Ok, so I know that law of conservation of linear momentum holds in a system in a particular direction, provided no net external force is acting in that direction. So, if we drop a ball on the earth surface from a height much less than earth's radius and then to analyze its momentum, we take ball+earth as the system. Now in the system no net external force is acting as gravity has become its internal force. Thus conservation of linear momentum will hold. Now Initial momentum of ball+earth: mu+0=mu ,where u is the velocity gained by ball in time t1. Final momentum of ball+earth system=mv+0=mv at time t2. Now conserving linear momentum: mv=mu=> v=u ???? Something is absurd. How can velocity of ball remain constant in spite of gravity acting on it?