CAPF AC 2023 Q33: Science & Technology – Physics
- (a) It moves with a uniform velocity.
- (b) It moves with a uniform speed.
- (c) It moves with a non-uniform acceleration.
- (d) It moves with a uniform acceleration.
Answer: (d) It moves with a uniform acceleration.
Explanation
Free fall
How to think: a freely falling body feels only gravity, and gravity gives the same pull on it at every moment near the Earth.
So its acceleration is constant, about 9.8 m/s², while its speed keeps growing.
| Statement | Holds for free fall? | Why |
|---|---|---|
| Uniform velocity | No | Velocity rises every second |
| Uniform speed | No | Speed rises every second |
| Non-uniform acceleration | No | Acceleration stays at g |
| Uniform acceleration | Yes | Same g throughout the fall |
Numbers to see it: speed after 1 s is about 9.8 m/s, after 2 s about 19.6 m/s, after 3 s about 29.4 m/s.
The speed gain per second is the same, which is what uniform acceleration means.
Answer: (d) It moves with a uniform acceleration.
Why each option is right or wrong
- (a) Incorrect. The velocity keeps growing, so it is not uniform.
- (b) Incorrect. The speed keeps growing, so it is not uniform.
- (c) Incorrect. The acceleration is constant (g), not changing.
- (d) CORRECT. A freely falling body has a uniform acceleration equal to g.
Key point
Free fall: only gravity acts; a = g = 9.8 m/s² downward (about 10 in quick sums); v = gt, s = ½gt²; mass does not matter (in vacuum a feather and a stone fall together).
Remember
Free fall means constant acceleration g, rising speed.
Common mistakes
- Mixing up uniform velocity (no acceleration) with uniform acceleration.
- Thinking heavier bodies fall faster; in the absence of air all fall with the same g.
- Reading 'uniform acceleration' as 'constant speed'.
Exam tip
Educated guess: a ball dropped from a roof clearly speeds up, so (a) and (b) fail at once. Between (c) and (d), the standard value g = 9.8 m/s² is a fixed number, so (d).
Sources
- NCERT Class IX Science — Gravitation, Chapter 10 (free fall and acceleration due to gravity)
- NCERT Class XI Physics Part I — Motion in a Straight Line, Chapter 3 (motion under gravity)