When we drop a body the momentum of the ball doesn't remain constant becuse an external force(gravity) is acting on it but when we consider both eath and ball to be one system the this force(gravity) becomes an internal force. Now eath also moves to meet the ball. But it moves very little as the mass of earth is huge as compared to earth. Hence tge momentum is conserved.
But when we again look at scenario and think over energy conservation, we say that the TOTAL MECHANICAL ENERGY OF BALL IS CONSERVED even though the external force (gravity) is acting. Pls enlighten.