Final answer:
Newton's third law maintains that for every action, there is an equal and opposite reaction, often leading to confusion about movement. The horse and cart example illustrates that forces act on different systems, which allows for motion without cancellation. Choosing different systems of interest can show how forces can either cancel out or result in movement.
Step-by-step explanation:
According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This principle may lead to some confusion when considering how a horse can pull a cart if the forces are always equal and opposite. The key to understanding this motion lies in considering the two separate systems involved: the horse with the ground, and the horse with the cart.When the horse pushes against the ground with its hooves, the ground pushes back with an equal and opposite force. This reaction force propels the horse forward. The action force of the horse on the ground and the reaction force of the ground on the horse occur in different pairs, and thus do not cancel out - they act on separate bodies. Simultaneously, the horse exerts a force on the cart, pulling it forward. The cart exerts an equal and opposite force on the horse, but this does not prevent movement because these forces are part of a different action-reaction pair.The choice of the system of interest can affect whether forces cancel out or not. For instance, if the system of interest includes both the horse and the cart, the internal forces that the horse and cart exert on each other cancel out because they are equal and opposite. However, looking only at the horse or the cart as the system, the forces do not cancel as they are exerted by and on different systems.