If action is always equal to the reaction, explain how a horse can pull a cart.

Explanation

According to the Newton's third law of motion, the horse exerts some force on the cart, and the cart exerts an equal and opposite force on the horse. So, at first glance it seems that the action and reaction forces being and opposite cancel out and hence the cart would not move. But it should be noted that it is 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. Thus, if the horse is able to apply enough force to overcome the frictional forces present, the cart will move. So, to make the cart move, the horse bends forward and pushes the ground with its feet. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.