A small smooth ball strikes a smooth vertical wall at right angles.

Its kinetic energy after impact is one-half of its initial kinetic energy.

Find the coefficient of restitution between the ball and the wall.

This is what I have got so far:

Let speed of approach be u and speed of separation be v

KE = 1/2 mv^2

so therefore

1/2 m (2)^2 = 1/2 * 1/2 m (v) ^2

Then I just do not have a clue what to do from there.

If it helps the textbook tells me the answer is e = 1/√2, I just don't know how to get there...

Any help will be appreciated