Final answer:
Newton's third law of motion explains that forces occur in equal and opposite pairs, known as action-reaction pairs. A horse pulls on a cart, but the cart's 'pull' on the horse is actually the horse exerting force on the ground, creating a reaction force that moves the horse (and therefore the cart) forward.
Step-by-step explanation:
The question about how a cart can pull a horse if Newton's third law states that forces occur in equal and opposite pairs can be explained by understanding the law itself. Newton's third law of motion states: Whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts. This law implies that for every action, there is an equal and opposite reaction, also known as action-reaction pairs.
When a horse pulls on a cart, it is exerting a force on the cart, and the cart pulls back with an equal and opposite force. This is Newton's third law in action. The cart does not actually pull the horse in the traditional sense. Instead, the horse moves forward due to the reaction force from the ground against the horse's hooves as it pushes backward against the ground. This allows the horse to move forward, pulling the cart along with it, despite the cart exerting an equal and opposite force on the horse.
Other examples such as a professor walking, a car accelerating, rockets in space, and helicopters creating lift all involve the same principle of Newton's third law. Forces always occur in pairs, and one object cannot exert a force without experiencing an equal and opposite force in return.