Question

Two tennis balls undergo a head-on elastic collision. Under which of these initial
conditions is it impossible for both balls to be moving in the same direction after the
collision?
(a) the lighter ball is stationary and the heavier ball
is in motion.
(b) the two balls have the same mass and are initially
moving in the same direction, and they collide
because the faster-moving ball overtakes the
slower-moving one.
(c) the two balls have the same mass and only one
is moving.

Answer 1

Velocity of ball after collision is given as:

For elastic collision

if bamm m2 has lower mass and in rest and ball m2 has higher mass in motion then equation of velocity after collision is given as..

Velosity of ball 2

so

So option one is correct.

the lighter ball is stationary and the heavier ball
is in motion..

Option a is correct.

the lighter ball is stationary and the heavier ball
is in motion.

Thanks for asking.