Question

a boy with mass 50kg running at 5m/s has a head-on collision with an identical ball Y at rest. X stops and Y moves off. what is Y velocity?

Answer 1

What is the mass of ball Y

Answer 2

Y velocity will be same as X velocity because energy of a system is constant

$kinetic \: energy \: = \frac{1}{2} \times mass \times velocity {}^{2}$

Before collision
energy of X = 1/2*50*5*5 = 625J
energy of Y = 1/2*50*0*0 = 0J

After collision
Energy of X = 1/2*50*0*0 = 0J
energy of Y = 1/2*50*25*25 = 625

total energy is conserved

Hope it helps!

This is the principle behind Newton's cradle