Newton's Third Law - College Physics

A baseball bat strikes a baseball. During the collision, the bat exerts $1500\text{ N}$ on the ball for $0.002\text{ s}$ . Which statement about the force the ball exerts on the bat is correct?
Answer: The ball exerts $1500\text{ N}$ on the bat for $0.002\text{ s}$ in the opposite direction When you encounter collision problems, you're dealing with Newton's Third Law of Motion, which states that for every action, there's an equal and opposite reaction. This fundamental principle applies to all interactions between objects, regardless of their masses, materials, or other properties. During the bat-ball collision, these objects form an interaction pair. Newton's Third Law requires that the forces they exert on each other must be equal in magnitude and opposite in direction, and they must act for exactly the same duration. Since the bat exerts $1500\text{ N}$ on the ball for $0.002\text{ s}$ , the ball must exert $1500\text{ N}$ on the bat for $0.002\text{ s}$ in the opposite direction. Let's examine why the other options are incorrect. Option B suggests the ball exerts force for a longer time because it's softer, but Newton's Third Law doesn't allow for different time durations—the forces must be simultaneous. Option C claims the ball exerts less force because the bat is more massive, but force magnitude in Newton's Third Law pairs is independent of mass differences. Option D states the ball exerts the same force magnitude but for less time due to its smaller mass, which again violates the requirement that Third Law force pairs act for identical time periods. Remember this key point: Newton's Third Law force pairs are always equal in magnitude, opposite in direction, and simultaneous in timing, regardless of the masses, materials, or other properties of the interacting objects. Don't let these physical differences distract you from this fundamental principle.
A rocket in space fires its engines, ejecting gas at high speed. The rocket has a mass of 1000 kg and ejects 10 kg of gas at 500 m/s relative to the rocket. What is the magnitude of the force that the ejected gas exerts on the rocket during the 2.0 s burn time?
Answer: 2500 N, because this equals the momentum change of the gas divided by the burn time By Newton's third law, the force the gas exerts on the rocket equals the force the rocket exerts on the gas. The gas gains momentum Δp = (10 kg)(500 m/s) = 5000 kg⋅m/s over 2.0 s. Using F = Δp/Δt gives F = 5000/2.0 = 2500 N. This is the magnitude of force in both directions of the third law pair. Choice B unnecessarily complicates with variable mass (not needed for this calculation). Choice C confuses the momentum with the force. Choice D calculates the rocket's acceleration but misses that third law forces are equal in magnitude.
Two ice skaters, initially at rest, push off against each other. Skater A (mass $60\text{ kg}$ ) moves away at $2.0\text{ m/s}$ . During the push-off, which statement about the forces is correct according to Newton's third law?
Answer: The force that A exerts on B equals the force that B exerts on A in magnitude When you encounter problems involving two objects interacting (like colliding, pushing off, or connected by strings), Newton's third law is often the key concept being tested. This law states that forces always come in pairs: when object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. In this ice skating scenario, during the brief moment when the skaters are pushing off against each other, they form an action-reaction pair. The force that skater A applies to skater B is exactly equal in magnitude to the force that skater B applies to skater A—they just point in opposite directions. This equality holds regardless of what happens after the interaction, making choice A correct. Choice B incorrectly assumes that the final velocities determine the forces during interaction. While skater A moves faster afterward, this is because of the mass difference, not because A exerted more force. Choice C represents a common misconception that forces depend on mass. Newton's third law guarantees equal force magnitudes regardless of the objects' masses—the different masses explain why the accelerations and final velocities differ. Choice D incorrectly suggests that force equality depends on the masses and velocities being equal, when in fact Newton's third law applies universally to all interactions. Remember this pattern: Newton's third law force pairs are always equal in magnitude during the interaction itself. The different outcomes (velocities, accelerations) result from Newton's second law ( $F = ma$ ) acting on different masses, not from unequal forces.
A student sits in a chair. The chair exerts an upward normal force of $600\text{ N}$ on the student. Identify the Newton's third law pair to this force.
Answer: The student exerts a downward force of $600\text{ N}$ on the chair When you encounter Newton's third law problems, focus on identifying action-reaction pairs between two specific objects that are in direct contact and exerting forces on each other. Newton's third law states that forces always come in pairs: when object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. The key is that these forces act on different objects and involve the same type of interaction. Here, the chair exerts an upward normal force of 600 N on the student. For the third law pair, you need the student exerting a force back on the chair. When the student sits, they press down on the chair with 600 N due to contact between their body and the chair surface. This makes choice A correct. Choice B describes Earth's gravitational pull on the student. While this force might equal 600 N, it's not the third law pair because it involves Earth pulling on the student, not the student pushing back on the chair. This is a different interaction entirely. Choice C describes the floor supporting the chair. This involves the floor and chair, not the student and chair, so it's not the relevant action-reaction pair for the given force. Choice D restates the gravitational force concept from B in different words. The student's weight is still Earth pulling on the student, not the student's response to the chair's normal force. Remember: Newton's third law pairs always involve the same two objects exerting forces on each other, with the forces being the same type of interaction (contact, gravitational, etc.).
A ball is thrown upward and caught at the same height. At the moment of release, the thrower's hand exerts $25\text{ N}$ upward on the ball. What is the Newton's third law pair to this force?
Answer: The ball exerts $25\text{ N}$ downward on the thrower's hand at the moment of release Newton's third law is one of the most fundamental principles in physics: for every action, there is an equal and opposite reaction. The key insight is that these force pairs always act on different objects and occur simultaneously. When the thrower's hand exerts $25\text{ N}$ upward on the ball, Newton's third law requires an immediate, equal, and opposite force. The ball must simultaneously exert $25\text{ N}$ downward on the thrower's hand. This makes choice A correct. Let's examine why the other options miss the mark. Choice B incorrectly identifies gravity as the third law pair. While gravity does act on the ball, it's a separate force entirely and doesn't equal $25\text{ N}$ unless the ball weighs exactly that much. More importantly, gravity acts between the ball and Earth, not between the ball and hand. Choice C suggests air resistance as the pair force, but air resistance is much smaller than $25\text{ N}$ for a typical thrown ball and acts between the ball and air molecules, not the hand. Choice D describes a force during catching, which occurs at a completely different time than the release. The crucial pattern to remember: Newton's third law pairs always involve the same two objects exerting equal and opposite forces on each other at the same instant. When you see a Newton's third law question, immediately ask yourself "what two objects are interacting?" and "what force does each exert on the other?" This will help you avoid the common trap of confusing third law pairs with other forces acting on the same object.
A rocket in space fires its engines, ejecting gas molecules backward with a total force of $10,000\text{ N}$ . What force do the ejected gas molecules exert on the rocket according to Newton's third law?
Answer: $10,000\text{ N}$ forward, providing the thrust that propels the rocket When you encounter rocket propulsion problems, you're dealing with Newton's third law: for every action, there's an equal and opposite reaction. The key insight is understanding which forces act on which objects and their directions. The rocket engines exert $10,000\text{ N}$ of force on the gas molecules, pushing them backward. By Newton's third law, the gas molecules must exert an equal and opposite force on the rocket. Since the rocket pushes gas backward with $10,000\text{ N}$ , the gas pushes the rocket forward with exactly $10,000\text{ N}$ . This forward force on the rocket is what we call thrust. Choice A correctly identifies both the magnitude ( $10,000\text{ N}$ ) and direction (forward) of this reaction force, which indeed provides the rocket's propulsion. Choice B has the right magnitude but wrong direction. The gas molecules don't push the rocket backward—that would prevent the rocket from accelerating forward. Choice C incorrectly assumes energy losses affect the force magnitude. While engines do lose energy as heat, Newton's third law pairs are always equal in magnitude regardless of efficiency losses. The $10,000\text{ N}$ refers to the actual force exerted, not the theoretical maximum. Choice D wrongly suggests the reaction force exceeds the action force. Newton's third law pairs are always exactly equal in magnitude—the absence of air resistance affects the rocket's acceleration but not the force pair itself. Remember: Newton's third law force pairs always have equal magnitudes and opposite directions, acting on different objects. In space propulsion, the rocket-gas force pair enables forward motion despite having no external surface to "push against."
A person jumps off a boat onto a dock. During the jump, the person's feet exert $600\text{ N}$ on the boat. According to Newton's third law, what force does the boat exert on the person during the jump?
Answer: $600\text{ N}$ in the direction opposite to the person's push, providing the reaction force When you encounter problems involving forces between objects, Newton's third law is the key principle: for every action, there is an equal and opposite reaction. This means forces always come in pairs that are equal in magnitude but opposite in direction. In this scenario, when the person's feet push against the boat with $600\text{ N}$ of force, the boat simultaneously pushes back on the person's feet with exactly $600\text{ N}$ of force in the opposite direction. This reaction force is what propels the person toward the dock. The forces are equal in magnitude regardless of the masses of the objects or how they subsequently move. Option A correctly identifies that the boat exerts $600\text{ N}$ opposite to the person's push, which is the reaction force described by Newton's third law. Option B incorrectly suggests the forces act in the same direction. Newton's third law specifically states the forces are opposite in direction - if they were in the same direction, both the person and boat would accelerate in the same direction, which contradicts what we observe. Option C wrongly assumes the boat's backward motion reduces the force magnitude. Newton's third law guarantees equal force magnitudes regardless of how the objects subsequently accelerate or move. Option D mistakenly thinks the force must be larger to accelerate the person. The $600\text{ N}$ reaction force is what causes the person's acceleration - no additional force is needed. Remember: Newton's third law pairs are always equal in magnitude and opposite in direction, regardless of object masses or resulting motion. Focus on identifying the action-reaction pair, not the consequences of those forces.
Two astronauts in space are connected by a rope. Astronaut A pulls on the rope with $20\text{ N}$ of force. According to Newton's third law, what force does the rope exert on astronaut A?
Answer: $20\text{ N}$ in the direction opposite to A's pull, acting as tension When you encounter problems involving forces and Newton's laws, focus on identifying action-reaction pairs and remembering that these forces are always equal in magnitude but opposite in direction. Newton's third law states that for every action, there is an equal and opposite reaction. When astronaut A pulls on the rope with $20\text{ N}$ , the rope simultaneously exerts $20\text{ N}$ back on astronaut A in the opposite direction. This isn't a consequence of the pull—it happens instantaneously as part of the same interaction. The rope's tension force on astronaut A is exactly $20\text{ N}$ directed opposite to A's pulling force. Let's examine why the other options fail: Option B incorrectly suggests the rope assists astronaut A by pulling in the same direction, which would violate Newton's third law. Action-reaction pairs must be opposite in direction. Option C makes the common error of thinking an object's mass affects the magnitude of action-reaction forces. While the rope's low mass means it will accelerate easily, this doesn't change the force magnitudes in the action-reaction pair. Option D incorrectly implies that space somehow amplifies forces, but Newton's laws work the same way in space as on Earth. The correct answer is A: the rope exerts $20\text{ N}$ opposite to A's pull. Study tip: Action-reaction forces always occur in pairs with equal magnitudes and opposite directions, regardless of the masses involved or the environment. When you see Newton's third law problems, immediately identify what's pushing or pulling on what, then find the equal and opposite partner force.
The student is confused because they expected each scale to read 10 N (half the applied force). Using Newton's third law, explain why both scales read 20 N and identify the correct force pairs in this system.
Answer: Each scale reads 20 N because they are in series, so the same tension exists throughout. The force pairs include the 20 N tension forces between the scales and the 20 N forces between each scale and its attachments When you encounter tension problems with objects connected in series, the key insight is that tension is the same throughout a continuous system in equilibrium. Think of tension as being "transmitted" through the entire chain of connections. In this setup, both spring scales experience the same 20 N tension force. Here's why: when you pull with 20 N, that force must be balanced by an equal and opposite 20 N force from the wall (since the system is in equilibrium). This 20 N tension exists throughout the entire series connection - it's not divided between the scales, but rather each scale experiences the full tension. Both scales read 20 N because they're each measuring this same tension force. The Newton's third law pairs include: the 20 N tension force between the scales (each scale pulls on the other with 20 N), the 20 N force between scale 1 and the wall, and the 20 N force between scale 2 and your hand. Choice A incorrectly describes the force pairs - it doesn't properly identify the tension forces between the scales themselves. Choice B incorrectly invokes "resonance effects," which don't apply to static force measurements, and misidentifies the third law pairs as internal spring forces rather than the external interaction forces. Choice C contains the fundamental misconception that forces divide in series connections - this would only be true if the scales had significant mass and were accelerating, but in equilibrium, tension remains constant throughout. Remember: in series connections under equilibrium, tension is transmitted, not divided. Always identify all the action-reaction pairs when analyzing force systems.
A rope is used to pull a sled across ice. The rope exerts a force of $80\text{ N}$ on the sled. According to Newton's third law, what force does the sled exert on the rope?
Answer: $80\text{ N}$ in the direction opposite to the rope's pull on the sled When you encounter questions about forces between objects, you're dealing with Newton's third law: for every action, there is an equal and opposite reaction. This fundamental principle states that forces always come in pairs between interacting objects. The rope pulls on the sled with $80\text{ N}$ of force in some direction. By Newton's third law, the sled must simultaneously pull back on the rope with exactly $80\text{ N}$ of force in the opposite direction. These forces are equal in magnitude but opposite in direction—this is what makes them an action-reaction pair. Looking at the wrong answers: Choice B incorrectly suggests the sled pulls in the same direction as the rope, which would violate the "opposite" requirement of Newton's third law. Choice C falls into the trap of thinking that because the sled is being pulled, it somehow exerts less force—but Newton's third law guarantees equal magnitudes regardless of which object is "doing the pulling." Choice D incorrectly brings friction into the action-reaction pair. While friction between the sled and ice is certainly present, it's a separate force that doesn't affect the magnitude of the action-reaction forces between rope and sled. Choice A correctly identifies that the sled exerts $80\text{ N}$ on the rope in the direction opposite to the rope's pull. Remember this key insight: Newton's third law action-reaction pairs always involve the same two objects exerting equal and opposite forces on each other. Don't let other forces in the problem (like friction or gravity) distract you from identifying the specific pair.
A person stands on a bathroom scale inside an elevator that is accelerating upward at 2.0 m/s². The scale reads 750 N. According to Newton's third law, what force does the person exert on the scale?
Answer: 750 N downward, because the person pushes down on the scale with the same force the scale pushes up Newton's third law states that forces come in equal and opposite pairs. If the scale exerts an upward force of 750 N on the person, then by Newton's third law, the person must exert a downward force of exactly 750 N on the scale. This is true regardless of whether the elevator is accelerating. Choice B gives the person's weight at rest, which is not relevant to the third law pair. Choice C incorrectly adds effects that are already included in the scale reading. Choice D gives only the 'extra' force from acceleration, missing the complete interaction force.
A swimmer pushes water backward with her hands, exerting a force of $30\text{ N}$ on the water. According to Newton's third law, what happens to the swimmer?
Answer: Water exerts $30\text{ N}$ forward on the swimmer, propelling her through the water When you encounter questions about forces and motion, Newton's third law is often the key concept being tested. This law states that for every action, there is an equal and opposite reaction - forces always come in pairs. In this swimming scenario, when the swimmer pushes backward on the water with $30\text{ N}$ of force, Newton's third law requires that the water simultaneously pushes forward on the swimmer with exactly $30\text{ N}$ of force. These forces are equal in magnitude but opposite in direction, and they act on different objects (the swimmer acts on the water, the water acts on the swimmer). Choice A correctly identifies this force pair - the water exerts $30\text{ N}$ forward on the swimmer, which is what propels her through the water. This is exactly what Newton's third law predicts. Choice B gets the magnitude right ( $30\text{ N}$ ) but has the direction wrong. The reaction force must be opposite to the action force - since the swimmer pushes backward, the water pushes forward on her. Choice C reflects a common misconception that the reaction force is somehow diminished by the properties of the medium. Newton's third law applies regardless of whether you're pushing on a solid wall or flowing water - the reaction force is always equal in magnitude. Choice D incorrectly suggests that water's incompressibility increases the reaction force. While incompressibility affects how efficiently force is transmitted, it doesn't change the fundamental equality demanded by Newton's third law. Remember: Newton's third law forces are always equal in magnitude and opposite in direction, regardless of the materials involved.
Two ice skaters, Maria (60 kg) and Carlos (80 kg), are initially at rest and push off each other. After pushing off, Maria moves at 4.0 m/s to the right. During the push-off, Maria exerts an average force on Carlos for 0.50 s. What is the magnitude of the average force that Carlos exerts on Maria during this interaction?
Answer: 480 N, because this accounts for the force needed to accelerate Maria based on her final momentum By Newton's third law, the forces Maria and Carlos exert on each other are equal in magnitude. To find this force, we can analyze Maria's motion: her momentum change is Δp = (60 kg)(4.0 m/s) = 240 kg⋅m/s. Using impulse-momentum theorem: F⋅t = Δp, so F = 240/(0.50) = 480 N. This same force acts on both skaters by Newton's third law. Choice A uses Maria's average acceleration but calculates incorrectly. Choice C incorrectly uses Carlos's momentum change divided by time. Choice D incorrectly averages separate force calculations rather than recognizing the forces are equal by the third law.
A spring is compressed between two identical blocks on a frictionless surface. When released, the spring pushes the left block with $40\text{ N}$ toward the left. According to Newton's third law, what force does the spring exert on the right block?
Answer: $40\text{ N}$ toward the right, because action-reaction forces are equal and opposite When you encounter problems involving springs and multiple objects, focus on Newton's third law: forces between interacting objects are always equal in magnitude and opposite in direction. This fundamental principle applies regardless of the objects' masses, the spring's properties, or energy considerations. As the spring expands, it simultaneously pushes against both blocks. The spring exerts $40\text{ N}$ on the left block toward the left, so by Newton's third law, it must exert an equal and opposite force on the right block. This means the spring pushes the right block with $40\text{ N}$ toward the right. The forces form an action-reaction pair—they're internal forces within the spring-block system that are always equal and opposite. Looking at the wrong answers: Choice B incorrectly suggests both forces point in the same direction, violating Newton's third law's "opposite" requirement. Choice C falls into the trap of thinking energy loss affects force magnitude—while the spring does lose potential energy as it expands, this doesn't change the instantaneous forces between objects. Choice D misunderstands inertia; the spring doesn't need to "overcome" inertia with different forces. Inertia describes an object's resistance to acceleration, but Newton's third law still requires equal and opposite forces regardless of the masses involved. Remember this key insight: Newton's third law creates force pairs that are always equal in magnitude and opposite in direction, regardless of masses, energy changes, or other system properties. When you see spring problems with multiple objects, immediately look for these action-reaction pairs.
Two magnets are placed near each other. Magnet A exerts a repulsive force of $0.5\text{ N}$ on magnet B. What can be concluded about the force that magnet B exerts on magnet A?
Answer: Magnet B exerts $0.5\text{ N}$ repulsive force on magnet A in the opposite direction When you encounter forces between objects, you're dealing with Newton's Third Law of Motion, which states that forces always come in pairs. For every action force, there's an equal and opposite reaction force. In this magnetic interaction, magnet A exerts a $0.5\text{ N}$ repulsive force on magnet B. According to Newton's Third Law, magnet B must simultaneously exert an equal and opposite force on magnet A. Since the magnets are repelling each other, both forces must be repulsive - magnet A pushes away from magnet B with $0.5\text{ N}$ , and magnet B pushes away from magnet A with exactly $0.5\text{ N}$ in the opposite direction. Choice A correctly identifies this equal and opposite force relationship. Choice B incorrectly suggests the force would be attractive - this misunderstands that Newton's Third Law doesn't create "balance" by reversing the type of force, but rather creates equal magnitudes in opposite directions. Choice C falls into the misconception that one magnet is "responding" to the other with a weaker force - in reality, both forces exist simultaneously and are always equal in magnitude. Choice D incorrectly assumes magnet strength affects the force pairing - while stronger magnets create larger forces, Newton's Third Law guarantees the action-reaction pair will always be equal regardless of which magnet is "stronger." Remember: Newton's Third Law applies to all force interactions, not just collisions. Whenever you see forces between two objects, immediately look for the equal and opposite pair - the magnitudes are always identical, and the directions are always opposite.
A horse pulls a cart with a force of $800\text{ N}$ . The cart experiences friction with the ground. According to Newton's third law, what force does the cart exert on the horse?
Answer: $800\text{ N}$ backward, opposing the horse's pull regardless of friction When you encounter questions about forces between interacting objects, Newton's third law is the fundamental principle at work. This law states that for every action, there is an equal and opposite reaction—and these force pairs always act between the same two objects. The horse exerts $800\text{ N}$ on the cart in the forward direction. By Newton's third law, the cart must exert exactly $800\text{ N}$ on the horse in the backward direction. This is a direct consequence of how forces work in nature—they always come in equal and opposite pairs between the same two objects. Choice A correctly identifies this $800\text{ N}$ backward force regardless of friction, because Newton's third law pairs are independent of other forces in the system. Choice B incorrectly suggests the cart pulls the horse forward, which would violate the "opposite" requirement of Newton's third law. Choice C represents a common misconception that friction somehow reduces the third-law force pair—but friction is a separate force between the cart and ground, not between the horse and cart. Choice D similarly confuses friction's role, suggesting it increases the reaction force, when friction doesn't affect the magnitude of third-law pairs at all. The key insight is that Newton's third law force pairs exist between specific objects and are completely independent of other forces in the system. Friction affects the cart's motion, but it doesn't change the force interaction between horse and cart. Remember: Newton's third law pairs are always equal in magnitude and opposite in direction, regardless of what other forces might be present in the problem.
A bird flies horizontally at constant velocity. The bird's wings push air downward with a force of $2\text{ N}$ . According to Newton's third law, what is the reaction force?
Answer: Air pushes upward on the bird's wings with a force of $2\text{ N}$ When you encounter Newton's third law problems, focus on identifying the specific interaction between two objects and remember that action-reaction pairs always involve the same force magnitude acting on different objects in opposite directions. In this problem, the bird's wings are the first object and the air is the second object. The action force is the wings pushing air downward with $2\text{ N}$ . By Newton's third law, the reaction force must be the air pushing back on the wings with equal magnitude but opposite direction. Since the wings push down on the air, the air must push up on the wings with $2\text{ N}$ , making choice A correct. Let's examine why the other options fail. Choice B suggests air pushes horizontally on the bird with $2\text{ N}$ . While horizontal forces do exist to maintain constant velocity, this isn't the reaction force to the downward wing push—reaction forces must be in the opposite direction of the action force. Choice C identifies gravity pulling down on the bird. Even if gravity happens to equal $2\text{ N}$ , this isn't part of the wing-air interaction we're analyzing; gravity involves the Earth-bird system. Choice D describes the bird's body pushing on its wings. This represents an internal force within the bird's anatomy, not the reaction to the external wing-air interaction. Remember that Newton's third law pairs always involve two different objects exerting forces on each other. When identifying reaction forces, ask yourself: "What object is being acted upon, and what force does it exert back on the original object?"
A box sits on an inclined plane. The inclined plane exerts a normal force of $50\text{ N}$ on the box perpendicular to the surface. Which force is the Newton's third law pair to this normal force?
Answer: The box exerts $50\text{ N}$ on the inclined plane perpendicular to the surface When you encounter Newton's third law questions, focus on identifying action-reaction pairs between two objects that are in direct contact and exert equal and opposite forces on each other. Newton's third law states that for every action, there's an equal and opposite reaction. The key is that these paired forces act on different objects. Here, the inclined plane exerts a $50\text{ N}$ normal force on the box. By Newton's third law, the box must exert an equal and opposite force back on the inclined plane. Choice A correctly identifies this relationship: the box exerts $50\text{ N}$ on the inclined plane perpendicular to the surface. This force has the same magnitude, acts in the opposite direction, and involves the same two objects in contact. Choice B incorrectly identifies gravity as the third law pair. While Earth's gravitational pull on the box might equal $50\text{ N}$ in certain orientations, gravity acts between Earth and the box, not between the box and inclined plane. The gravitational force pair would be the box pulling on Earth. Choice C mentions friction, which is a separate force entirely. Friction acts parallel to the surface and isn't related to the normal force's third law pair, even if it happens to have the same magnitude. Choice D describes another force the inclined plane exerts on the box. This can't be a third law pair because both forces would act on the same object (the box), violating the fundamental requirement that paired forces act on different objects. Remember: Newton's third law pairs always involve the same two objects exerting equal and opposite forces on each other.
A person pushes horizontally on a wall with a force of $50\text{ N}$ . The wall does not move. Which statement best describes the forces involved according to Newton's third law?
Answer: The wall exerts $50\text{ N}$ horizontally back on the person, forming an action-reaction pair When you encounter questions about forces and objects that aren't moving, you're dealing with Newton's third law, which states that forces always come in action-reaction pairs that are equal in magnitude and opposite in direction. The correct answer is A because Newton's third law applies to all interactions between objects, regardless of whether they move. When you push on the wall with $50\text{ N}$ horizontally, the wall simultaneously pushes back on you with exactly $50\text{ N}$ horizontally in the opposite direction. This happens instantly and automatically - it's a fundamental property of how forces work in nature. Option B reflects a common misconception that stationary objects don't exert forces. Motion has nothing to do with Newton's third law - the action-reaction pairs exist whether objects move or not. The wall doesn't move because other forces (like friction with the ground) keep it in equilibrium. Option C confuses different force interactions. While the wall does support weight through normal forces, this vertical support force is separate from the horizontal action-reaction pair created by your push. These are two different force interactions happening simultaneously. Option D incorrectly suggests that material properties affect the magnitude of action-reaction forces. Newton's third law guarantees equal and opposite forces regardless of whether the materials are rigid, flexible, moving, or stationary. Remember: Newton's third law is absolute - every force has an equal and opposite reaction force. Don't let an object's motion (or lack thereof) fool you into thinking action-reaction pairs don't exist.
A car accelerates forward. The engine provides a force that turns the wheels, and the wheels push backward on the road with a force of $3000\text{ N}$ . According to Newton's third law, what is the reaction force?
Answer: The road pushes forward on the wheels with a force of $3000\text{ N}$ Newton's third law states that forces always come in pairs: for every action force, there's an equal and opposite reaction force acting on a different object. When analyzing force pairs, you need to identify which two objects are directly interacting and determine the forces they exert on each other. In this scenario, the wheels and the road are the two objects directly interacting. The wheels push backward on the road with $3000\text{ N}$ (the action force). According to Newton's third law, the road must push forward on the wheels with an equal force of $3000\text{ N}$ (the reaction force). This forward force from the road is actually what propels the car forward. Looking at the wrong answers: Choice B describes a force between the wheels and car, but this isn't the reaction pair to the wheel-road interaction described in the problem. Choice C mentions air resistance, which is a completely different force interaction between the car and air molecules, not related to the wheel-road contact. Choice D refers to the engine pushing on the wheels, which is part of the car's internal mechanics and again not the reaction to the wheel-road force. Remember that Newton's third law pairs always involve the same two objects exerting forces on each other. When you see a force described in a problem, ask yourself: "What object is being pushed or pulled, and what object is doing the pushing or pulling?" The reaction force will be the second object pushing back on the first with equal magnitude but opposite direction.

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