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Newton's Third Law (Action/Reaction)

22 questions

All types22Multiple choice10True / False6Matching6
Question 1Question →

A mechanic swings a 2 lb2 \text{ lb} hammer, striking a 100 lb100 \text{ lb} stationary steel anvil. During the brief fraction of a second of contact, the hammer rapidly decelerates to a stop while the anvil remains practically motionless. Which of the following statements correctly compares the interaction forces between the hammer and the anvil during the exact moment of impact?

Show answer & explanation

Answer: The force the hammer exerts on the anvil is exactly equal in magnitude to the force the anvil exerts on the hammer.

Answer

The force the hammer exerts on the anvil is exactly equal in magnitude to the force the anvil exerts on the hammer.
According to Newton's Third Law of Motion, forces always occur in equal and opposite pairs. When the hammer strikes the anvil, the interaction consists of the hammer pushing on the anvil and the anvil pushing on the hammer. These two forces are simultaneously created and are always perfectly equal in magnitude, regardless of the fact that the anvil is 5050 times heavier than the hammer or that the hammer is rapidly decelerating. The difference in their observed movement is due to their differing masses (Newton's Second Law), not a difference in the force applied.

Step-by-Step Solution

1
Identify the two interacting bodies in the mechanical system.
The interacting bodies are the 2 lb2 \text{ lb} hammer and the 100 lb100 \text{ lb} anvil.
Newton's Third Law always applies to the mutual interaction between exactly two distinct objects.
2
Apply Newton's Third Law to the specific moment of impact.
The force of the hammer on the anvil (FH→AF_{H \to A}) must equal the negative force of the anvil on the hammer (−FA→H-F_{A \to H}).
For every action force, there is an equal and opposite reaction force, regardless of mass or velocity.
3
Differentiate between the interaction force and the resulting acceleration.
Both objects experience the exact same magnitude of force, but the hammer decelerates rapidly because it has a much smaller mass.
Newton's Second Law (F=maF = ma) explains the differing accelerations, while Newton's Third Law dictates the equal forces.

Key Concept

Action and reaction forces between two interacting bodies are always perfectly equal in magnitude and opposite in direction, entirely independent of the objects' masses or rates of acceleration.
Estimated Time:1m 30s
Question 2Question →

According to Newton's Third Law of Motion, mechanical forces always occur in equal and opposite action-reaction pairs acting on different objects. Match each described action force in the left column to its corresponding reaction force in the right column.

Click a left item, then click its matching right item

Items

A construction worker's boots exert a backward frictional force on a concrete floor.
A descending hammer head exerts a downward contact force on a steel nail.
A locomotive's rotating drive wheel exerts a backward force on the steel track.
A crane's active electromagnet exerts an upward magnetic pull on a scrap iron block.

Matches

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Answer

The correct matches pair the action force on the second object with an equal and opposite reaction force acting back on the first object. Worker's boots pair with the concrete floor's forward friction. The hammer head pairs with the nail's upward contact force. The locomotive wheel pairs with the track's forward force. The electromagnet pairs with the iron block's downward magnetic pull.
According to Newton's Third Law, for every action force there is an equal and opposite reaction force. These forces must act on two different, interacting bodies. By identifying the two bodies in the action (e.g., boots pushing on the floor), the exact reaction is simply the reverse physical interaction (the floor pushing on the boots).

Step-by-Step Solution

1
Identify the two interacting objects in each action force description.
The identified pairs are: boots/floor, hammer/nail, wheel/track, and electromagnet/block.
Newton's Third Law states that forces only exist in pairs between exactly two interacting bodies.
2
Determine the direction of the original action force.
The action directions are backward (boots), downward (hammer), backward (wheel), and upward (magnet).
The reaction force must be in the exact opposite direction of the action force.
3
Locate the corresponding reaction force that acts on the first object in the opposite direction.
Boots receive a forward force; the hammer receives an upward force; the wheel receives a forward force; the magnet receives a downward force.
Reaction forces act on the source object, opposite to the action force's direction.

Key Concept

Newton's Third Law of Motion (Action and Reaction Forces)
Estimated Time:1m 30s
Question 3Question →

A tugboat is pushing a heavy cargo barge steadily forward through a shipping channel. The bow of the tugboat is in direct contact with the stern of the barge, exerting a constant forward pushing force of 10,000 N10,000 \text{ N} on the barge. According to Newton's Third Law of Motion, which of the following accurately describes the reaction force in this specific mechanical interaction?

Show answer & explanation

Answer: The barge exerts a 10,000 N10,000 \text{ N} backward pushing force directly against the bow of the tugboat.

Answer

The barge exerts a 10,000 N10,000 \text{ N} backward pushing force directly against the bow of the tugboat.
The correct answer accurately applies the principle that interacting objects exert equal and opposite forces on each other. If the tugboat pushes the barge forward with a specific force, the barge simultaneously pushes the tugboat backward with the exact same amount of force.

Step-by-Step Solution

1
Identify the primary action force and the two interacting bodies described in the scenario.
The action force is the tugboat pushing forward on the barge with 10,000 N10,000 \text{ N}. The two interacting bodies are the tugboat and the barge.
Newton's Third Law applies specifically to a pair of interacting objects.
2
Determine the magnitude and direction of the reaction force.
The reaction force must be exactly equal in magnitude (10,000 N10,000 \text{ N}) and exactly opposite in direction (backward).
According to the law of action and reaction, forces always occur in equal and opposite pairs.
3
Identify which body the reaction force acts upon.
Since the tugboat acts on the barge, the barge must act back on the tugboat.
Action and reaction forces act on different bodies, never on the same body.

Key Concept

Newton's Third Law states that forces occur in simultaneous pairs acting on different objects with equal magnitude and opposite direction.
Estimated Time:1m 0s
Question 4Question →

According to the principle of action and reaction, interacting objects apply forces to one another. Pair the following initial mechanical forces with their direct reaction forces.

Click a left item, then click its matching right item

Items

A jet engine forces hot exhaust gases backward out of its nozzle.
A swimmer's feet push forcefully backward against the concrete pool wall.
The spinning blades of a helicopter push a large volume of air downward.
A high-pressure fire hose accelerates water forward out of its nozzle.

Matches

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Answer

The correct pairings directly flip the interacting objects and reverse the direction of the force. The jet engine pushing gas backward matches with gas pushing the engine forward. The swimmer pushing the wall backward matches with the wall pushing the swimmer forward. The blades pushing air downward matches with air pushing the blades upward. The hose pushing water forward matches with water pushing the hose backward.
Each correct match demonstrates Newton's Third Law by identifying the exact same two interacting bodies from the action statement, reversing their roles as the object applying the force and the object receiving the force, and reversing the physical direction of that force.

Step-by-Step Solution

1
Identify the two interacting objects in each initial force description.
In the jet engine scenario, the objects are the engine and the exhaust gases. In the swimmer scenario, they are the feet and the wall. In the helicopter scenario, they are the blades and the air. In the fire hose scenario, they are the hose and the water.
Newton's Third Law always involves two distinct objects interacting with one another.
2
Identify the physical direction of the applied initial force.
The initial forces are directed backward (gas), backward (wall), downward (air), and forward (water).
Reaction forces act in the exact opposite direction of the initial action force.
3
Formulate the reaction force by swapping the objects and reversing the direction.
The correct reactions are: gas pushing engine forward, wall pushing feet forward, air pushing blades upward, and water pushing hose backward.
If the first object exerts a force on the second object, the second object must exert an equal and opposite force on the first object.

Key Concept

For every action force, there is a simultaneous reaction force that is equal in magnitude, opposite in direction, and acts on a different object.
Estimated Time:1m 0s
Question 5Question →

An astronaut floating motionless in space throws a heavy wrench directly forward. As a result, the astronaut is pushed backward. According to Newton's Third Law of Motion, what causes the astronaut to move backward?

Show answer & explanation

Answer: The wrench exerts a backward force on the astronaut that is equal to the forward force applied to the wrench.

Answer

The wrench exerts an equal and opposite backward force on the astronaut.
Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. These forces always act on two different interacting bodies. When the astronaut (Body 1) applies a forward force to the wrench (Body 2), the wrench simultaneously applies a backward force of exactly the same magnitude to the astronaut. This equal and opposite push is what causes the astronaut to move backward.

Step-by-Step Solution

1
Identify the initial action force described in the scenario.
The action force is the astronaut's hand pushing the heavy wrench forward.
To apply Newton's Third Law, the first interacting body and its applied force must be defined.
2
Apply Newton's Third Law of Motion to find the reaction force.
For the forward force on the wrench, there must be an equal and opposite backward force.
Newton's Third Law dictates that every action has an equal and opposite reaction occurring on the other interacting body.
3
Determine which object applies this reaction force to the astronaut.
The wrench applies the backward force onto the astronaut.
Forces in a Third Law pair always act on opposite objects: Astronaut pushes Wrench, therefore Wrench pushes Astronaut.

Key Concept

Newton's Third Law (Action/Reaction)
Estimated Time:45s
Question 6Question →

When a massive steel cargo ship collides with a small wooden fishing boat, the impact force exerted by the cargo ship on the wooden boat is significantly greater in magnitude than the reaction force exerted by the wooden boat on the cargo ship, due to the ship's vastly greater mass and momentum.

Show answer & explanation

Answer: False

Answer

The statement is false.
The statement is false because Newton's Third Law mandates that forces between two interacting objects are always exactly equal in magnitude and opposite in direction, irrespective of their differing masses or momentums.

Step-by-Step Solution

1
Analyze the interacting bodies and the nature of the forces described in the scenario.
The two bodies are a massive cargo ship and a small wooden boat experiencing a collision, which constitutes an interacting force pair.
Identifying the system and the interaction is necessary to apply the correct physical laws.
2
Apply Newton's Third Law to the interaction.
The law states that force pairs between two interacting objects are always equal in magnitude and opposite in direction (Fship=−FboatF_{ship} = -F_{boat}).
This fundamental law dictates how forces operate between any two interacting bodies, regardless of their physical properties.
3
Evaluate the role of mass and momentum mentioned in the statement.
While the ship has significantly greater mass and momentum, these factors determine the resulting acceleration (a=Fma = \frac{F}{m}) and subsequent damage, not the magnitude of the interacting force pair itself.
It is critical to distinguish between the contact force itself and the effects (acceleration, deformation) of that force.

Key Concept

Newton's Third Law of Motion (Equal and Opposite Reaction)
Question 7Question →

In the context of Newton's Third Law of Motion, every action force has a specific, equal, and opposite reaction force acting on a different body. Match each described action force on the left with its exact corresponding reaction force on the right.

Click a left item, then click its matching right item

Items

The downward aerodynamic force exerted by a hovering helicopter's rotor blades on the surrounding air.
The upward magnetic pull exerted by an industrial crane's electromagnet on a piece of scrap steel.
The downward gravitational force exerted by the Earth's mass on a descending skydiver.
The forward mechanical force exerted by the internal walls of a fire hose nozzle on an exiting stream of high-pressure water.

Matches

Show answer & explanation

Answer

The correct pairings strictly follow the A-on-B / B-on-A rule, where the reaction force is always the exact same type of force exerted in the opposite direction by the receiving object back onto the originating object.
According to Newton's Third Law, forces always come in pairs. If body A exerts a force on body B, then body B simultaneously exerts an equal and opposite force on body A. The correct matches precisely swap the 'exerter' and the 'receiver' of the force while maintaining the identical fundamental type of force (gravitational, magnetic, aerodynamic, or mechanical).

Step-by-Step Solution

1
Analyze the action force to identify the two interacting bodies (Object A and Object B).
For example, in the skydiver scenario, Object A is the Earth and Object B is the skydiver.
Newton's Third Law strictly dictates that action and reaction forces must occur between two interacting objects.
2
Identify the type and direction of the force exerted by Object A on Object B.
The Earth exerts a downward gravitational force on the skydiver.
You must know the exact nature of the action force to find its true paired reaction.
3
Reverse the roles of the objects and the direction of the force to find the reaction.
The reaction must be Object B (the skydiver) exerting an upward gravitational force on Object A (the Earth).
Reaction forces are always equal in magnitude, opposite in direction, of the same type, and act on the opposing body.
4
Repeat this reversal process for all given scenarios and select the matching right-side items.
Helicopter on air pairs with air on helicopter; Magnet on steel pairs with steel on magnet; Nozzle on water pairs with water on nozzle.
This guarantees an accurate mapping of all Newton's Third Law pairs without falling for common distractors like balancing forces.

Key Concept

Newton's Third Law (Action/Reaction Pairs)
Question 8Question →

When a person pushes forward against a stalled car to move it, the force the person exerts on the car is greater than the force the car exerts backward on the person.

Show answer & explanation

Answer: False

Answer

False
The statement is false because Newton's Third Law states that the mutual forces of action and reaction between two interacting bodies are always equal in magnitude and opposite in direction. The person and the car exert exactly equal forces on each other.

Step-by-Step Solution

1
Identify the two interacting objects in the given scenario.
The two interacting objects are the person pushing and the stalled car.
Newton's Third Law describes the mutual forces between two interacting bodies.
2
Determine the action and reaction forces occurring between the objects.
The action force is the person pushing forward on the car. The reaction force is the car pushing backward against the person's hands.
Identifying the specific action-reaction pair is necessary to evaluate their magnitudes.
3
Apply Newton's Third Law to compare the magnitudes of the two forces.
Newton's Third Law dictates that these two forces must be equal in magnitude and opposite in direction.
Because the forces must be equal, the statement claiming the person's force is greater is false.

Key Concept

Newton's Third Law (Action/Reaction)
Estimated Time:45s
Question 9Question →

Match each described action force with its corresponding reaction force according to Newton's Third Law of Motion.

Click a left item, then click its matching right item

Items

A swimmer's hands push backward against the water.
A descending hammer exerts a downward force on a nail.
A rocket engine forcefully expels hot exhaust gases backward.
A heavy box exerts a downward contact force on the floor.

Matches

Show answer & explanation

Answer

Each action force must be paired with a reaction force that is equal in magnitude, opposite in direction, and acting on the original body.
Newton's Third Law states that for every action, there is an equal and opposite reaction. To correctly identify the corresponding pair, you simply reverse the 'pusher' and the 'receiver' objects, and reverse the physical direction of the force.

Step-by-Step Solution

1
Identify the two interacting objects in each scenario.
The paired objects are identified (e.g., swimmer/water, hammer/nail, rocket/gas, box/floor).
Reaction forces always occur between the exact same two interacting bodies.
2
Determine the direction of the action force.
The initial force directions are identified (e.g., backward, downward).
The reaction force must be in the exact opposite direction.
3
Select the reaction force that swaps the acting object and reverses the direction.
The correct matches are successfully established.
This satisfies the fundamental definition of action and reaction pairs.

Key Concept

Newton's Third Law (Action/Reaction)
Question 10Question →

A person wearing roller skates stands facing a solid concrete wall. When they push forward against the wall with their hands, they immediately roll backward. According to Newton's Third Law of Motion, which of the following best explains why the person rolls backward?

Show answer & explanation

Answer: The wall exerts a reaction force on the person that is equal in magnitude and opposite in direction to the person's push.

Answer

The wall exerts a reaction force on the person that is equal in magnitude and opposite in direction to the person's push.
According to Newton's Third Law, forces always occur in pairs. When the person applies an action force forward onto the wall, the wall simultaneously applies an equal reaction force backward onto the person. This backward force acts on the person and causes them to roll backward on their low-friction skates.

Step-by-Step Solution

1
Identify the two interacting objects in the given scenario.
The interacting objects are the person (specifically their hands) and the concrete wall.
Newton's Third Law analyzes forces as pairs between two distinct objects.
2
Identify the action force.
The action force is the person pushing forward against the solid wall.
This is the initial physical interaction described.
3
Determine the reaction force based on Newton's Third Law.
The wall exerts a reaction force pushing backward on the person. This force is equal in strength to the person's push.
Newton's Third Law dictates that every action force generates an equal and opposite reaction force on the interacting body.

Key Concept

Newton's Third Law states that when one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object.
Estimated Time:45s
Question 11Question →

When a heavy, fast-moving bowling ball strikes a light, stationary wooden pin, the impact force exerted by the ball on the pin is greater than the reaction force exerted by the pin on the ball.

Show answer & explanation

Answer: False

Answer

The statement is false. The action and reaction forces are exactly equal in magnitude.
The statement is false because Newton's Third Law dictates that for every action, there is an equal and opposite reaction. The force the bowling ball exerts on the pin is exactly equal in magnitude to the force the pin exerts back on the bowling ball. The reason the pin flies away while the ball continues moving forward is due to their differences in mass (inertia), not a difference in the force applied.

Step-by-Step Solution

1
Identify the two interacting objects in the collision scenario.
The two objects are the heavy bowling ball and the light wooden pin.
To determine the action-reaction force pair correctly, you must isolate the interacting bodies.
2
Apply Newton's Third Law of Motion to the interaction.
The law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
This fundamental physical law applies to all mechanical interactions regardless of size, weight, or speed.
3
Evaluate the statement's claim about differing forces based on mass.
The mass and velocity only affect how much each object accelerates (Newton's Second Law), but the impact force itself is identical for both.
Force is an interaction between two objects; they share the exact same force magnitude.

Key Concept

Newton's Third Law of Motion (Action and Reaction)
Estimated Time:45s
Question 12Question →

A canoeist pulls a paddle backward through the water to propel the canoe. According to Newton's Third Law of Motion, which specific force directly causes the canoe and paddler to move forward?

Show answer & explanation

Answer: The water pushing forward against the paddle

Answer

The water pushing forward against the paddle
According to Newton's Third Law, every action has an equal and opposite reaction. The action is the paddle pushing backward on the water. Therefore, the reaction is the water pushing forward on the paddle, which drives the canoe forward.

Step-by-Step Solution

1
Identify the action force described in the scenario.
The paddle pushes backward against the water.
Newton's Third Law requires identifying the initial interacting bodies and the direction of the applied force.
2
Apply Newton's Third Law to find the reaction force.
The water pushes forward against the paddle.
Forces always occur in equal and opposite pairs acting on different bodies. If the action is backward on the water, the reaction must be forward on the paddle.

Key Concept

Action and reaction forces occur in equal and opposite pairs between two interacting bodies.
Question 13Question →

According to Newton's Third Law of Motion, every action has an equal and opposite reaction acting on different bodies. Match each described mechanical action force in the left column with its precise corresponding reaction force in the right column.

Click a left item, then click its matching right item

Items

Action: A ship's propeller pushes water backward.
Action: A pneumatic nail gun drives a nail forward.
Action: A forklift's drive wheels push backward against the warehouse floor.
Action: A hydraulic ram presses downward on a piece of sheet metal.

Matches

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Answer

The correct matches pair the specific interacting bodies in each scenario by reversing the direction of the force and swapping the subject and object.
In every mechanical interaction, forces occur in equal and opposite pairs. To correctly identify the reaction force, you must isolate the exact two interacting bodies and simply reverse both the direction of the force and their roles. If Body A pushes Body B forward, the precise reaction is Body B pushing Body A backward.

Step-by-Step Solution

1
Identify the two distinct interacting objects in the given action force.
The subject (the pusher) and the object (the pushed) are isolated for each scenario.
Newton's Third Law requires that action-reaction pairs act on two different, specific interacting bodies.
2
Reverse the roles of the two interacting objects.
The object being pushed becomes the new subject exerting the reaction force.
The reaction force must be exerted by the receiving body back onto the originating body.
3
Invert the direction of the applied force.
A backward action becomes a forward reaction; a downward action becomes an upward reaction.
Reaction forces are always exactly opposite in direction to their corresponding action forces.

Key Concept

Newton's Third Law states that force pairs consist of the exact same two interacting bodies, with the roles of the 'pusher' and 'pushed' swapped, and the direction of the force inverted.
Estimated Time:1m 30s
Question 14Question →

An artillery cannon fires a heavy projectile horizontally across a testing ground. As expanding gases force the projectile rapidly forward out of the barrel, the entire cannon assembly recoils backward. Which of the following statements correctly describes the relationship between the forces acting on the projectile and the cannon?

Show answer & explanation

Answer: The force accelerating the projectile forward is exactly equal in magnitude to the reaction force pushing the cannon backward.

Answer

The force acting on the projectile and the reaction force acting on the cannon are exactly equal in magnitude.
Newton's Third Law dictates that all forces exist in pairs. During the firing sequence, the forward action force applied to the projectile generates a simultaneous, equal, and opposite backward reaction force on the cannon. The cannon moves backward much slower only because of its massive size, not because it experiences less force.

Step-by-Step Solution

1
Identify the interacting physical bodies in the system.
The expanding explosive gases interact simultaneously with both the projectile and the cannon barrel.
Newton's Third Law applies to the mutual interaction forces between interacting objects.
2
Apply the principle of action and reaction to the interacting bodies.
The action force pushes the projectile forward, and a simultaneous reaction force pushes the cannon backward.
Forces always occur in equal and opposite pairs.
3
Evaluate the relationship between force, mass, and the resulting motion.
The forces are perfectly identical in magnitude. The cannon accelerates much less than the projectile only because it has significantly more mass.
Newton's Second Law (F=maF=ma) dictates that the same magnitude of force applied to a larger mass results in a smaller acceleration, which often creates the illusion of unequal forces.

Key Concept

Newton's Third Law (Action/Reaction)
Question 15Question →

Evaluate the accuracy of the following claim: During a head-on collision between a heavy commercial transport truck and a lightweight passenger car, the impact force that the truck exerts on the car is greater in magnitude than the force that the car exerts on the truck.

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Answer: False

Answer

The claim is false. The forces exerted by the two vehicles upon each other are identical in magnitude.
The statement is false because Newton's Third Law of Motion dictates that any two interacting bodies exert equal and opposite forces upon one another. Even though the truck possesses a much larger mass, the actual force of impact it applies to the car is exactly the same magnitude as the force the car applies to the truck.

Step-by-Step Solution

1
Identify the two interacting objects and the physical law governing their interaction.
The objects are the heavy truck and the lightweight car. The mutual interaction is governed by Newton's Third Law of Motion (action and reaction).
Determining the correct physical principle is necessary to evaluate the relationship between the forces during the collision.
2
Apply Newton's Third Law to the collision scenario.
The law states that for every action force, there is an equal and opposite reaction force. Therefore, the force the truck exerts on the car must be identical in magnitude to the force the car exerts on the truck.
This direct application of the law disproves the claim made in the question stem.
3
Differentiate between the applied force and the resulting acceleration to avoid common misconceptions.
While the impact forces are exactly equal, the car experiences a significantly greater acceleration (and consequently more structural damage) because its mass is much smaller (a=Fma = \frac{F}{m}).
Clarifying why the car suffers more damage confirms that the unequal effects are due to mass differences under Newton's Second Law, not unequal forces.

Key Concept

Newton's Third Law states that all forces exist in pairs, and whenever two objects interact, they exert forces upon each other that are equal in magnitude and opposite in direction.

Alternative Method

You can logically verify this by considering Newton's Second Law combined with the Third Law. The perfectly equal forces cause different accelerations inversely proportional to the masses (a=Fma = \frac{F}{m}). The car's massive acceleration often misleads observers into incorrectly assuming the force applied to it was larger.
Estimated Time:45s
Question 16Question →

A motorized boat is accelerating forward across a body of water. According to Newton's Third Law of Motion, which specific force is the reaction force that directly propels the boat forward?

Show answer & explanation

Answer: The force of the water pushing forward against the propeller blades

Answer

The force of the water pushing forward against the propeller blades directly propels the boat.
According to Newton's Third Law, forces always come in pairs. When the boat's propeller pushes water backward (action), the water simultaneously exerts an equal and opposite force forward on the propeller (reaction). Because the propeller is attached to the boat, this reaction force is what propels the entire vessel forward.

Step-by-Step Solution

1
Identify the interacting bodies in the mechanical system.
The two primary interacting bodies are the boat's propeller and the surrounding water.
Newton's Third Law requires identifying the two distinct bodies exerting forces on each other.
2
Determine the action force created by the boat's mechanism.
The turning propeller exerts a force backward against the water.
To move forward, a marine propulsion system must push fluid in the opposite direction.
3
Apply Newton's Third Law to find the equal and opposite reaction force.
If the propeller pushes the water backward, the water must push the propeller forward with an equal magnitude of force.
Forces always occur in simultaneous pairs acting on different objects.
4
Isolate the force that acts specifically on the boat.
The forward force from the water acting on the propeller blades is what accelerates the boat forward.
An object's acceleration is determined only by the external forces acting upon it, not the forces it exerts on other things.

Key Concept

Newton's Third Law requires that for every action force, there is a simultaneous reaction force equal in magnitude and opposite in direction, acting on two different bodies.
Question 17Question →

Two warehouse workers are sitting on identical, frictionless wheeled carts facing each other. Worker X and his cart have a combined mass of 120 kg120 \text{ kg}, while Worker Y and his cart have a combined mass of 60 kg60 \text{ kg}. Worker X pushes against Worker Y's cart with a steady force of 240 N240 \text{ N}. Which of the following best describes the forces and resulting accelerations experienced by both workers during the push?

Show answer & explanation

Answer: Both workers experience an equal force of 240 N240 \text{ N} in opposite directions, but Worker Y accelerates twice as fast as Worker X.

Answer

Both workers experience an equal force of 240 N240 \text{ N} in opposite directions, but Worker Y accelerates twice as fast as Worker X.
The correct answer accurately combines Newton's Third and Second Laws. By the Third Law, the action force of 240 N240 \text{ N} exerted by Worker X creates an equal and opposite reaction force of 240 N240 \text{ N} on Worker X. By the Second Law (F=maF = ma), acceleration is inversely proportional to mass. Since Worker Y has half the mass (60 kg60 \text{ kg}) of Worker X (120 kg120 \text{ kg}), the same 240 N240 \text{ N} force will cause Worker Y to accelerate exactly twice as fast.

Step-by-Step Solution

1
Determine the force acting on both workers using Newton's Third Law.
Both Worker X and Worker Y experience a 240 N240 \text{ N} force in opposite directions.
Newton's Third Law states that for every action, there is an equal and opposite reaction. The push of 240 N240 \text{ N} generates a reaction force of exactly 240 N240 \text{ N} on the person pushing.
2
Calculate the acceleration of Worker X using Newton's Second Law (a=Fma = \frac{F}{m}).
Acceleration of Worker X = 240 N120 kg=2 m/s2\frac{240 \text{ N}}{120 \text{ kg}} = 2 \text{ m/s}^2.
Acceleration depends on the net force applied and the total mass of the object.
3
Calculate the acceleration of Worker Y using Newton's Second Law (a=Fma = \frac{F}{m}).
Acceleration of Worker Y = 240 N60 kg=4 m/s2\frac{240 \text{ N}}{60 \text{ kg}} = 4 \text{ m/s}^2.
Worker Y has the same magnitude of force applied but half the mass, resulting in a different acceleration.
4
Compare the accelerations of the two workers.
Worker Y (4 m/s24 \text{ m/s}^2) accelerates exactly twice as fast as Worker X (2 m/s22 \text{ m/s}^2).
The inversely proportional relationship between mass and acceleration means the object with half the mass achieves twice the acceleration under an identical force.

Key Concept

Interaction of Newton's Third Law (Force Pairs) and Second Law (F=maF = ma)
Question 18Question →

An industrial electromagnet attracts a piece of scrap steel. The magnet pulls the steel forward, and the steel pulls the magnet backward with an equal and opposite force. Because these two forces are equal in magnitude and opposite in direction, they cancel each other out, preventing the steel from accelerating.

Show answer & explanation

Answer: False

Answer

The statement is False.
The forces in an action-reaction pair never cancel each other out because they act on completely different objects. The net force determining the acceleration of the scrap steel depends only on the magnetic pull acting directly on the steel, completely independent of the reaction force the steel exerts back on the magnet.

Step-by-Step Solution

1
Identify the two forces described in the scenario.
Force 1 is the magnet pulling on the steel. Force 2 is the steel pulling on the magnet.
Mapping the specific forces is required to apply Newton's Laws of Motion correctly.
2
Determine which object receives each force.
The first force acts exclusively on the scrap steel, while the second force acts exclusively on the electromagnet.
Net force calculations must be done for a single object at a time.
3
Evaluate whether the forces can cancel each other out.
Because the forces are applied to completely different objects, they do not cancel each other.
Forces can only cancel out and result in zero net force if they are acting in opposite directions on the exact same body.

Key Concept

Action and reaction force pairs act on completely different objects, so they never cancel each other out when determining the net force on a single object.
Estimated Time:45s
Question 19Question →

According to Newton's Third Law of Motion, forces always exist in action-reaction pairs acting between interacting objects. For each mechanical action described below, identify its exact corresponding reaction force.

Click a left item, then click its matching right item

Items

Helicopter rotors push a mass of air downward.
A pneumatic nail gun forcefully drives a nail forward.
The metal treads of an advancing bulldozer push backward against the dirt.
A compressed suspension spring pushes downward on a vehicle's axle.

Matches

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Answer

Match each action force with the reaction force that reverses the roles of the exact same two interacting bodies and points in the opposite direction.
Newton's Third Law dictates that forces always come in pairs. If Object A exerts a force on Object B, then Object B simultaneously exerts a force of equal magnitude in the exact opposite direction on Object A. The correct matches precisely swap the interacting bodies and reverse the direction of the applied force for each mechanical scenario.

Step-by-Step Solution

1
Identify the two specific interacting objects (Object A and Object B) in each action scenario.
The interacting pairs are: Rotors and Air; Gun mechanism and Nail; Treads and Dirt; Spring and Axle.
Newton's Third Law force pairs strictly occur between the two specific bodies that are interacting.
2
Determine the direction of the initial action force applied by Object A onto Object B.
Rotors push down; Gun pushes forward; Treads push backward; Spring pushes down.
The corresponding reaction force must be exactly opposite in direction.
3
Identify the statement that correctly reverses the interacting objects (Object B acting on Object A) and reverses the direction.
Air pushes rotors up; Nail pushes gun backward; Dirt pushes treads forward; Axle pushes spring up.
This guarantees an equal and opposite force acting on the counterpart body, completely fulfilling the action-reaction criteria.

Key Concept

Newton's Third Law (Action/Reaction force pairs)
Question 20Question →

A 75 kg75 \text{ kg} mechanic stands on a slick, oil-covered garage floor with negligible friction. The mechanic pushes forward horizontally against a stationary 150 kg150 \text{ kg} tool cabinet with a constant force of 200 N200 \text{ N}. Which of the following statements correctly describes the forces involved in this interaction?

Show answer & explanation

Answer: The cabinet experiences a 200 N200 \text{ N} forward force, and the mechanic experiences a 200 N200 \text{ N} backward force simultaneously.

Answer

The statement accurately describing the interaction is that the cabinet experiences a 200 N200 \text{ N} forward force, and the mechanic simultaneously experiences a 200 N200 \text{ N} backward force.
According to Newton's Third Law, any force exerted by one object on another is accompanied by an equal and opposite force exerted back by the second object on the first. Because the mechanic pushes the cabinet with 200 N200 \text{ N} forward, the cabinet simultaneously pushes the mechanic with 200 N200 \text{ N} backward. The differing masses do not alter the force magnitudes, though they result in different accelerations.

Step-by-Step Solution

1
Identify the interacting bodies in the given scenario.
The two interacting bodies are the mechanic and the tool cabinet.
Newton's Third Law applies to the forces exerted between two interacting objects.
2
Determine the magnitude and direction of the action force.
The mechanic exerts a forward force of 200 N200 \text{ N} on the tool cabinet.
This is the initial action provided in the problem description.
3
Apply Newton's Third Law to find the reaction force.
The tool cabinet must exert an equal and opposite reaction force of 200 N200 \text{ N} backward on the mechanic.
Forces always exist in equal and opposite pairs acting on different objects, regardless of their respective masses or whether they accelerate.

Key Concept

Newton's Third Law of Motion states that when two objects interact, they apply forces to each other of equal magnitude and opposite direction.
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Newton's Third Law (Action/Reaction) Practice Questions — Armed Services Vocational Aptitude Battery (ASVAB) | Examkin