* Momentum is a measure of an object's mass in motion. It's calculated as mass (m) times velocity (v): momentum (p) = mv.

* As the ball travels upwards, its velocity decreases. This is due to the force of gravity acting against its motion.

* Since velocity is a factor in momentum, a decrease in velocity means a decrease in momentum.

Important Note: While the ball's momentum decreases as it moves upwards, its total mechanical energy remains constant (ignoring air resistance). This energy is transformed between kinetic energy (energy of motion) and potential energy (energy due to position). At the peak of its trajectory, the ball momentarily has zero kinetic energy (zero velocity) but maximum potential energy.