Ball Through the Air
Rowden Fullen (1970’s)
As we know from previous articles a topspin ball travels in an arc over the net, drops quite sharply before contact with the table and then shoots forward fast and low after the bounce.
Lets us look however a little more closely at the ball in the air and before the bounce. What we must first understand is that the ball surface is not smooth and contains pockets of air in the surface which react with the flow of air against the ball. We do know that in the case of the top part of a topspinning ball, this spins against the oncoming air while the bottom part is in the same direction. Therefore we have an area of high turbulence at the top and low turbulence at the bottom.
However the air flow round a ball moving at high speed changes from turbulent to laminar as it slows down in the air and this is what causes the ball to dip. Just what do we mean by this?
At the ‘static point’ which is the leading point of the ball at speed there will be an ‘eye’ like at the centre of a hurricane where there is an area of pressure. The flow of air around the ball however will in the initial stages of flight as the ball leaves the racket at speed, be chaotic or ‘turbulent’ in nature. By this we mean there is no smooth pattern of air molecules flowing around the surface of the moving ball.
It is only as the ball slows down that a pattern starts to emerge and the air flow around the ball forms a more ordered outline. We call this a ‘laminar’ effect.
It is of course at this stage that the high and low pressure areas forming on different parts of the ball’s surface have a direct effect and as a result the ball is forced to dip sharply downwards on to the table.