$A$ horse continues to apply a force in order to move a cart with a constant speed. Explain why?

(N/A) When a cart moves on the road,it encounters frictional force between the wheels and the ground.
According to Newton's first law of motion,an object in motion will continue to move at a constant velocity only if the net external force acting on it is zero.
Since friction acts as an opposing force that tends to slow down the cart,the horse must apply an external force equal and opposite to the frictional force to maintain a net force of zero.
Therefore,the horse continues to apply a force to overcome friction and maintain a constant speed.