# Oblique impact on ground of a ping ping ball

1. Aug 15, 2011

### abcd8989

When a ping pong ball strikes the ground obliquely without spinning, the angle of incidence is not equal to that of reflection, am I correct? Here is my thoughts: Assume the ground is smooth and negligible air resistance. When the ball touches the ground, it starts to deform, delivering its momentum to the ground(is it right to say so??) . After a split second, it retains its shape due to its elasticity, applying a force to the ground. A reaction force appears which causes an upward acceleration to the ball. However, due to energy loss as sound, the loss in elastic pe is less than the loss in gravitational pe before the ball reaches the ground. Therefore the upward acceleration is smaller in magnitude than the deceleration during impact. The resultant velocity of the ball after impact thus does not equal that before impact.
Please correct my mistakes, if any.

2. Aug 15, 2011

### xts

That's one reason.
And other - dominant and having reverse effect - is that ball after such collision starts spinning. Point on the bottom of the ball was moving with some horizontal speed, the same, as whole ball. After touch, in the very short time, friction stops it. In effect the ball starts to spin.

You may see the same effect much more spactacularily: watch the wheels of landing airplane at the moment of touchdown.

Last edited: Aug 15, 2011