If action is always equal to reaction, explain how a horse can pull a cart?
By Newton’s 3rd Law, the horse exerts a force on the cart, and the cart exerts an equal but opposite reaction on the horse.
There are only two forces: the force exerted on it by the cart, and the force exerted by the ground.
The force exerted on the horse by the ground is equal and opposite to the force exerted on the ground by the horse. So long as the horse is able to exert a force greater than the force exerted by the cart on him , then there will be a net force and that’s why horses are able to pull carts!