Which one of the following statements with regard to Newton's third law of motion is NOT correct?

Q: What is the answer to CAPF 2020 Paper I Q6?

C. Correct — C, There is a cause-effect relation implied in the third law. The third law says that if a body A exerts a force on a body B, then B exerts an equal and opposite force on A at the same instant. Neither force comes first and neither causes the other; they are two aspects of one interaction. Textbook treatments make the point explicitly, which is why the paired statement (d) is the correct one and (c) is the statement the question asks you to reject.

(a)Force never occurs singly in nature
(b)When the earth pulls a stone downwards due to gravity, the stone exerts a force on the earth
(c)There is a cause-effect relation implied in the third law
(d)There is no cause-effect relation implied in the third law

Answer

Why

Correct — C, There is a cause-effect relation implied in the third law. The third law says that if a body A exerts a force on a body B, then B exerts an equal and opposite force on A at the same instant. Neither force comes first and neither causes the other; they are two aspects of one interaction. Textbook treatments make the point explicitly, which is why the paired statement (d) is the correct one and (c) is the statement the question asks you to reject.

Why the others are wrong

(a)Force never occurs singly in nature — Forces genuinely never occur singly. Every force is one side of an interaction between two bodies, which is a true consequence of the third law and so not the answer to a NOT question.
(b)When the earth pulls a stone downwards due to gravity, the stone exerts a force on the earth — The stone really does pull the Earth up with a force equal to its own weight. The Earth's acceleration is unmeasurably small only because its mass is so enormous; the force is not.
(d)There is no cause-effect relation implied in the third law — This is the correct reading of the law — the two forces are simultaneous and neither is the cause of the other — so it cannot be the incorrect statement the question wants.

Concept

Newton's third law is a statement about a single interaction viewed from its two ends. The action and the reaction are equal in magnitude, opposite in direction, of the same type, and act on two different bodies. That last clause is what stops them from cancelling: they never appear together in the force balance of one body, so they cannot add to zero on it.

The trap in this item is temporal. Ordinary speech makes 'action' sound like the first event and 'reaction' like the answer to it, and the terms invite a story in which one force provokes the other. Physics makes no such claim — the two arise together and describing either as the cause is a mistake in the reading of the law rather than in arithmetic. Options (c) and (d) are exact contradictions, which is the giveaway: one of them has to be the statement being tested.

Key facts

Action and reaction are equal in magnitude, opposite in direction, and act on two different bodies.
Because they act on different bodies they never cancel each other in the force balance of a single body.
The two forces are simultaneous, so neither is the cause and neither the effect.
Conservation of linear momentum for an isolated system follows from the third law.
Walking, swimming, rocket propulsion and the recoil of a gun are all third-law pairs at work.

The stone and the Earth, force by force

Earth pulls the stone→Force mg downward, acting on the stone

Stone pulls the Earth→Force mg upward, acting on the Earth — equal in size, opposite in direction

Why only the stone appears to move→Equal forces, wildly unequal masses, so the Earth's acceleration is far too small to notice

Which came first→Neither. The two are one interaction seen from its two ends

The last row is the tested point. Options (c) and (d) contradict each other outright, so the item reduces to deciding which of the two physics endorses.

Study next

Conservation of linear momentum and its derivation from the third lawFree-body diagrams, and identifying which forces act on which bodyRecoil of a gun and the propulsion of a rocketImpulse and the momentum change during a collisionThe first and second laws, and how the three fit together

Common traps

Reading 'action' and 'reaction' as first and second in time.
Cancelling an action-reaction pair against each other, which is invalid because they act on different bodies.
Assuming that equal forces must produce equal accelerations, when the masses are what settle that.

A NOT-correct item built from a contradictory pair. Locate the two options that cannot both be true and decide between them; the other two options are usually decoration.

Related PYQs

CAPF_GAI_2023_Q352023Open & attempt →

Which one of the following conservation laws is a consequence of the Newton's third law of motion?

(a) Conservation of energy
(b) Conservation of momentum
(c) Conservation of charge
(d) Conservation of mass

Answer(b) Conservation of momentum

CAPF returned to the same law three years later and asked what follows from it rather than what it asserts. Equal and opposite internal forces are exactly why the momentum of an isolated system holds steady.

Practice

practice — not a real PYQ
A book lies at rest on a table. The reaction to the weight of the book, in the sense of Newton's third law, is
- (a)the normal force exerted by the table on the book
- (b)the gravitational pull exerted by the book on the Earth
- (c)the weight of the table
- (d)the friction between the book and the table
Answer(b) the gravitational pull exerted by the book on the Earth — the pair must be the same type of force acting on the other body of the interacting pair.
practice — not a real PYQ
Which one of the following is a direct consequence of Newton's third law of motion?
- (a)Conservation of energy
- (b)Conservation of linear momentum
- (c)Conservation of angular momentum only
- (d)Conservation of mass
Answer(b) Conservation of linear momentum — equal and opposite internal forces mean the total momentum of an isolated system cannot change.