Explain why?
(i) a horse cannot pull a cart and run in empty space,
(ii) passengers are thrown forward from their seats when a speeding bus stops suddenly,
(iii) it is easier to pull a lawn mower than to push it,
(iv) a cricketer moves his hands backwards while holding a catch.
(i) When a horse is trying to pull a cart, he pushes the ground backward at an angle from the horizontal. According to Newton’s third law of motion, the ground also apply equal reaction force on the feet of the horse in opposite direction. The vertical component of reaction balances the weight of the horse and horizontal component is responsible for motion of the cart. In empty space, there is no reaction force, therefore a horse cannot pull a cart.
(ii) When bus is moving, the passengers sitting in it are also in motion and moving with the speed of the bus. When bus stops suddenly then lower part of the body of passengers which is in contact with the bus slower down with the bus but upper part of the bodies of the passengers continue remain in motion in initial direction due to its inertia of motion and hence are thrown forward.
(iii) In pulling a lawn mower, a force F is applied in upward direction, making an angle \theta from the horizontal [Fig. (a)]. Its vertical component in upward and decreasing the effective weight of the mower.
In pushing a lawn mower, a force F is applied in downward direction, making an angle 0 from the horizontal [Fig. (b)]. It vertical component is in downward and increasing the effective weight of the mower. Therefore, it is easier to pull a lawn mower than to push it.
(iv) In holding a catch the impulse imparted to the hands = F X ∆t = change in momentum of the ball when a cricketer lowers his hands to take a catch, he increases the time taken to stop the ball. As time t increases the force F applied on the hands of the cricketer by the ball decreases and his hands feel less hurt.