Since collisions occur at two separate surfaces it is essential to
formulate a clear notation for describing the progress of the ball.
Let un, vn be the horizontal and vertical velocity components of
the centre of the ball immediately after the nth bounce on the floor,
and let on, measured positive in the direction from Ox to Oy, be
the angular velocity of the ball. Clearly vn40 by definition and we
require uno0 in order for the ball to proceed towards the wall.
Further, let xn+a denote the horizontal distance of the ball centre
C from the wall at the nth bounce on the floor, and yn+a the height
of C above the floor at the subsequent impact with the wall. After
launch from the floor the centre of the ball describes a parabolic
trajectory, and elementary mechanics shows that