Practical Illustration to understand Newtons third law of motion

When the driver of horse cart, Mr. SAM whipped the horse named chetak, in order to make a move; chetak(the horse) looked back at Sam and said, "Sam, it seems you do not know Newton's third law of motion. According to this law, with whatever force F₁ Would i pull the cart forward, the cart would also pull me backward with the force F₂ and these two forces are equal in magnitude and hence the resultant of them would be zero, so there is no way can i make the cart move."

Chetak knew only that action and reaction are equal and opposite. But since Sam had secured 100/100 marks in physics, he very well knew that action and reaction act on different bodies. So he patiently explained to Chetak about forces which is as under....

Suppose the horse pulls the cart by a force F₁ in the forward direction and the cart exerts the force F₂ on the horse in the backward direction Here F₁ =F₂


We should first decide the system. If we take the horse as our system then we should consider only those forces which are acting on the horse. Since force f1 is on the cart we do not consider it but we have to consider the force f2 which acts on the horse. Then a question arises that why the horse goes forward when the force f2 on it is acting backward? The answer to this question is that still we have not considered all the forces acting on the horse.

In these process three action-reaction pairs are there...

1. a pair of forces exerted by the horse on the cart and that by the cart on the horse.
2. a pair of forces exerted by the horse on the ground and that by the ground on the horse.
3. a pairs of forces exerted by the cart on the ground and that by the ground on the cart.

When a horse walks, it pushes the ground and as a result the ground exerts force on the horse in the opposite direction. The horse accelerates forward if the forward components exceeds the force. Thus, the acceleration of the horse and that of the cart are equal in magnitude and hence they move together.