CAPF AC 2023 Q35: Science & Technology – Physics
- (a) Conservation of energy
- (b) Conservation of momentum
- (c) Conservation of charge
- (d) Conservation of mass
Answer: (b) Conservation of momentum
Explanation
Newton's third law and conservation
How to think: the third law says forces come in equal and opposite pairs acting for the same time.
Equal and opposite forces for equal time give equal and opposite changes in momentum, so the total momentum of the pair does not change.
| Conservation law | Comes from |
|---|---|
| Momentum | Newton's third law (action-reaction pair) |
| Energy | The first law of thermodynamics; time symmetry of nature |
| Charge | A basic property of electric charge, not a law of motion |
| Mass | Chemical and nuclear conservation, not a law of motion |
Everyday example: a gun and its bullet push on each other equally and oppositely, so the gun recoils while the total momentum stays zero.
Answer: (b) Conservation of momentum.
Why each option is right or wrong
- (a) Incorrect. Energy conservation is a separate principle, not derived from the third law.
- (b) CORRECT. Equal and opposite forces over equal time conserve the total momentum.
- (c) Incorrect. Charge conservation is a property of charge itself.
- (d) Incorrect. Mass conservation is a chemistry and nuclear idea, not a result of the third law.
Key point
Newton III: F(AB) = −F(BA). Momentum conserved when no external force acts. Examples: recoil of a gun, rocket propulsion, a boat pushed away from the shore.
Remember
Third law gives conservation of momentum.
Common mistakes
- Picking energy because it is the most famous conservation law.
- Linking momentum to Newton's second law only; the pair of forces comes from the third.
- Thinking action and reaction act on the same body; they act on two different bodies.
Exam tip
Educated guess: the words 'Newton's laws' go with 'motion', and the only motion quantity in the options is momentum. Energy, charge and mass are not motion quantities.
Sources
- NCERT Class IX Science — Force and Laws of Motion, Chapter 9 (third law and conservation of momentum)
- NCERT Class XI Physics Part I — Laws of Motion, Chapter 5 (conservation of momentum)