Posted on: Thu,Mar 17th 2011
Suppose that a wicket keeper needs to move as fast as possible to the right to catch a ball.
Which foot should move first, and in what direction?
It seems obvious that his left foot should stay on the ground and his right foot should move to the right while pushing as hard as possible to the left with the left foot.
That way, his whole body and every part of it moves rapidly to the right. But suppose he pushes to the left with his left foot and moves his right foot to the left. That way, he will tend to fall over to the right and his upper body moves even faster to the right. Such a step is called a gravity-step and it is counter-intuitive.
The same situation arises when a tennis player is facing a 200 km/hr serve and needs to move as fast as possible to the right. High speed film shows that players who move their right foot to the left, before moving it to the right, get their racquet to the ball faster.
The Physics explanation is that keeping the feet together reduces the moment of inertia and the upper body will therefore rotate faster. The same sort of thing happens when a diver wants to do a double or triple somersault. Tucking the arms and legs in reduces the moment of inertia and the diver spins faster.
Wicket keeping tips
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