# A ball with a mass of # 2 kg# is rolling at #2 m/s# and elastically collides with a resting ball with a mass of #9 kg#. What are the post-collision velocities of the balls?

##### 1 Answer

The initially moving ball would move

#### Explanation:

(NOTE: **velocities with apostrophes** denotes the arbitrary object's **final velocity** )

Consider the total momentum of the system.

Note that final velocities are positive because I assumed them to move to the right after the impact.

Moreover, the problem states that the impact involves an elastic collision. Therefore, the coefficient of restitution of the system,

To solve the post-velocities solve the system of equations:

Where

and

Since