A 1.0 kg red superball moving at 5.0 m/s collides head-on with stationary blue superball of mass 4.0 kg in an elastic collision. What are the final velocities of the two superballs after the collision?

Initial conditions:

m1 = 1.0; v1 = 5

m2 = 4.0; v2 = 0

In the case where the second object (sometimes called the target) is at rest the velocities after the condition are

v1' = v1 * (m1-m2) / (m1+m2)

v2' = 2v1*m1 / (m1+m2)

For this we get

v1' = 5 * (-3) / 5 = - 3m/s (moving in the opposite direction as before at 3m/s

v2' = 2*5 * (1) / 5 = 2m/s in the same direction as the original ball was moving

you can see these directions by looking at the signs. The momenta also add to the initial momentum as required.