question_answer 22)

Explain why (a) a horse cannot pull a cart and run in empty space, (b) passengers are thrown forward from their seats when a speeding bus stops suddenly, (c) it is easier to pull a lawn mower than to push it, (d) a cricketer moves his hands backwards while holding a catch.  

Answer:

(a) For pulling a cart or for running, the horse pushes the earth with its feet and reaction of the earth makes it move in the forward direction. Since in empty space there is no reaction force, therefore the horse cannot run in empty space. (b) Refer to solution of Problem 7 on page 5.62. (c) Refer to Figs. 5.96 and 5.97. A lawn mower is pulled or pushed by applying force at an angle. The vertical component of the applied force reduces the effective weight of the mower in case of pull and increases the effective weight in case of push. Consequently, the normal reaction and hence the force of friction is less in case of pull than that in push. Hence it is easier to pull a lawn mower than to push it. (d) When the ball is caught, the impulse received by the hands is equal to the product of the force exerted by the ball and the time taken to complete the catch. By moving the hands backwards, the cricketer increases the time of catch. The force exerted on his hands becomes much smaller and it does not hurt him.