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**Velocity and Center of mass question???**

## Homework Statement

Having some issues with this one.

"Ball A has a mass of .200kg and a velocity of 1.50 m/s. Ball B has a mass of .300kg and a celocity of -0.400 m/s. The balls meet in a head on collision. After the collision, the velocity of ball A is -0.780 m/s.

A) Assuming that the two balls form an isolated system, determine whether or not this was an elastic collision.

and

B) (Mainly this one) - Find the velocity of the centre of mass of the system, before and after the collision. ????

## Homework Equations

## The Attempt at a Solution

How do you find the velocity of the centre of mass of a system??

Any help would be great!