When a bull pulls a cart, the force that helps the bull to move forward is the force exerted by the ground on the bull.why?
To understand the motion of the bull we must consider of all the forces acting on it. When the bull pulls the cart, the force (T) on the bull by the cart in the backward direction gets balanced by the force (T) exerted by bull on the cart in the forward direction. Also, horse pushes the ground backwards with a force F inclined at some angle ? with the horizontal. As a reaction, the ground exerts a force of R on the bull. Its vertical component balances the weight of the horse; while the horizontal component (which is the only unbalanced force left) tends to move the cart.