4 A small ball \(P\), of mass 40 grams, is dropped from rest at a point \(A\) which is 10 m above a fixed horizontal plane. At the same instant an identical ball \(Q\) is dropped from rest at the point \(B\), which is vertically below \(A\) and at a height of 5 m above the plane. The coefficient of restitution between \(Q\) and the plane is \(\frac { 1 } { 2 }\). Find the magnitude of the impulse exerted on \(Q\) by the plane. The balls collide after \(Q\) rebounds from the plane and before \(Q\) hits the plane again. Find the height above the plane of the point at which the collision occurs.