When a ball bounces off the ground, which of the following changes suddenly? (Assume no loss of energy to the floor)
- A.Its speed
- B.Its momentum
- C.Its kinetic energy
- D.Its potential energy
Correct answer
Explanation
The correct answer is B, Its momentum. Momentum is a vector, and on bouncing the ball's direction of motion reverses at once, from downward to upward.
Just before impact the ball moves down with velocity v, so its momentum is mv downward. With no energy lost to the floor, it leaves with the same speed v but upward, so its momentum becomes mv upward. The change is 2mv, delivered in a very short time, which is why the floor exerts a large force on the ball during the bounce. Speed and kinetic energy are scalars that depend only on how fast the ball moves, so they come back to the same values.
A is wrong: in an elastic bounce the speed after impact equals the speed before it.
C is wrong: kinetic energy, ½mv², stays the same because the speed is unchanged.
D is wrong: the ball is at the same height, the ground, just before and just after the bounce, so its potential energy does not jump.
Exam tip: when only the direction of motion reverses, momentum changes by 2mv while speed and kinetic energy stay the same.
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