If action is always equal to the reaction, explain how a horse can pull a cart
Action is always equal to the reaction but it acts on two different bodies.
So, if a horse pulls a cart with some force.
Same force is applied on the ground.
Since the force is on two different objects, it cannot cancel
For a horse to move a cart, it applies muscular force on the cart so that it overcomes the frictional force on the wheels. When this muscular force is greater than frictional force, the horse cart moves.