When a horse pulls a cart, the force exerted by horse on the cart is always equal and opposite to the force exerted by cart on the horse. So, at first glance, it seems that the forces being equal and opposite cancel each other out and hence, the cart would not move. It is a fact that the cart moves.
Then the question arises why does the cart move? This is possible only when the horse is able to apply enough force to overcome the frictional forces.
Thus, to make the cart move, the horse bends forward and pushes the ground backward with its feet. When the forward force to the backward push of the horse is greater than the opposing frictional forces between tyres of cart and ground, the cart moves.

How a Horse Can Pull a Cart

Horses have been used for centuries to pull carts, carriages, and other heavy loads. Despite the fact that action is always equal to the reaction, a horse is able to pull a cart due to several factors that come into play. Let's delve into the details of how a horse can successfully pull a cart.

Understanding Newton's Third Law of Motion
According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction. This law implies that when a horse pulls a cart, the cart exerts an equal and opposite force on the horse. However, it is important to note that the reaction force does not impede the horse's ability to pull the cart.

The Role of Friction
Friction plays a crucial role in enabling a horse to pull a cart. When the horse exerts a forward force on the ground, the ground pushes back with an equal and opposite force due to friction. This reaction force allows the horse to gain traction and prevent slipping. Without friction, the horse would struggle to move the cart, as the force exerted on the cart would cause the horse's hooves to slide backward.

Utilizing the Horse's Muscular Power
Horses are incredibly strong animals with well-developed muscles, particularly in their legs and shoulders. These muscles generate the necessary force to pull the cart. When the horse contracts its muscles, it exerts a force on the cart's harness, propelling it forward. The horse's muscles convert chemical energy from the food it consumes into mechanical energy, which is then used to overcome the resistance of the cart.

Proper Harnessing and Leverage
The way the horse is harnessed and the use of leverage also contribute to its ability to pull a cart. The harness is designed to distribute the pulling force across the horse's body, minimizing strain on specific areas. Additionally, the design of the cart and the position of the horse's harness attachment points create leverage, allowing the horse to utilize its strength more efficiently.

Conclusion
Despite the action being equal to the reaction, a horse is able to pull a cart through a combination of factors. Friction between the horse's hooves and the ground provides traction, allowing the horse to push against the cart without slipping. The horse's powerful muscles generate the necessary force, converting chemical energy into mechanical energy. Proper harnessing and leverage also contribute to the horse's ability to pull the cart effectively. By understanding these principles, we can appreciate the remarkable abilities of horses in performing tasks such as pulling carts.