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.

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) Due to inertia of motion.

C)While pulling a lawn mower, a force at an angle is applied on it, as shown in the given figure.

The vertical component of this applied force acts upward. This reduces the effective weight of the mower.

On the other hand, while pushing a lawn mower, a force at an angle is applied to it, as shown in the following figure.

In this case, the vertical component of the applied force acts in the direction of the weight of the mower. This increases the effective weight of the mower.

Since the effective weight of the lawn mower is lesser in the first case, pulling the lawn mower is easier than pushing it.

D) From Newton's second law of motion,

F= ma

=m× v/t

When mass and velocity are constant,

F  ∝ 1 / t

While taking a catch, a cricketer moves his hand backward so as to increase the time of impact (t). This is turn results in the decrease in the stopping force, thereby preventing the hands of the cricketer from getting hurt.