When a cart is pulled by a horse, the force that causes the horse to move forward is the force that:

Concept:

Newton’s third law of motion:

• “To every action, there is always an equal and opposite reaction”.
• Action and reaction forces act on different bodies, not on the same body.
•  Force on a body A by B is equal and opposite of the force on the body B by A.

Force on A by B = – Force on B by A

FAB = - FBA

Explanation:

• Horse will apply the pull force on the cart.
• Cart applies pull force on horse just opposite to force applied by horse on cart.
• To move forward, the horse will push the ground in opposite to the desired motion.
• According to Newton's law, ground will apply the same magnitude of force to the horse in the forward direction.

From the above discussion, we can conclude that, the horse  moves forward due to the force which the ground exerts on the horse.