When a horse pulls a cart, Newton's third law states that the action force exerted by the horse on the cart is equal and opposite to the reaction force exerted by the cart on the horse. However, these forces act on different bodies and therefore do not cancel each other out. The key to understanding how the horse can pull the cart lies in the interaction between the horse's hooves and the ground. The horse pushes backward against the ground with its hooves. According to Newton's third law, the ground exerts an equal and opposite forward force on the horse. This forward force from the ground propels the horse forward. As the horse moves forward, it pulls the cart along. The cart resists motion due to friction, but because the friction between the horse's hooves and the ground (static friction) is greater than the rolling resistance of the cart's wheels, the horse-cart system experiences a net forward force. This net force causes both the horse and the cart to accelerate forward. In summary:
- The horse pulls the cart with a force.
- The cart pulls back on the horse with an equal and opposite force (internal forces within the system).
- The horse pushes backward on the ground.
- The ground pushes forward on the horse with an equal and opposite force (external force).
- The forward force from the ground on the horse is greater than the backward frictional force on the cart, resulting in net forward motion.
Thus, the horse moves forward and pulls the cart despite action and reaction forces being equal and opposite because these forces act on different bodies, and the crucial external force from the ground enables the horse to move and pull the cart
