Answer:
A horse pushes the ground in a backward direction. According to Newton’s third law of motion, a reaction force is exerted on the horse in the forward direction. When the horse walks in the forward direction (with the cart attached to it), it exerts a force in the backward direction on the Earth. An equal force in the opposite direction (forward direction) is applied on the horse by the Earth. This force moves the horse and the cart forward. As a result, the cart moves forward.