CAPF AC 2020 Q1: Science & Technology – Physics

CAPF AC 2020, Paper I (General Ability and Intelligence) · Science & Technology

Which one of the following statements for an object falling freely under the influence of gravity is correct?

(a) Zero acceleration always implies zero velocity
(b) Zero acceleration has no relation with the velocity of the object
(c) Zero velocity at any instant necessarily means zero acceleration at that instant
(d) Acceleration is constant all throughout the free fall

Answer: (d) Acceleration is constant all throughout the free fall

Explanation

Free fall under gravity

How to think: test every option at the top of a throw.

There the velocity is zero, yet the ball still comes down.

So the acceleration is not zero there.

In free fall, the only force on the object is gravity.

So the acceleration is g (about 9.8 m/s²), pointing downwards, at every instant.

The velocity keeps changing, but the acceleration stays the same.

Option	Verdict	Fact
(a) Zero acceleration always implies zero velocity	Incorrect	Zero acceleration means constant velocity, which need not be zero
(b) Zero acceleration has no relation with the velocity of the object	Incorrect	A freely falling object never has zero acceleration, and acceleration is the rate of change of velocity
(c) Zero velocity at any instant necessarily means zero acceleration at that instant	Incorrect	At the top of a throw the velocity is zero but the acceleration is still g
(d) Acceleration is constant all throughout the free fall	Correct	It equals g throughout

Ball thrown straight up:

Answer: (d) Acceleration is constant all throughout the free fall.

(a) Incorrect. Zero acceleration only means the velocity is not changing; it can be any constant value.
(b) Incorrect. Acceleration is the rate of change of velocity, so the two are related.
(c) Incorrect. At the highest point the velocity is zero but the acceleration is still g.
(d) CORRECT. In free fall the acceleration is g, constant in size and direction.

Free fall: acceleration = g (about 9.8 m/s²), constant and downward throughout; velocity changes, acceleration does not.

Free fall = constant acceleration g, even when velocity is zero.

Thinking the acceleration becomes zero at the highest point because the velocity is zero there.
Mixing up zero acceleration with zero velocity.
Believing a heavier object falls with a larger acceleration; without air, all fall at g.

Educated guess: three options talk of zero acceleration, which never happens in free fall. Only (d) is left.