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

A horse pushes the ground in the backward direction. According to Newton's third law of motion, a reaction force is exerted by the Earth on the horse in the forward direction. As a result, the cart moves forward.

In order to pull the cart horse pushes the ground with its foot in the backward direction by pressing the ground. As a reaction of this force, the ground pushes the horse in forward direction. As a result the cart is pulled by the horse in the forward direction.

When a horse pulls a cart with a certain force, the cart also pulls the horse with the same force. But, in an effort to pull the cart, the horse pushes the ground backwards with his feet. To this action of the horse, the ground offers reaction. A part of this reaction from the ground balances the weight of the cart, while the other part makes the cart move in the forward direction.

To pull a cart, the horse pushes the ground backwards. Therefore, the ground exerts an equal reaction on the horse in the forward direction. The cart moves due to forward component of this force of reaction.

The cart is pulled by the horse, when the horse pushes the ground backwards with its feet. The ground offers an equal reaction forward on the feet horse. This is responsible for forward motion of the cart. The actual motion occurs when forward reaction exceeds the opposing force of friction between the cart and the ground.