Question

If action is equal to reaction then how is a horse able to pull a cart.

Answer 1

In the situation of a horse pulling a cart, the horse exerts some force on the cart and the cart exerts an equal and opposite force on the horse. At first it appears that the forces being equal and opposite cancel out each other and hence the cart would not move. But, it is not only the force on the cart which determines whether the cart will move or not, and that the force exerted by the cart on the horse affects the horse alone. To make the cart move, the horse must be able to apply enough force to overcome the frictional forces. Thus, when a horse pushes the ground backward with its feet, the ground reacts and exerts a force on the horse in the forward direction. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.