Newton's Third Law

Question

When a rifle of large mass fires a small bullet, both experience forces of the same magnitude, yet the bullet flies forward fast while the rifle barely moves backward. What is the correct reason for this difference?

RankGuru · Accepted Answer

As described in NCERT Class 11, Chapter 5 (Laws of Motion), Newton's Third Law states that for every action there is an equal and opposite reaction, so the force the rifle exerts on the bullet equals in magnitude the force the bullet exerts on the rifle. Although the forces are equal, acceleration is not, because a = F / m. The bullet has a very small mass, so the same force gives it a huge acceleration and high muzzle velocity. The rifle has a much larger mass, so the identical force produces only a tiny backward acceleration, felt as recoil. The option that the forces are unequal directly contradicts the Third Law and is wrong. The option that momentum is not conserved is incorrect because total momentum of the isolated rifle-bullet system stays zero; the forward bullet momentum balances the backward rifle momentum. The option that friction stops the rifle instantly is wrong because recoil occurs during firing, before friction acts significantly. Plausibility check: equal momentum magnitudes with unequal masses force the lighter body to move faster, which matches the observed bullet-rifle behaviour exactly.

Newton's Third Law

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