Newton's third law of motion
1
According to Newton's third law of motion, when one object exerts a force on another object, what happens?
Practice
A
The second object exerts an equal and opposite force on the first object.
B
The second object exerts a larger force on the first object.
C
The second object exerts a smaller force on the first object.
D
The second object does not exert any force on the first object.
✅ Show Answer
✔ Correct Answer:
A
(The second object exerts an equal and opposite force on the first object.)
💡 Explanation
Newton's third law states that forces always occur in pairs.
When object A exerts a force on object B, object B simultaneously exerts a force on object A.
The two forces:
• have equal magnitude
• act in opposite directions
• act on different objects
Therefore, if A pushes B with a certain force, B pushes A with an equal force in the opposite direction.
When object A exerts a force on object B, object B simultaneously exerts a force on object A.
The two forces:
• have equal magnitude
• act in opposite directions
• act on different objects
Therefore, if A pushes B with a certain force, B pushes A with an equal force in the opposite direction.
🎯 Conclusion
For every action force, there is an equal and opposite reaction force acting on the other object.
2
A student pushes a wall with a force of 50 N. According to Newton's third law, what force does the wall exert on the student?
Practice
A
0 N
B
25 N in the same direction
C
50 N in the opposite direction
D
100 N in the opposite direction
✅ Show Answer
✔ Correct Answer:
C
(50 N in the opposite direction)
💡 Explanation
The student exerts a force of 50 N on the wall.
According to Newton's third law, the wall simultaneously exerts an equal and opposite force on the student.
Therefore:
Reaction force = 50 N
The direction is opposite to the force exerted by the student.
According to Newton's third law, the wall simultaneously exerts an equal and opposite force on the student.
Therefore:
Reaction force = 50 N
The direction is opposite to the force exerted by the student.
🎯 Conclusion
The reaction force has the same magnitude as the action force but acts in the opposite direction.
3
A book rests on a table. Which pair represents a Newton's third-law force pair?
Practice
A
Weight of the book and normal force on the book
B
Force of the book on the table and force of the table on the book
C
Weight of the book and force of the book on the table
D
Normal force on the book and weight of the book
✅ Show Answer
✔ Correct Answer:
B
(Force of the book on the table and force of the table on the book)
💡 Explanation
Newton's third-law forces must:
• be equal in magnitude
• be opposite in direction
• act on different objects
The book exerts a downward force on the table.
The table exerts an upward force on the book.
These forces act on different objects and form a third-law pair.
The weight of the book and the normal force on the book are not a third-law pair because both act on the same object—the book.
• be equal in magnitude
• be opposite in direction
• act on different objects
The book exerts a downward force on the table.
The table exerts an upward force on the book.
These forces act on different objects and form a third-law pair.
The weight of the book and the normal force on the book are not a third-law pair because both act on the same object—the book.
🎯 Conclusion
Third-law pairs always act on two different interacting objects.
4
A hammer strikes a nail with a force of 100 N. What is the force exerted by the nail on the hammer?
Practice
A
0 N
B
50 N
C
100 N in the opposite direction
D
200 N in the same direction
✅ Show Answer
✔ Correct Answer:
C
(100 N in the opposite direction)
💡 Explanation
The hammer exerts 100 N on the nail.
By Newton's third law, the nail exerts an equal and opposite force on the hammer.
Therefore:
Force of nail on hammer = 100 N
Its direction is opposite to the force exerted by the hammer.
By Newton's third law, the nail exerts an equal and opposite force on the hammer.
Therefore:
Force of nail on hammer = 100 N
Its direction is opposite to the force exerted by the hammer.
🎯 Conclusion
The interaction between two objects produces equal and opposite forces on the two objects.
5
A person pushes a box with a force of 80 N. Why does the person experience a force from the box?
Practice
A
Because the box has greater mass
B
Because the box exerts an equal and opposite reaction force
C
Because friction always doubles the applied force
D
Because the person's force disappears
✅ Show Answer
✔ Correct Answer:
B
(Because the box exerts an equal and opposite reaction force)
💡 Explanation
When the person pushes the box, the person exerts a force on the box.
At the same time, the box exerts a force back on the person.
According to Newton's third law, this reaction force is equal in magnitude and opposite in direction.
Therefore, if the person pushes with 80 N, the box pushes back with 80 N in the opposite direction.
At the same time, the box exerts a force back on the person.
According to Newton's third law, this reaction force is equal in magnitude and opposite in direction.
Therefore, if the person pushes with 80 N, the box pushes back with 80 N in the opposite direction.
🎯 Conclusion
Whenever two objects interact, each object exerts a force on the other.
6
A person pushes a wall with 200 N, but the wall does not move. What is the magnitude of the force exerted by the wall on the person?
Practice
A
0 N
B
100 N
C
200 N
D
It depends on the person's mass
✅ Show Answer
✔ Correct Answer:
C
(200 N)
💡 Explanation
The wall does not need to move for Newton's third law to apply.
The person exerts 200 N on the wall.
Therefore, the wall exerts an equal and opposite force of 200 N on the person.
The fact that the wall remains stationary does not mean that the reaction force is zero.
The person exerts 200 N on the wall.
Therefore, the wall exerts an equal and opposite force of 200 N on the person.
The fact that the wall remains stationary does not mean that the reaction force is zero.
🎯 Conclusion
An object can exert a reaction force even when it does not move.
7
A swimmer pushes water backward with a force of 60 N. What force does the water exert on the swimmer?
Practice
A
60 N backward
B
30 N forward
C
60 N forward
D
120 N forward
✅ Show Answer
✔ Correct Answer:
C
(60 N forward)
💡 Explanation
The swimmer pushes the water backward.
According to Newton's third law, the water exerts an equal and opposite force on the swimmer.
Therefore:
Force exerted by water = 60 N
Direction = forward
This forward reaction force helps the swimmer move forward.
According to Newton's third law, the water exerts an equal and opposite force on the swimmer.
Therefore:
Force exerted by water = 60 N
Direction = forward
This forward reaction force helps the swimmer move forward.
🎯 Conclusion
Swimming is possible because the swimmer pushes water backward and the water pushes the swimmer forward.
8
A rocket pushes exhaust gases downward. According to Newton's third law, what happens to the rocket?
Practice
A
The rocket experiences no force.
B
The rocket experiences an upward force.
C
The rocket experiences a downward force.
D
The rocket experiences a force in the same direction as the gases.
✅ Show Answer
✔ Correct Answer:
B
(The rocket experiences an upward force.)
💡 Explanation
The rocket engine pushes exhaust gases downward.
The exhaust gases simultaneously exert an equal and opposite force on the rocket.
Therefore, the reaction force on the rocket is upward.
This upward force can accelerate the rocket upward.
The exhaust gases simultaneously exert an equal and opposite force on the rocket.
Therefore, the reaction force on the rocket is upward.
This upward force can accelerate the rocket upward.
🎯 Conclusion
Rocket propulsion is an example of Newton's third law: gases are pushed downward and the rocket is pushed upward.
9
A person jumps from a boat onto a dock. The boat moves backward as the person moves forward. Which principle explains the backward motion of the boat?
Practice
A
Newton's first law only
B
Newton's second law only
C
Newton's third law
D
The law of conservation of mass
✅ Show Answer
✔ Correct Answer:
C
(Newton's third law)
💡 Explanation
When the person pushes against the boat while jumping toward the dock, the person exerts a force on the boat.
The boat exerts an equal and opposite force on the person.
As a result, the person moves forward while the boat moves backward.
This interaction is explained by Newton's third law.
The boat exerts an equal and opposite force on the person.
As a result, the person moves forward while the boat moves backward.
This interaction is explained by Newton's third law.
🎯 Conclusion
The forward motion of one interacting object can be accompanied by an opposite motion of the other object.
10
A car tyre pushes the road backward while the car accelerates forward. What force mainly provides the forward interaction force on the car?
Practice
A
The car's weight
B
The road's frictional force on the tyre
C
The car's engine pulling the road
D
The air pressure inside the tyre
✅ Show Answer
✔ Correct Answer:
B
(The road's frictional force on the tyre)
💡 Explanation
The tyre pushes backward against the road.
According to Newton's third law, the road exerts an equal and opposite force on the tyre.
Therefore, the road exerts a forward frictional force on the tyre.
This forward force contributes to the car's acceleration.
According to Newton's third law, the road exerts an equal and opposite force on the tyre.
Therefore, the road exerts a forward frictional force on the tyre.
This forward force contributes to the car's acceleration.
🎯 Conclusion
A car accelerates forward because the road exerts a forward frictional force on the tyres when the tyres push backward on the road.
11
A person pulls a rope with a force of 40 N. According to Newton's third law, what force does the rope exert on the person?
Practice
A
0 N
B
20 N in the same direction
C
40 N in the opposite direction
D
80 N in the opposite direction
✅ Show Answer
✔ Correct Answer:
C
(40 N in the opposite direction)
💡 Explanation
The person pulls the rope with a force of 40 N.
The rope simultaneously pulls the person with an equal force in the opposite direction.
Therefore:
Reaction force = 40 N
Direction = opposite to the person's pull.
The two forces form a Newton's third-law pair.
The rope simultaneously pulls the person with an equal force in the opposite direction.
Therefore:
Reaction force = 40 N
Direction = opposite to the person's pull.
The two forces form a Newton's third-law pair.
🎯 Conclusion
For a direct interaction between two objects, the forces they exert on each other are equal in magnitude and opposite in direction.
12
A 2 kg ball exerts a force of 10 N on a 5 kg ball during a collision. What force does the 5 kg ball exert on the 2 kg ball?
Practice
A
2 N in the opposite direction
B
5 N in the opposite direction
C
10 N in the opposite direction
D
25 N in the opposite direction
✅ Show Answer
✔ Correct Answer:
C
(10 N in the opposite direction)
💡 Explanation
During the collision, the two balls exert forces on each other.
The 2 kg ball exerts:
F = 10 N
According to Newton's third law, the 5 kg ball simultaneously exerts an equal and opposite force.
Therefore:
F = 10 N in the opposite direction.
The 2 kg ball exerts:
F = 10 N
According to Newton's third law, the 5 kg ball simultaneously exerts an equal and opposite force.
Therefore:
F = 10 N in the opposite direction.
🎯 Conclusion
Third-law forces are equal in magnitude even when the interacting objects have different masses.
13
A small ball and a large ball collide. Which statement about the forces during the collision is correct?
Practice
A
The large ball exerts a greater force.
B
The small ball exerts a greater force.
C
Both balls exert equal forces on each other.
D
Only the moving ball exerts a force.
✅ Show Answer
✔ Correct Answer:
C
(Both balls exert equal forces on each other.)
💡 Explanation
Newton's third law states that when two objects interact, they exert forces of equal magnitude and opposite direction on each other.
Therefore, the size or mass of the objects does not change the equality of the interaction forces.
The forces are equal, although the resulting accelerations may be different because the masses may be different.
Therefore, the size or mass of the objects does not change the equality of the interaction forces.
The forces are equal, although the resulting accelerations may be different because the masses may be different.
🎯 Conclusion
Different masses do not produce different third-law force magnitudes.
14
A 2 kg object and a 10 kg object interact and exert equal forces of 20 N on each other. Which object has the greater acceleration due to this interaction?
Practice
A
The 10 kg object
B
The 2 kg object
C
Both have the same acceleration
D
Neither object accelerates
✅ Show Answer
✔ Correct Answer:
B
(The 2 kg object)
💡 Explanation
The third-law forces have equal magnitude:
F = 20 N
For the 2 kg object:
a₁ = F / m
a₁ = 20 / 2
a₁ = 10 m/s²
For the 10 kg object:
a₂ = F / m
a₂ = 20 / 10
a₂ = 2 m/s²
Therefore, the smaller-mass object has the greater acceleration.
F = 20 N
For the 2 kg object:
a₁ = F / m
a₁ = 20 / 2
a₁ = 10 m/s²
For the 10 kg object:
a₂ = F / m
a₂ = 20 / 10
a₂ = 2 m/s²
Therefore, the smaller-mass object has the greater acceleration.
🎯 Conclusion
Third-law forces are equal, but the accelerations can be different because the masses are different.
15
Earth pulls an apple downward with a gravitational force of 5 N. What is the corresponding Newton's third-law force?
Practice
A
The apple pulls Earth upward with 5 N.
B
The apple pulls Earth downward with 5 N.
C
Earth pulls the apple upward with 5 N.
D
The apple exerts zero force on Earth.
✅ Show Answer
✔ Correct Answer:
A
(The apple pulls Earth upward with 5 N.)
💡 Explanation
Earth exerts a downward gravitational force of 5 N on the apple.
The apple simultaneously exerts an equal gravitational force on Earth in the opposite direction.
Therefore:
Force of apple on Earth = 5 N upward.
The apple simultaneously exerts an equal gravitational force on Earth in the opposite direction.
Therefore:
Force of apple on Earth = 5 N upward.
🎯 Conclusion
Gravitational interaction also forms a Newton's third-law pair: Earth pulls the apple and the apple pulls Earth.
16
A person standing on Earth is pulled downward by Earth's gravity. According to Newton's third law, what force does the person exert on Earth?
Practice
A
A smaller downward force
B
An equal upward gravitational force
C
An equal downward gravitational force
D
No force
✅ Show Answer
✔ Correct Answer:
B
(An equal upward gravitational force)
💡 Explanation
Earth exerts a gravitational force downward on the person.
The person also exerts a gravitational gravitational force on Earth.
The two forces have equal magnitude and opposite directions.
Therefore, if Earth pulls the person downward, the person pulls Earth upward with the same magnitude.
The person also exerts a gravitational gravitational force on Earth.
The two forces have equal magnitude and opposite directions.
Therefore, if Earth pulls the person downward, the person pulls Earth upward with the same magnitude.
🎯 Conclusion
The gravitational force between two objects is mutual: each object pulls the other with equal magnitude.
17
A gun fires a bullet forward. According to Newton's third law, what happens to the gun?
Practice
A
The gun experiences no force.
B
The gun experiences a backward force.
C
The gun experiences a forward force greater than the bullet's force.
D
The gun experiences a backward force smaller than the bullet's force.
✅ Show Answer
✔ Correct Answer:
B
(The gun experiences a backward force.)
💡 Explanation
The gun exerts a forward force on the bullet.
The bullet simultaneously exerts an equal and opposite force on the gun.
Therefore, the gun experiences a backward force, producing recoil.
The forces are equal in magnitude, but the accelerations are not necessarily equal because the masses are different.
The bullet simultaneously exerts an equal and opposite force on the gun.
Therefore, the gun experiences a backward force, producing recoil.
The forces are equal in magnitude, but the accelerations are not necessarily equal because the masses are different.
🎯 Conclusion
Gun recoil is a direct example of Newton's third law.
18
A 1 kg bullet and a 10 kg gun exert equal and opposite forces during firing. Which object generally has the greater acceleration?
Practice
A
The gun
B
The bullet
C
Both have the same acceleration
D
Neither accelerates
✅ Show Answer
✔ Correct Answer:
B
(The bullet)
💡 Explanation
The forces on the bullet and gun are equal in magnitude.
However, acceleration is given by:
a = F / m
The bullet has a much smaller mass than the gun.
Therefore, for the same force, the bullet experiences the greater acceleration.
However, acceleration is given by:
a = F / m
The bullet has a much smaller mass than the gun.
Therefore, for the same force, the bullet experiences the greater acceleration.
🎯 Conclusion
Equal third-law forces can produce very different accelerations because the interacting objects can have different masses.
19
Two skaters push each other apart. Skater A has a mass of 40 kg and Skater B has a mass of 80 kg. If they exert equal forces on each other, which skater has the greater acceleration?
Practice
A
Skater A
B
Skater B
C
Both have equal acceleration
D
Neither skater accelerates
✅ Show Answer
✔ Correct Answer:
A
(Skater A)
💡 Explanation
The skaters exert equal and opposite forces on each other according to Newton's third law.
Acceleration is:
a = F / m
Skater A has mass 40 kg, while Skater B has mass 80 kg.
For the same force, the smaller mass experiences the greater acceleration.
Therefore, Skater A has the greater acceleration.
Acceleration is:
a = F / m
Skater A has mass 40 kg, while Skater B has mass 80 kg.
For the same force, the smaller mass experiences the greater acceleration.
Therefore, Skater A has the greater acceleration.
🎯 Conclusion
In a mutual interaction, the lighter object can have greater acceleration even though the forces are equal.
20
Two students standing on frictionless carts push each other apart. Student A pushes Student B with 30 N. What is the force exerted by Student B on Student A?
Practice
A
0 N
B
15 N
C
30 N in the opposite direction
D
60 N in the same direction
✅ Show Answer
✔ Correct Answer:
C
(30 N in the opposite direction)
💡 Explanation
Student A exerts a force of 30 N on Student B.
Newton's third law requires Student B to exert an equal and opposite force on Student A.
Therefore:
F = 30 N
Direction = opposite to the force exerted by Student A.
Newton's third law requires Student B to exert an equal and opposite force on Student A.
Therefore:
F = 30 N
Direction = opposite to the force exerted by Student A.
🎯 Conclusion
The interaction forces between two objects are always equal in magnitude and opposite in direction.
21
Two objects exert equal and opposite forces on each other. Why do these forces not cancel each other when finding the acceleration of either individual object?
Practice
A
Because the forces have different magnitudes
B
Because the forces act on different objects
C
Because the forces act in the same direction
D
Because one force occurs later
✅ Show Answer
✔ Correct Answer:
B
(Because the forces act on different objects)
💡 Explanation
Newton's third-law forces are equal and opposite, but they act on different objects.
For example, if object A pushes object B, the force of A on B acts on B, while the force of B on A acts on A.
When calculating the net force on one object, only forces acting on that object are included.
Therefore, the two third-law forces do not cancel each other on a single object.
For example, if object A pushes object B, the force of A on B acts on B, while the force of B on A acts on A.
When calculating the net force on one object, only forces acting on that object are included.
Therefore, the two third-law forces do not cancel each other on a single object.
🎯 Conclusion
Third-law forces do not cancel each other because they act on different objects.
22
A block A is placed on a horizontal table. Block A exerts a downward force on the table. Which force is the Newton's third-law partner of this force?
Practice
A
The weight of block A
B
The normal force exerted by the table on block A
C
The gravitational force exerted by Earth on block A
D
The frictional force on block A
✅ Show Answer
✔ Correct Answer:
B
(The normal force exerted by the table on block A)
💡 Explanation
Block A exerts a downward contact force on the table.
The Newton's third-law partner must:
• have the same magnitude,
• act in the opposite direction,
• act on the other object involved in the interaction.
Therefore, the table exerts an upward normal force on block A.
The weight of block A is not the third-law partner because weight is the gravitational force exerted by Earth on the block.
The Newton's third-law partner must:
• have the same magnitude,
• act in the opposite direction,
• act on the other object involved in the interaction.
Therefore, the table exerts an upward normal force on block A.
The weight of block A is not the third-law partner because weight is the gravitational force exerted by Earth on the block.
🎯 Conclusion
For a contact force, its third-law partner is the equal and opposite contact force exerted by the other object.
23
A block of mass 5 kg rests on a horizontal table. Which pair of forces is a Newton's third-law pair?
Practice
A
Weight of the block and normal force on the block
B
Weight of the block and gravitational force exerted by the block on Earth
C
Normal force on the block and weight of the block
D
Weight of the block and frictional force on the block
✅ Show Answer
✔ Correct Answer:
B
(Weight of the block and gravitational force exerted by the block on Earth)
💡 Explanation
The weight of the block is the gravitational force exerted by Earth on the block.
Its Newton's third-law partner is the gravitational force exerted by the block on Earth.
These two forces act on different objects:
• Earth pulls the block downward.
• The block pulls Earth upward.
The weight and normal force are not a third-law pair because both act on the block.
Its Newton's third-law partner is the gravitational force exerted by the block on Earth.
These two forces act on different objects:
• Earth pulls the block downward.
• The block pulls Earth upward.
The weight and normal force are not a third-law pair because both act on the block.
🎯 Conclusion
The third-law partner of a gravitational force on an object is the gravitational force exerted by that object on the source of gravity.
24
A block of mass 2 kg is pulled horizontally by a string. The string exerts a force of 10 N on the block. What is the force exerted by the block on the string?
Practice
A
0 N
B
5 N
C
10 N in the opposite direction
D
20 N in the same direction
✅ Show Answer
✔ Correct Answer:
C
(10 N in the opposite direction)
💡 Explanation
The string exerts a 10 N force on the block.
The block simultaneously exerts an equal and opposite force on the string.
Therefore:
Force exerted by block on string = 10 N
Direction = opposite to the force exerted by the string on the block.
The block simultaneously exerts an equal and opposite force on the string.
Therefore:
Force exerted by block on string = 10 N
Direction = opposite to the force exerted by the string on the block.
🎯 Conclusion
The tension force exerted by a string on an object has an equal and opposite third-law partner exerted by the object on the string.
25
A person pulls a rope with a force of 100 N. The rope is connected to a wall. If the rope remains at rest, what is the force exerted by the wall on the rope?
Practice
A
0 N
B
50 N
C
100 N in the opposite direction
D
200 N in the same direction
✅ Show Answer
✔ Correct Answer:
C
(100 N in the opposite direction)
💡 Explanation
The person pulls the rope with 100 N.
If the rope remains at rest, the wall must exert a force that balances the pull.
Therefore, the wall exerts 100 N on the rope in the opposite direction.
This is also consistent with Newton's third-law interaction between the rope and the wall.
If the rope remains at rest, the wall must exert a force that balances the pull.
Therefore, the wall exerts 100 N on the rope in the opposite direction.
This is also consistent with Newton's third-law interaction between the rope and the wall.
🎯 Conclusion
A stationary rope can still experience substantial forces; zero acceleration does not imply zero force.
26
A man of mass 60 kg stands on a weighing machine in a lift accelerating upward at 2 m/s². Which statement about the forces between the man and the weighing machine is correct?
Practice
A
The machine exerts a force of 600 N and the man exerts 600 N on it.
B
The machine exerts a force greater than 600 N and the man exerts an equal force on it.
C
The machine exerts a force less than 600 N and the man exerts a larger force.
D
The two forces are unequal because the man is accelerating.
✅ Show Answer
✔ Correct Answer:
B
(The machine exerts a force greater than 600 N and the man exerts an equal force on it.)
💡 Explanation
The man's weight is approximately:
mg = 60 × 10 = 600 N
Since the lift accelerates upward:
N - mg = ma
N = m(g + a)
N = 60(10 + 2)
N = 720 N
The machine therefore exerts 720 N upward on the man.
By Newton's third law, the man exerts an equal 720 N downward force on the machine.
mg = 60 × 10 = 600 N
Since the lift accelerates upward:
N - mg = ma
N = m(g + a)
N = 60(10 + 2)
N = 720 N
The machine therefore exerts 720 N upward on the man.
By Newton's third law, the man exerts an equal 720 N downward force on the machine.
🎯 Conclusion
Even when the object accelerates, third-law forces remain equal and opposite; acceleration depends on the net force acting on each object.
27
A 50 kg person stands in a lift accelerating downward at 2 m/s². Take g = 10 m/s². What is the force exerted by the person on the floor?
Practice
A
300 N
B
400 N
C
500 N
D
600 N
✅ Show Answer
✔ Correct Answer:
B
(400 N)
💡 Explanation
For the person, taking upward as positive:
N - mg = -ma
Therefore:
N = m(g - a)
N = 50(10 - 2)
N = 400 N
The floor exerts 400 N upward on the person.
By Newton's third law, the person exerts 400 N downward on the floor.
N - mg = -ma
Therefore:
N = m(g - a)
N = 50(10 - 2)
N = 400 N
The floor exerts 400 N upward on the person.
By Newton's third law, the person exerts 400 N downward on the floor.
🎯 Conclusion
The force between two interacting objects is always equal and opposite, even when the objects are accelerating.
28
A 10 kg block is pulled horizontally by a force of 30 N on a frictionless surface. What is the magnitude of the force exerted by the block on the pulling agent?
Practice
A
0 N
B
3 N
C
10 N
D
30 N
✅ Show Answer
✔ Correct Answer:
D
(30 N)
💡 Explanation
The pulling agent exerts 30 N on the block.
Newton's third law states that the block simultaneously exerts an equal and opposite force on the pulling agent.
Therefore:
Reaction force = 30 N.
Newton's third law states that the block simultaneously exerts an equal and opposite force on the pulling agent.
Therefore:
Reaction force = 30 N.
🎯 Conclusion
The reaction force does not depend on whether the block is accelerating; it remains equal in magnitude to the applied interaction force.
29
Two blocks A and B are in contact on a frictionless horizontal surface. A force is applied to block A, causing both blocks to accelerate together. The force exerted by A on B is 12 N. What is the force exerted by B on A?
Practice
A
0 N
B
6 N
C
12 N in the opposite direction
D
24 N in the same direction
✅ Show Answer
✔ Correct Answer:
C
(12 N in the opposite direction)
💡 Explanation
Block A exerts a force of 12 N on block B.
The contact interaction between A and B produces an equal and opposite force.
Therefore, block B exerts:
12 N on A
The direction is opposite to the force exerted by A on B.
The contact interaction between A and B produces an equal and opposite force.
Therefore, block B exerts:
12 N on A
The direction is opposite to the force exerted by A on B.
🎯 Conclusion
The contact forces between two touching bodies form a Newton's third-law pair.
30
A 2 kg block A pushes a 3 kg block B with a force of 15 N. Which statement is correct?
Practice
A
B pushes A with less than 15 N because B has greater mass.
B
B pushes A with more than 15 N because B has greater mass.
C
B pushes A with exactly 15 N in the opposite direction.
D
B exerts no force on A because A initiates the motion.
✅ Show Answer
✔ Correct Answer:
C
(B pushes A with exactly 15 N in the opposite direction.)
💡 Explanation
Newton's third law applies regardless of the masses of the objects.
If A exerts 15 N on B, then B exerts exactly 15 N on A in the opposite direction.
The different masses can produce different accelerations, but they do not make the third-law forces unequal.
If A exerts 15 N on B, then B exerts exactly 15 N on A in the opposite direction.
The different masses can produce different accelerations, but they do not make the third-law forces unequal.
🎯 Conclusion
Mass affects acceleration, not the equality of the third-law force pair.
31
A bullet of mass 20 g is fired from a gun of mass 2 kg. During firing, the bullet experiences a force of 100 N from the gun. What is the magnitude of the force exerted by the bullet on the gun?
Practice
A
1 N
B
20 N
C
100 N
D
5000 N
✅ Show Answer
✔ Correct Answer:
C
(100 N)
💡 Explanation
The gun exerts a force of 100 N on the bullet.
According to Newton's third law, the bullet exerts an equal and opposite force on the gun.
Therefore:
F = 100 N
The different masses affect their accelerations, not the magnitude of the interaction forces.
According to Newton's third law, the bullet exerts an equal and opposite force on the gun.
Therefore:
F = 100 N
The different masses affect their accelerations, not the magnitude of the interaction forces.
🎯 Conclusion
During recoil, the gun and bullet experience equal and opposite forces.
32
A 1 kg ball moving toward a wall collides with it. During the collision, the wall exerts a force of 500 N on the ball. What is the force exerted by the ball on the wall?
Practice
A
0 N
B
250 N
C
500 N in the opposite direction
D
1000 N in the same direction
✅ Show Answer
✔ Correct Answer:
C
(500 N in the opposite direction)
💡 Explanation
The wall exerts 500 N on the ball.
By Newton's third law, the ball simultaneously exerts an equal force on the wall in the opposite direction.
Therefore:
Force of ball on wall = 500 N.
By Newton's third law, the ball simultaneously exerts an equal force on the wall in the opposite direction.
Therefore:
Force of ball on wall = 500 N.
🎯 Conclusion
In a collision, the interacting objects exert equal and opposite forces on each other.
33
A 4 kg block is pressed against a vertical wall by a horizontal force. The wall exerts a normal force of 80 N on the block. What is the force exerted by the block on the wall?
Practice
A
0 N
B
40 N
C
80 N
D
160 N
✅ Show Answer
✔ Correct Answer:
C
(80 N)
💡 Explanation
The wall exerts a normal force of 80 N on the block.
The block simultaneously exerts an equal and opposite normal force on the wall.
Therefore:
Force exerted by block on wall = 80 N.
The block simultaneously exerts an equal and opposite normal force on the wall.
Therefore:
Force exerted by block on wall = 80 N.
🎯 Conclusion
Normal forces between two surfaces form an equal and opposite third-law pair.
34
A person pulls a suitcase with a force of 40 N. The suitcase pulls the person backward with 40 N. If the person accelerates forward, why do these two forces not cancel?
Practice
A
They are unequal forces.
B
They act on the same object.
C
They act on different objects.
D
They act in the same direction.
✅ Show Answer
✔ Correct Answer:
C
(They act on different objects.)
💡 Explanation
The person exerts 40 N on the suitcase.
The suitcase exerts 40 N backward on the person.
These are equal and opposite forces, but they act on different objects.
Therefore, they cannot be directly added together as forces acting on one object.
The suitcase exerts 40 N backward on the person.
These are equal and opposite forces, but they act on different objects.
Therefore, they cannot be directly added together as forces acting on one object.
🎯 Conclusion
Third-law forces never cancel each other when calculating the net force on a single object because they act on different objects.
35
A horse pulls a cart forward. The cart pulls the horse backward with an equal force. Why can the horse-cart system still accelerate forward?
Practice
A
Newton's third law is violated.
B
The two forces act on different objects, and the ground can exert an external forward force on the horse.
C
The cart's force is actually smaller.
D
The horse does not experience the backward force.
✅ Show Answer
✔ Correct Answer:
B
(The two forces act on different objects, and the ground can exert an external forward force on the horse.)
💡 Explanation
The horse pulls the cart forward, and the cart pulls the horse backward with an equal force.
These two forces act on different objects and therefore do not cancel when considering the horse alone or the cart alone.
The horse pushes the ground backward through its hooves.
The ground then exerts a forward frictional force on the horse. This external force can accelerate the horse-cart system forward.
These two forces act on different objects and therefore do not cancel when considering the horse alone or the cart alone.
The horse pushes the ground backward through its hooves.
The ground then exerts a forward frictional force on the horse. This external force can accelerate the horse-cart system forward.
🎯 Conclusion
A system can accelerate even though internal third-law forces are equal and opposite, because external forces determine the system's net force.
36
A person jumps upward from the ground. Which force is the Newton's third-law partner of the upward force exerted by the ground on the person?
Practice
A
The weight of the person
B
The upward force exerted by the person on the ground
C
The downward gravitational force exerted by Earth on the person
D
The normal force exerted by the ground on the person
✅ Show Answer
✔ Correct Answer:
B
(The upward force exerted by the person on the ground)
💡 Explanation
When the person jumps, the person pushes the ground downward.
The ground exerts an upward force on the person.
These two forces form a Newton's third-law pair:
• ground on person → upward
• person on ground → downward
The person's weight is not the third-law partner because it is a gravitational interaction, not the contact interaction with the ground.
The ground exerts an upward force on the person.
These two forces form a Newton's third-law pair:
• ground on person → upward
• person on ground → downward
The person's weight is not the third-law partner because it is a gravitational interaction, not the contact interaction with the ground.
🎯 Conclusion
To identify a third-law partner, first identify the interaction involved and then reverse the two interacting objects.
37
A block rests on a horizontal floor. The floor exerts a normal force N on the block. The third-law partner of this normal force is:
Practice
A
The weight mg of the block
B
The gravitational force of the block on Earth
C
The force exerted by the block on the floor, equal to N and downward
D
The frictional force on the block
✅ Show Answer
✔ Correct Answer:
C
(The force exerted by the block on the floor, equal to N and downward)
💡 Explanation
The normal force N is the force exerted by the floor on the block.
Its third-law partner must be the force exerted by the block on the floor.
Therefore, the block exerts a force of magnitude N downward on the floor.
The weight mg is a separate gravitational interaction.
Its third-law partner must be the force exerted by the block on the floor.
Therefore, the block exerts a force of magnitude N downward on the floor.
The weight mg is a separate gravitational interaction.
🎯 Conclusion
The third-law partner of a normal force is the equal and opposite force exerted by the object on the supporting surface.
38
A 5 kg block is pulled upward by a string with tension 60 N. What is the force exerted by the block on the string?
Practice
A
0 N
B
30 N
C
60 N downward
D
110 N downward
✅ Show Answer
✔ Correct Answer:
C
(60 N downward)
💡 Explanation
The string exerts an upward tension force of 60 N on the block.
By Newton's third law, the block exerts an equal and opposite force on the string.
Therefore:
Force exerted by block on string = 60 N downward.
By Newton's third law, the block exerts an equal and opposite force on the string.
Therefore:
Force exerted by block on string = 60 N downward.
🎯 Conclusion
The force exerted by a string on an object and the force exerted by the object on the string are an equal and opposite third-law pair.
39
A person pushes a box with 50 N to the right. The box pushes the person with 50 N to the left. If friction on the box is 20 N to the left, what is the net force on the box?
Practice
A
0 N
B
20 N left
C
30 N right
D
50 N right
✅ Show Answer
✔ Correct Answer:
C
(30 N right)
💡 Explanation
The 50 N force exerted by the person acts on the box to the right.
The 50 N reaction force acts on the person, not on the box. Therefore, it must not be included in the net force on the box.
Friction on the box is 20 N to the left.
Therefore:
Fₙₑₜ = 50 - 20
Fₙₑₜ = 30 N right.
The 50 N reaction force acts on the person, not on the box. Therefore, it must not be included in the net force on the box.
Friction on the box is 20 N to the left.
Therefore:
Fₙₑₜ = 50 - 20
Fₙₑₜ = 30 N right.
🎯 Conclusion
When calculating the net force on an object, include only forces acting on that object; its third-law partner acts on another object.
40
A 10 kg block is pushed against a wall with a horizontal force of 100 N. The wall pushes back with 100 N. If the block has no horizontal acceleration, which statement is correct?
Practice
A
Newton's third law is violated because the forces cancel.
B
The two 100 N forces are a third-law pair acting on the same object.
C
The 100 N force of the person on the block and the 100 N force of the wall on the block are different forces acting on the block.
D
The wall does not exert any force because the block does not move.
✅ Show Answer
✔ Correct Answer:
C
(The 100 N force of the person on the block and the 100 N force of the wall on the block are different forces acting on the block.)
💡 Explanation
The 100 N applied force acts on the block.
The wall's 100 N normal force also acts on the block in the opposite direction.
These two forces cancel in the horizontal direction, giving zero horizontal net force.
However, they are NOT a Newton's third-law pair because both forces act on the same object—the block.
The third-law partner of the wall's force on the block is the force exerted by the block on the wall.
The wall's 100 N normal force also acts on the block in the opposite direction.
These two forces cancel in the horizontal direction, giving zero horizontal net force.
However, they are NOT a Newton's third-law pair because both forces act on the same object—the block.
The third-law partner of the wall's force on the block is the force exerted by the block on the wall.
🎯 Conclusion
Equal and opposite forces acting on the same object may cancel, but they are not a Newton's third-law pair.
41
Two bodies A and B interact. The force exerted by A on B is 20 N east. If body B has an acceleration of 5 m/s² west due only to this interaction, what is the mass of B?
Practice
A
2 kg
B
4 kg
C
5 kg
D
10 kg
✅ Show Answer
✔ Correct Answer:
B
(4 kg)
💡 Explanation
The force exerted by A on B is 20 N east.
Therefore, the force exerted by B on A is 20 N west.
For body B, the stated acceleration is 5 m/s² west, so the force causing that acceleration must be 20 N west in magnitude.
Using Newton's second law:
F = ma
20 = m × 5
m = 4 kg.
Therefore, the force exerted by B on A is 20 N west.
For body B, the stated acceleration is 5 m/s² west, so the force causing that acceleration must be 20 N west in magnitude.
Using Newton's second law:
F = ma
20 = m × 5
m = 4 kg.
🎯 Conclusion
Newton's third law determines the interaction-force pair, while Newton's second law determines the acceleration produced on each object.
42
A block is pulled by a string with tension T. Which statement correctly identifies the third-law partner of the tension force exerted by the string on the block?
Practice
A
The weight of the block
B
The normal force on the block
C
The force exerted by the block on the string with magnitude T in the opposite direction
D
The frictional force on the block
✅ Show Answer
✔ Correct Answer:
C
(The force exerted by the block on the string with magnitude T in the opposite direction)
💡 Explanation
The tension force T is the force exerted by the string on the block.
Its Newton's third-law partner is the force exerted by the block on the string.
This force has:
• magnitude T
• opposite direction
• action on the string rather than on the block.
Its Newton's third-law partner is the force exerted by the block on the string.
This force has:
• magnitude T
• opposite direction
• action on the string rather than on the block.
🎯 Conclusion
The third-law partner of a tension force acts on the string, not on the same block.
43
A person standing on a weighing machine in an accelerating lift exerts a force of 700 N downward on the machine. What is the force exerted by the machine on the person?
Practice
A
0 N
B
350 N upward
C
700 N upward
D
It depends on the person's mass
✅ Show Answer
✔ Correct Answer:
C
(700 N upward)
💡 Explanation
The person exerts a downward force of 700 N on the weighing machine.
By Newton's third law, the machine exerts an equal and opposite force on the person.
Therefore:
Force of machine on person = 700 N upward.
By Newton's third law, the machine exerts an equal and opposite force on the person.
Therefore:
Force of machine on person = 700 N upward.
🎯 Conclusion
Third-law forces are equal and opposite regardless of whether the interacting objects are accelerating.