College Physics Quiz: Newtons Third Law

20 questions · exam conditions

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Newtons Third LawQuestion 1 of 20

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?

The ball exerts

25\text{ N}

downward on the thrower's hand at the moment of release

Gravity exerts

25\text{ N}

downward on the ball throughout its flight

Air resistance exerts

25\text{ N}

downward on the ball during its upward motion

The thrower's hand exerts

25\text{ N}

downward when catching the ball

The ball exerts

25\text{ N}

upward on the thrower's hand due to the ball's inertia

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College Physics Quiz

College Physics Quiz: Newtons Third Law

Practice Newtons Third Law in College Physics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

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

How to use this quiz

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

All questions

Question 1

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?

The ball exerts $25\text{ N}$ downward on the thrower's hand at the moment of release (correct answer)
Gravity exerts $25\text{ N}$ downward on the ball throughout its flight
Air resistance exerts $25\text{ N}$ downward on the ball during its upward motion
The thrower's hand exerts $25\text{ N}$ downward when catching the ball
The ball exerts $25\text{ N}$ upward on the thrower's hand due to the ball's inertia

Explanation: 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.

Question 2

A book rests on a table. The book exerts a downward force of $12\text{ N}$ on the table. According to Newton's third law, what force does the table exert on the book?

$12\text{ N}$ upward, because the forces form an action-reaction pair (correct answer)
$12\text{ N}$ downward, because both objects exert gravitational force
Zero, because the book is at rest and has no acceleration
Less than $12\text{ N}$ upward, because the table is supporting the book
Greater than $12\text{ N}$ upward, because the table must overcome gravity

Explanation: When you encounter questions about forces between objects in contact, you're dealing with Newton's third law of motion, which states that for every action force, there's an equal and opposite reaction force. These forces always occur in pairs between two interacting objects. In this scenario, the book pushes down on the table with

12\text{ N}

of force. Newton's third law requires that the table simultaneously pushes back up on the book with exactly

12\text{ N}

of force. These forces are equal in magnitude, opposite in direction, and act on different objects—this is what makes them an action-reaction pair. Choice A correctly identifies this

12\text{ N}

upward force and properly explains it as an action-reaction pair. Choice B incorrectly suggests the table pushes downward, which would violate Newton's third law—reaction forces must be opposite in direction to action forces. Choice C falls into the common trap of confusing Newton's third law with Newton's first law; while the book isn't accelerating, this doesn't mean forces don't exist—it means the net force is zero because the upward and downward forces on the book balance out. Choice D incorrectly assumes the reaction force is somehow diminished, but Newton's third law guarantees that action-reaction pairs are always equal in magnitude. Remember that Newton's third law forces always act on different objects and are always equal and opposite. When you see force interaction problems, immediately ask yourself: "What object is pushing/pulling on what other object, and what's the equal and opposite reaction?"

Question 3

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?

The road pushes forward on the wheels with a force of $3000\text{ N}$ (correct answer)
The wheels push forward on the car with a force of $3000\text{ N}$
Air resistance pushes backward on the car with a force of $3000\text{ N}$
The engine pushes forward on the wheels with a force of $3000\text{ N}$
Friction between the tires and road creates a backward force of $3000\text{ N}$

Explanation: 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.

Question 4

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?

$80\text{ N}$ in the direction opposite to the rope's pull on the sled (correct answer)
$80\text{ N}$ in the same direction as the rope's pull on the sled
Less than $80\text{ N}$ because the sled is being pulled, not pulling
More than $80\text{ N}$ because the sled must overcome friction with the ice
Zero, because the rope is doing work on the sled, not vice versa

Explanation: 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.

Question 5

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?

Air pushes upward on the bird's wings with a force of $2\text{ N}$ (correct answer)
Air pushes horizontally forward on the bird with a force of $2\text{ N}$
Earth's gravity pulls downward on the bird with a force of $2\text{ N}$
The bird's body pushes downward on the wings with a force of $2\text{ N}$
Air resistance pushes backward on the bird with a force of $2\text{ N}$

Explanation: 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?"

Question 6

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?

The ball exerts $1500\text{ N}$ on the bat for $0.002\text{ s}$ in the opposite direction (correct answer)
The ball exerts $1500\text{ N}$ on the bat for a longer time because balls are softer
The ball exerts less than $1500\text{ N}$ on the bat because the bat is more massive
The ball exerts $1500\text{ N}$ on the bat, but for a shorter time due to its smaller mass
The ball exerts more than $1500\text{ N}$ on the bat to change the bat's momentum

Explanation: 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.

Question 7

A person stands in an elevator that is accelerating upward at $2\text{ m/s}^2$ . The elevator floor exerts $700\text{ N}$ upward on the person. What is the Newton's third law pair to this force?

The person exerts $700\text{ N}$ downward on the elevator floor (correct answer)
Earth exerts $700\text{ N}$ downward on the person due to gravity
The elevator cable exerts $700\text{ N}$ upward on the elevator car
The person's inertia creates $700\text{ N}$ downward resistance to acceleration
The elevator motor exerts $700\text{ N}$ upward to create the acceleration

Explanation: 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. These paired forces act on different objects and are called action-reaction pairs. In this elevator scenario, you need to identify what force pairs with the floor pushing up on the person with 700 N. Since the floor exerts an upward force on the person, Newton's third law requires the person to exert an equal and opposite force on the floor. This means the person pushes down on the floor with exactly 700 N. Let's examine why the other options miss the mark. Option B (Earth's gravitational force) is incorrect because gravity and the normal force from the floor are completely separate forces - they're not a Newton's third law pair. The gravitational force would actually be less than 700 N since the person is accelerating upward. Option C (cable tension) involves forces between the cable and elevator car, which has nothing to do with the person-floor interaction we're analyzing. Option D incorrectly invokes "inertia creating resistance," but inertia isn't a force - it's an object's tendency to resist changes in motion. The correct answer is A: the person exerts 700 N downward on the elevator floor. Study tip: When identifying Newton's third law pairs, always ask "what two objects are directly touching and pushing/pulling on each other?" The paired forces will always act on these two different objects with equal magnitude but opposite directions.

Question 8

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?

$20\text{ N}$ in the direction opposite to A's pull, acting as tension (correct answer)
$20\text{ N}$ in the same direction as A's pull to assist the motion
Less than $20\text{ N}$ because the rope has negligible mass in space
More than $20\text{ N}$ because space conditions amplify forces
Zero, because astronaut A is the one doing the pulling action

Explanation: 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.

Question 9

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?

Water exerts $30\text{ N}$ forward on the swimmer, propelling her through the water (correct answer)
Water exerts $30\text{ N}$ backward on the swimmer, creating resistance to motion
Water exerts less than $30\text{ N}$ on the swimmer because water flows around her hands
Water exerts more than $30\text{ N}$ on the swimmer due to water's incompressibility
Water exerts force only when the swimmer stops pushing, not during the stroke

Explanation: 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.

Question 10

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?

$40\text{ N}$ toward the right, because action-reaction forces are equal and opposite (correct answer)
$40\text{ N}$ toward the left, because the spring pushes in both directions equally
Less than $40\text{ N}$ toward the right, because the spring loses energy as it expands
More than $40\text{ N}$ toward the right, because the spring must overcome both blocks' inertia
Zero toward the right, because all the spring's force is used on the left block

Explanation: 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.

Question 11

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?

$600\text{ N}$ in the direction opposite to the person's push, providing the reaction force (correct answer)
$600\text{ N}$ in the same direction as the person's push, helping propel the person forward
Less than $600\text{ N}$ because the boat moves backward and absorbs some force
More than $600\text{ N}$ because the boat must accelerate the person toward the dock
Zero, because the person is leaving the boat and breaking contact

Explanation: 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.

Question 12

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?

$10,000\text{ N}$ forward, providing the thrust that propels the rocket (correct answer)
$10,000\text{ N}$ backward, opposing the rocket's intended motion
Less than $10,000\text{ N}$ forward, because some energy is lost as heat
More than $10,000\text{ N}$ forward, because space has no air resistance
Zero, because the gas molecules are expelled and no longer interact with the rocket

Explanation: 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."

Question 13

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?

Magnet B exerts $0.5\text{ N}$ repulsive force on magnet A in the opposite direction (correct answer)
Magnet B exerts $0.5\text{ N}$ attractive force on magnet A to balance the system
Magnet B exerts a force less than $0.5\text{ N}$ because it's responding to magnet A
The force depends on which magnet is stronger and could be any magnitude
Magnet B exerts no force because magnet A is the source of the magnetic field

Explanation: 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.

Question 14

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?

$800\text{ N}$ backward, opposing the horse's pull regardless of friction (correct answer)
$800\text{ N}$ forward, assisting the horse's pulling effort
Less than $800\text{ N}$ backward, because friction reduces the reaction force
More than $800\text{ N}$ backward, because friction increases the resistance
A force equal to the friction force, because that determines the cart's resistance

Explanation: 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.

Question 15

Two identical cars collide head-on. During the collision, car A exerts $8000\text{ N}$ on car B for $0.1\text{ s}$ . Which statement about the force car B exerts on car A is correct according to Newton's third law?

Car B exerts $8000\text{ N}$ on car A for $0.1\text{ s}$ in the opposite direction (correct answer)
Car B exerts $8000\text{ N}$ on car A for $0.1\text{ s}$ in the same direction
Car B exerts $8000\text{ N}$ on car A for less than $0.1\text{ s}$ because it's being hit
Car B exerts the same force only if both cars have identical masses and speeds
Car B exerts $4000\text{ N}$ on car A because the force is shared between two objects

Explanation: When you encounter collision problems, Newton's third law is the fundamental principle at work: forces between interacting objects are always equal in magnitude, opposite in direction, and occur simultaneously. During this head-on collision, car A and car B form an action-reaction pair. Newton's third law states that for every action, there's an equal and opposite reaction. This means when car A exerts

8000\text{ N}

on car B, car B must simultaneously exert

8000\text{ N}

back on car A in the opposite direction. Crucially, these forces exist for exactly the same duration—

0.1\text{ s}

—because they're two sides of the same interaction. Choice A correctly captures this: car B exerts

8000\text{ N}

on car A for

0.1\text{ s}

in the opposite direction. Choice B gets the magnitude and time right but fails on direction—the forces must be opposite to each other. Choice C incorrectly suggests the duration differs, but action-reaction pairs are always simultaneous and last exactly the same time. Choice D introduces an irrelevant condition about masses and speeds; Newton's third law applies regardless of the objects' properties—even if a truck hits a bicycle, the forces between them are equal in magnitude. Remember this key insight for collision problems: Newton's third law guarantees that interacting objects always exert equal and opposite forces on each other, regardless of their sizes, masses, or velocities. The law is about the forces between objects during their interaction, not about the resulting accelerations or damage.

Question 16

A hammer strikes a nail with a force of $200\text{ N}$ . According to Newton's third law, the nail exerts a force on the hammer. Which statement about these forces is most accurate?

Both forces have the same magnitude and exist for the same duration during contact (correct answer)
The hammer's force on the nail is larger because the hammer is the active object
The nail's force on the hammer is larger because the nail resists deformation
Both forces are equal, but the hammer's force lasts longer than the nail's force
The forces are only equal when the nail stops moving into the wood

Explanation: When you encounter questions about forces between interacting objects, you're dealing with Newton's third law: "For every action, there is an equal and opposite reaction." This law describes force pairs that always exist simultaneously between two objects in contact. Newton's third law tells us that when the hammer exerts

200\text{ N}

on the nail, the nail must exert exactly

200\text{ N}

back on the hammer. These forces are equal in magnitude, opposite in direction, and exist for exactly the same time period—they appear and disappear together during the contact interaction. Think of it this way: forces always come in pairs, and you can't have one without the other. Looking at the wrong answers: Option B incorrectly suggests the "active" object exerts a larger force, but Newton's third law makes no distinction between active and passive objects—the forces are always equal regardless of which object initiated the motion. Option C falls into the trap of thinking resistance creates larger forces, but the nail's resistance doesn't change the fundamental equality of the force pair. Option D correctly identifies that the forces are equal but wrongly claims they have different durations—this violates the basic principle that third-law force pairs must exist simultaneously. Study tip: Remember that Newton's third law force pairs are always equal in magnitude and simultaneous in time, regardless of the objects' sizes, masses, or roles in the interaction. When you see phrases like "active object" or "resistance creates larger forces," these often signal incorrect reasoning about third-law pairs.

Question 17

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.

The student exerts a downward force of $600\text{ N}$ on the chair (correct answer)
Earth exerts a downward gravitational force of $600\text{ N}$ on the student
The floor exerts an upward force of $600\text{ N}$ on the chair
The student's weight creates a downward force of $600\text{ N}$ toward Earth's center
Air pressure exerts forces totaling $600\text{ N}$ on the student from all directions

Explanation: 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.).

Question 18

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?

The wall exerts $50\text{ N}$ horizontally back on the person, forming an action-reaction pair (correct answer)
The wall exerts no force on the person because the wall doesn't move
The wall exerts $50\text{ N}$ vertically upward to support the person's weight
The wall exerts less than $50\text{ N}$ on the person because it's made of rigid material
The wall exerts more than $50\text{ N}$ on the person to prevent the wall from moving

Explanation: 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.

Question 19

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?

The box exerts $50\text{ N}$ on the inclined plane perpendicular to the surface (correct answer)
Gravity exerts $50\text{ N}$ downward on the box toward Earth's center
Friction exerts $50\text{ N}$ parallel to the inclined surface on the box
The inclined plane exerts $50\text{ N}$ parallel to its surface due to the box's weight component
Earth exerts $50\text{ N}$ on the inclined plane to support both objects

Explanation: 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.

Question 20

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?

The force that A exerts on B equals the force that B exerts on A in magnitude (correct answer)
A exerts a larger force on B because A is moving faster after the interaction
B exerts a larger force on A because forces depend on the masses involved
The forces are equal only if both skaters have the same mass and velocity
No forces act between the skaters since they separate and move independently

Explanation: 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.