A ball of mass 0.1kg moving with a velocity of 20 ms^ -1 is hit by a force which acts on it for 0.02s. If the…

Analyze the question: A ball is moving, gets hit by a force for a short time, and reverses direction. We need the magnitude of the force. This is an impulse-momentum problem.

Concept: Impulse equals change in momentum:

F × t = m(v − u)

Direction matters. Since the ball reverses, the initial and final velocities have opposite signs.

Solution:

• Mass, m = 0.1 kg
• Initial velocity, u = +20 m/s
• Final velocity, v = −25 m/s (opposite direction)
• Time, t = 0.02 s

Change in momentum: Δp = m(v − u) = 0.1 × (−25 − 20) = 0.1 × (−45) = −4.5 kg m/s

Magnitude of force: F = |Δp| / t = 4.5 / 0.02 = 225 N

Answer: The force is 225 N, which is option C.

Tip: When an object bounces back or reverses, add the two speeds (20 + 25 = 45) instead of subtracting them. The most common mistake here is computing 25 − 20 = 5, which gives 0.5/0.02 = 25 N (option A), a trap option.