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?
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Physics Quiz
Practice Identify Action Reaction Force Pairs in Physics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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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?
This quiz focuses on Identify Action Reaction Force Pairs, giving you a quick way to practice the rules, question types, and explanations that matter most for Physics.
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.
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?
Explanation: 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).
A ball is dropped and is falling toward Earth (ignore air resistance). The only interaction is gravitational attraction between the ball and Earth. Which pair of forces is an action-reaction pair?
Explanation: 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 downward on the ball with force F, and the ball pulls upward 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, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice D suggests one force is larger than the other, but action-reaction pairs are always equal in magnitude regardless of object masses or which object wins in a competition. 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.
In a warehouse, a person pushes a box to the right across a level floor. The person’s hands are in contact with the box, so the interaction is a contact push. Which pair of forces is an action-reaction pair according to Newton’s Third Law for the person–box interaction?
Explanation: 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 box are in contact, so the person exerts a rightward push force on the box, while simultaneously the box exerts an equal push force on the person in the leftward direction—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 C 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?
Explanation: 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 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?
Explanation: 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 chandelier hangs at rest from the ceiling by a chain. Consider the chain–chandelier interaction. Which pair of forces is an action-reaction pair?
Explanation: 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 upward on the chandelier with tension force, and the chandelier pulls downward on the chain with an 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 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. 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?
Explanation: 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 person stands still on a floor. Consider only the person–floor contact interaction (support force). Which pair of forces is an action-reaction pair?
Explanation: 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 contact force on the floor, while simultaneously the floor exerts an equal upward normal force on the person—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 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 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?
Explanation: 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 book rests motionless on a table. Consider only the book–table contact interaction. Which pair of forces is an action-reaction pair?
Explanation: 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 contact force on the table, while simultaneously the table exerts an equal upward normal force on the book—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 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 car tows a trailer using a rigid hitch. Consider the interaction at the hitch while the car pulls the trailer forward. Which pair of forces is an action-reaction pair?
Explanation: 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. The car pulls forward on the trailer through the hitch with force F, and the trailer pulls backward on the car through the same hitch with equal force F—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 (car and trailer), (2) involve the same interaction (the pull at the hitch), (3) have equal magnitudes, and (4) point in opposite directions (forward and backward)—all required characteristics of action-reaction pairs. Choice A pairs forces from different interactions (the car-trailer interaction and the car-road interaction), 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 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?
Explanation: 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).
In a warehouse, a worker pushes a heavy crate across a smooth floor. The worker pushes the crate to the right (→) with their hands. Based on this interaction, which pair of forces is an action-reaction pair according to Newton’s Third Law (for the push between the worker and the crate)?
Explanation: 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 worker and the crate are in contact, so the worker exerts a rightward force on the crate, while simultaneously the crate exerts an equal force on the worker in the opposite (leftward) direction—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 satellite orbits Earth. Earth’s gravity pulls the satellite toward Earth. In this gravitational interaction, which pair of forces is an action-reaction pair?
Explanation: 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 F toward Earth, 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, (2) involve the same interaction, (3) have equal magnitudes, and (4) point in opposite directions—all required characteristics of action-reaction pairs. Choice D identifies two forces that both act on the same object (Earth), which makes them invalid as 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 A is held near magnet B so that they repel each other. Magnet A experiences a force to the left (←) due to magnet B. In this situation, which pair of forces is an action-reaction pair?
Explanation: 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, magnet B exerts a leftward magnetic force on magnet A, while simultaneously magnet A exerts an equal magnetic force on magnet B in the opposite (rightward) direction—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. 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 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?
Explanation: 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).
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?
Explanation: 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.
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?
Explanation: 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.
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?
Explanation: 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 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?
Explanation: 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.