ground interaction during the push?
Answer: The backward friction force of the skater on the ground and the forward friction force of the ground on the skater
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the skater and the ground are in contact, so the skater exerts a backward friction force on the ground, while simultaneously the ground exerts an equal forward friction force on the skater—these two forces form an action-reaction pair. Choice B is correct because it identifies forces that (1) act on different objects (skater and ground), (2) involve the same interaction (the push), (3) have equal magnitudes, and (4) point in opposite directions (backward and forward)—all required characteristics of action-reaction pairs. Choice A identifies two forces that both point forward, when action-reaction forces are always opposite in direction by Newton's Third Law. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- A sign hangs at rest from the ceiling by a vertical cable. Consider only the interaction between the cable and the sign (tension). Which pair of forces is an action-reaction pair?
Answer: The upward force of the cable on the sign and the downward force of the sign on the cable
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. For tension/support: the cable pulls upward on the sign through the tension, and the sign pulls downward on the cable through the same tension—the forces are transmitted through the connecting element but act on the two different objects. Choice B is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A pairs a contact force with a gravitational force, but action-reaction pairs must be the same type of force resulting from the same interaction. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- A book rests motionless on a table. Consider only the contact interaction between the book and the table. Which pair of forces is an action-reaction pair?
Answer: The upward normal force of the table on the book and the downward normal force of the book on the table
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the book and the table are in contact, so the table exerts an upward normal force on the book, while simultaneously the book exerts an equal downward normal force on the table—these two forces form an action-reaction pair. Choice B is correct because it identifies forces that (1) act on different objects (book and table), (2) involve the same interaction (the contact), (3) have equal magnitudes, and (4) point in opposite directions (upward and downward)—all required characteristics of action-reaction pairs. Choice A identifies two forces that both act on the same object (the book), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- A book rests motionless on a horizontal table. The book and table are in contact (support force). In this situation, which two forces form an action-reaction pair for the book–table interaction?
Answer: The downward force of the book on the table and the upward force of the table on the book
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the book and the table are in contact, so the book exerts a downward normal force on the table, while simultaneously the table exerts an equal normal force on the book in the upward direction—these two forces form an action-reaction pair. Choice A is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice B identifies two forces that both act on the same object (the book), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- A chandelier hangs from the ceiling by a chain. The chain pulls up on the chandelier, and the chandelier pulls down on the chain. In this situation, which pair of forces is an action-reaction pair for the interaction between the chandelier and the chain?
Answer: The upward tension force of the chain on the chandelier and the downward force of the chandelier on the chain
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. For tension/support: the chain pulls up on the chandelier with tension force, and the chandelier pulls down on the chain through the same chain with equal force—the forces are transmitted through the connecting element but act on the two different objects. Choice B is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A identifies two forces that both act on the same object (the chandelier), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- Two blocks are in contact on a horizontal surface. Block 1 is pushed to the right (→) and it presses against Block 2, causing Block 2 to accelerate to the right. In this situation, which pair of forces is an action-reaction pair for the contact between Block 1 and Block 2?
Answer: The rightward force of Block 1 on Block 2 and the leftward force of Block 2 on Block 1
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, Block 1 and Block 2 are in contact, so Block 1 exerts a rightward force on Block 2, while simultaneously Block 2 exerts an equal force on Block 1 in the opposite (leftward) direction—these two forces form an action-reaction pair. Choice A is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice B claims the forces act in the same direction, when action-reaction forces are always opposite in direction by Newton's Third Law. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- Two students, Student A and Student B, pull on opposite ends of the same rope in a tug-of-war. Focus on the interaction between Student A and the rope at A's hands. Which pair of forces is an action-reaction pair?
Answer: The force of Student A on the rope and the force of the rope on Student A
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. For tension/support: Student A pulls on the rope with a force, and the rope pulls back on Student A through the same tension—the forces are transmitted through the connecting element but act on the two different objects. Choice B is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A pairs forces from different interactions (such as Student A on rope and Student B on rope), when action-reaction pairs must involve the same interaction between two specific objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- A book rests motionless on a table. The book pushes down on the table, and the table pushes up on the book. In this situation, which two forces form an action-reaction pair for the contact interaction between the book and the table?
Answer: The upward normal force of the table on the book and the downward normal force of the book on the table
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the book and the table are in contact, so the book exerts a downward normal force on the table, while simultaneously the table exerts an equal normal force on the book in the opposite (upward) direction—these two forces form an action-reaction pair. Choice C is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A identifies two forces that both act on the same object (the book), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- A moving soccer ball strikes a stationary wall. During the brief contact, the ball pushes on the wall. Based on this collision, which pair of forces is an action-reaction pair according to Newton's Third Law?
Answer: The force of the ball on the wall and the force of the wall on the ball
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the soccer ball and the wall are in contact, so the ball exerts a force on the wall in one direction, while simultaneously the wall exerts an equal force on the ball in the opposite direction—these two forces form an action-reaction pair. Choice C is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A pairs forces from different interactions (such as the contact force and gravitational force), when action-reaction pairs must involve the same interaction between two specific objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Quick check for action-reaction pairs: (1) Do they act on different objects? (2) Are they the same type of force? (3) Do they have equal magnitudes? (4) Do they point in opposite directions? All four must be true.
- A person stands still on a horizontal floor. Focus on the contact interaction between the person and the floor. Which pair of forces is an action-reaction pair?
Answer: The downward force of the person on the floor and the upward force of the floor on the person
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the person and the floor are in contact, so the person exerts a downward normal force on the floor, while simultaneously the floor exerts an equal normal force on the person in the upward direction—these two forces form an action-reaction pair. Choice C is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A identifies two forces that both act on the same object (the person), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- A small satellite orbits Earth. Ignoring all other objects, which pair of forces is the Newton's Third Law action-reaction pair for their gravitational interaction?
Answer: The gravitational force of Earth on the satellite and the gravitational force of the satellite on Earth
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. Gravitational forces are mutual: Earth pulls on the satellite with force F, and the satellite pulls on Earth with the same force F in the opposite direction—even though Earth is much more massive, the forces are equal. Choice A is correct because it identifies forces that (1) act on different objects (Earth and satellite), (2) involve the same interaction (gravity), (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice B pairs a gravitational force with a normal force, but action-reaction pairs must be the same type of force resulting from the same interaction. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Quick check for action-reaction pairs: (1) Do they act on different objects? (2) Are they the same type of force? (3) Do they have equal magnitudes? (4) Do they point in opposite directions? All four must be true.
- A satellite orbits Earth. The only significant interaction is gravity. In this situation, which pair of forces is an action-reaction pair?
Answer: The gravitational force of Earth on the satellite (toward Earth), and the gravitational force of the satellite on Earth (toward the satellite)
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. For gravitational forces: Gravitational forces are mutual: Earth pulls on the satellite with force toward Earth, and the satellite pulls on Earth with the same force toward the satellite—even though Earth is much more massive, the forces are equal. Choice A is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice C describes only one force in the pair, or identifies both forces acting on the same object, missing the essential feature that action-reaction forces act on different interacting objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Quick check for action-reaction pairs: (1) Do they act on different objects? (2) Are they the same type of force? (3) Do they have equal magnitudes? (4) Do they point in opposite directions? All four must be true.
- A soccer ball moving to the right (→) collides with a stationary wall. During the brief contact, which pair of forces is an action-reaction pair?
Answer: The leftward force of the wall on the ball, and the rightward force of the ball on the wall
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the ball and the wall are in contact, so the ball exerts a rightward force on the wall, while simultaneously the wall exerts an equal leftward force on the ball—these two forces form an action-reaction pair. Choice B is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A claims the forces act in the same direction, when action-reaction forces are always opposite in direction by Newton's Third Law. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- A magnet on a table attracts a nearby paper clip without touching it. The paper clip accelerates toward the magnet. In this situation, which pair of forces is an action-reaction pair for the magnet–paper clip interaction?
Answer: The magnetic force of the magnet on the paper clip (toward the magnet), and the magnetic force of the paper clip on the magnet (toward the paper clip)
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. Among all the forces in this scenario, the action-reaction pair for the magnet-paper clip interaction involves the magnet exerting a magnetic force on the paper clip toward the magnet, and the paper clip exerting the same type of force back on the magnet toward the paper clip—these are the only two forces that are equal, opposite, and act on different objects in this particular interaction. Choice B is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A pairs forces from different interactions (such as magnetic and gravitational), when action-reaction pairs must involve the same interaction between two specific objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Quick check for action-reaction pairs: (1) Do they act on different objects? (2) Are they the same type of force? (3) Do they have equal magnitudes? (4) Do they point in opposite directions? All four must be true.
- A hammer strikes a nail and is in contact with the nail for a short time during the impact. Consider only the hammer–nail interaction during contact. Which pair of forces is an action-reaction pair?
Answer: The downward force of the hammer on the nail and the upward force of the nail on the hammer
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the hammer and the nail are in contact, so the hammer exerts a downward contact force on the nail, while simultaneously the nail exerts an equal contact force on the hammer in the upward direction—these two forces form an action-reaction pair. Choice A is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice C identifies two forces that both act on the same object (the nail), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- Two blocks are in contact on a frictionless surface. Block 1 is pushed to the right (→) and presses on Block 2, causing both to accelerate rightward. Considering only the Block 1–Block 2 contact interaction, which pair of forces is an action-reaction pair?
Answer: The rightward force of Block 1 on Block 2, and the leftward force of Block 2 on Block 1
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, Block 1 and Block 2 are in contact, so Block 1 exerts a rightward force on Block 2, while simultaneously Block 2 exerts an equal leftward force on Block 1—these two forces form an action-reaction pair. Choice B is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A claims the forces act in the same direction, when action-reaction forces are always opposite in direction by Newton's Third Law. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
- A person stands still on a flat floor. The person pushes down on the floor, and the floor pushes up on the person. Based on this contact interaction, which pair of forces is an action-reaction pair?
Answer: The upward normal force of the floor on the person and the downward force of the person on the floor
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. In this scenario, the person and the floor are in contact, so the person exerts a downward force on the floor, while simultaneously the floor exerts an equal normal force on the person in the opposite (upward) direction—these two forces form an action-reaction pair. Choice C is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A identifies two forces that both act on the same object (the person), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. Remember this key distinction: the normal force and weight on a book resting on a table are NOT an action-reaction pair (they both act on the book), but the normal force the book exerts downward on the table and the normal force the table exerts upward on the book ARE an action-reaction pair (they act on different objects).
- Two students, Student A and Student B, play tug-of-war using a rope. Student A pulls the rope to the left (←) while Student B pulls to the right (→). Focusing on the Student A–rope interaction, which pair of forces is an action-reaction pair?
Answer: The leftward force of Student A on the rope, and the rightward force of the rope on Student A
This question tests understanding of Newton's Third Law and the ability to identify action-reaction force pairs. Newton's Third Law states that whenever object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A—these two forces are called an action-reaction pair, and they always act on different objects. For tension/support: Student A pulls on the rope with a leftward force, and the rope pulls back on Student A with an equal rightward force—the forces are transmitted through the connecting element but act on the two different objects. Choice C is correct because it identifies forces that (1) act on different objects, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice A identifies two forces that both act on the same object (Student A or the rope), which makes them balanced forces, not an action-reaction pair—action-reaction forces must act on different objects. To identify action-reaction pairs: (1) find two objects that are interacting, (2) identify the force that Object A exerts on Object B, (3) identify the equal-but-opposite force that Object B exerts on Object A—these two forces form the pair. A common mistake is confusing action-reaction pairs with balanced forces: balanced forces act on one object and may cancel to produce zero net force, while action-reaction forces act on different objects and never cancel each other because they affect different objects' motions.
