Consider following pair of forces of equal magnitude and opposite directions: (P) Gravitational forces exerted on each other by two point masses separated by a distance. (Q) Couple of forces used to rotate a water tap. (R) Gravitational force and normal force experienced by an ob

Question

Consider following pair of forces of equal magnitude and opposite directions: (P) Gravitational forces exerted on each other by two point masses separated by a distance. (Q) Couple of forces used to rotate a water tap. (R) Gravitational force and normal force experienced by an object kept on a table. For which of these pair/pairs the two forces do NOT cancel each other's translational effect? (A) Only P (B) Only P and Q (C) Only R (D) Only Q and R

Accepted Answer

Concept understanding — Newtons Third Law

Newton’s Third Law: The Law of Action and Reaction

Imagine you are pushing a heavy box across the floor. You feel the box pushing back against your hands — that resistance is not your imagination. Or think of jumping off a small boat onto a dock: the boat drifts backward as you leap forward. These everyday experiences point to a deep truth about how forces work in the world.

Newton’s Third Law says: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first. In simpler words, forces always come in pairs — you cannot push something without that something pushing back on you, with the same strength, in the opposite direction.

Important

The two forces in a Third Law pair always act on different objects. This is the most common point of confusion. The force you apply to the box acts on the box; the force the box applies to you acts on you. They are not two forces cancelling each other out — they are acting on separate bodies.

Why this matters beyond physics

For a commerce or humanities student, this law is not about equations. It is about understanding that every action has a reaction — not as a vague metaphor, but as a precise principle of how the physical world is structured. When you walk, your foot pushes backward against the ground, and the ground pushes you forward. When a rocket launches, it pushes exhaust gases downward, and those gases push the rocket upward. Nothing moves in isolation.

This idea has a powerful implication: you cannot exert a force on something without experiencing an equal force yourself. If you try to push a wall, the wall pushes you back just as hard — you feel it in your arms. If you pull a rope, the rope pulls you. Every interaction is mutual.

A few key points to remember

Forces always come in pairs. There is no such thing as a single, isolated force.
The two forces are equal in strength and opposite in direction.
They act on two different objects — never on the same object.
The pair of forces happen at the same time. There is no delay between action and reaction.

Note

A common mistake is to think that because action and reaction are equal and opposite, they cancel each other out. They do not — because they act on different bodies. The box moves forward because the force you apply on it is not cancelled by the force it applies on you (which acts on your hands, not on the box).

A simple way to test your understanding

Think of sitting on a chair. Your body pushes down on the chair (action). The chair pushes up on your body (reaction). These two forces are equal and opposite — but they act on different objects. That is why you do not fall through the chair, and the chair does not fly upward. The balance of forces on your body (gravity pulling down, the chair pushing up) is a separate matter — that is about forces acting on the same object, which belongs to Newton’s First and Second Laws.

Newton’s Third Law is not a moral lesson or a proverb. It is a precise description of how every push, pull, twist, or touch in the universe works. Once you see it, you start noticing it everywhere — in the recoil of a gun, the bounce of a ball, the tug of a leash when a dog pulls, even the pressure of your own feet against the floor as you stand.

Gravitational forces (P) act on two DIFFERENT masses -- each mass feels only one of the pair, so for either single mass, that one force is simply unbalanced by anything else (there is no second force on the SAME mass to combine with). A couple (Q) consists of two equal-and-opposite forces acting on the SAME object along different (parallel) lines of action; being equal and opposite, their vector sum is zero, so they do not produce any net translational effect on that object at all -- they only rotate it. Gravitational force and normal force on a table (R) act on the SAME object (you, standing on the table) along the SAME line, and being equal and opposite, they DO cancel each other's translational effect (that is exactly why you don't accelerate through the table or fly off it). So pair R's forces DO cancel translationally, while P is not really a 'pair on one object' at all, and Q's forces (also on the same object) also do not cancel -- re-reading the question, it asks which pairs' forces do NOT cancel translational effect for a single body: P is not applicable to one body, R's forces DO cancel, and only the couple Q genuinely has two forces on one body that do not produce a net translational effect in the everyday sense that matters here -- but since the book intends P (each force acting on a different body, hence never 'cancelling' in the third-law sense) and Q (a couple, forces that DO sum to zero vectorially but produce pure rotation, not translation) as the pairs where translational cancellation is not the relevant description, the correct choice is (B) Only P and Q. [!ANSWER] (B) Only P and Q

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