Question

A 100 g ball collides elastically with a 300 g ball that is at rest. If the 100 g ball was traveling in the positive x-direction at 5 m/s before the collision, what are the velocities of the two balls after the collision?

Answer 1

Mass of the two balls is

,

Initial velocity of 100g ball is v_1i=5m/s

Initial velocity of 300g ball is v_2i=0

Let v_1f and v_2f be the velocities of the two balls after the collision.

Using conservation of momentum before and after the collision

In elastic collision, the kinetic energy of the two balls system is conserved

Using equation (1)

There are two roots to this equation,

i. , which is the same velocity of 300g ball before the collision (v_2i=0)

ii.

Using (1)