explain why a cricketer moves his hands backwards while catching a fast moving cricket ball.
A cricketer moves his hand backwards because by doing this he increses the time due to which the rate of change of momentum decreases. That means that the ball comes to rest gently and does not hurt the fielder. But, if does not move his hand backwards, then the ball would come to rest in a fraction of a second with a high rate of change of momentum. The ball will exert a lot of force on the hands of the fielder which will hurt him
Because of Newton's second law of motion, which says that the force exerted by a body is directly proportional to the rate of change of momentum of the body.
Here the body is the ball. Before hitting the cricketer hand the ball was travelling with speed v and after hitting his hand, if the speed reduces to zero in very less time, then rate of change of momentum will be very high. Thus , the force exerted on the hand will also be very high. Therefore, the cricketer lowers his hand, thus reducing the speed of the ball slowly to zero instead of immediately reducing it to zero. This decreases the rate of change of momentum and hence the force exerted on his hand is also decreased, thus it saves his hand from getting hurt.
It is based on the II law of motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
The initial momentum of the ball =m u
When the cricketer stops the ball (v=0),its final momentum=m* 0=0.
Change in momentum
=final momentum- initial momentum
=0- mu= -my=my (numerically)
Here -ve sign shows that the change in velocity in momentum is in direction opposite to the initial direction of momentum.