* Momentum is a vector quantity: It has both magnitude (how much) and direction. When a ball is thrown upwards, its momentum changes in both magnitude *and* direction.
* External forces act on the ball: The main external force acting on the ball is gravity. As the ball travels upwards, gravity acts against its motion, decreasing its upward velocity and therefore its momentum. On the way down, gravity acts in the same direction as the ball's motion, increasing its downward velocity and momentum.
* The system is not isolated: We need to consider the Earth as part of the system. When the ball's momentum decreases, the Earth's momentum increases by an equal and opposite amount. Similarly, when the ball's momentum increases, the Earth's momentum decreases.
* Conservation of momentum is about the *total* momentum: The total momentum of the ball-Earth system remains constant throughout the flight. While the momentum of the ball changes, it's being transferred to or from the Earth.
In summary: While the ball's momentum changes during its flight, the *total* momentum of the ball-Earth system remains constant, thus satisfying the conservation of momentum principle.
