Newton's Third Law
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?
Select the correct option:
Solution
The rifle's larger mass gives it smaller acceleration
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.
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About This Question
- Subject
- physics
- Chapter
- laws of motion
- Topic
- newton's third law
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
The rifle's larger mass gives it smaller acceleration
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.
This easy difficulty physics question is from the chapter laws of motion, covering the topic of newton's third law. It appeared in the 2025 exam.
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