Question

When someone falls from a tall building, the most effective way to save him is to provide a net at the bottom so that the person slows down himself conveniently without hurting himself, the impulse here is same that would have been without the net but the net provides extra time for the change in momentum. Another example is, in the game of cricket when the ball is hit by the bat the time to change the momentum is expected to be the least by the batsman to get an excellent shot.

Support your claim using equations and provide two scenarios with numbers and calculations.

Answer 1

According to Newton's second law , Force is rate of change of momentum

F = d(mv) / dt ....................(1)

when the person is falling from tall building directly on ground , bouncing time will be very short if his body hits the hard ground. Hence for given change of momentum takes place in short time, the reaction force given by ground to falling man will be enormous. But if he falls on net, due to elastic nature of net, considerable time is taken by man to reverse his speed . Then for the same change in momentum takes place in long time , the reaction force to his body is reduced appreciably

When the ball is coming towards batsman with high speed, if batsman deflects the ball in short time , then change in momentum takes place in short time, hence force applied to the deflected ball is huge.

Hence to increase the force , we need to decrease the time of changing momentum.

To decrease the force, we need to increase the time of changing momentum.