The momentum of a ball at the highest point of its trajectory is zero. Here's why:

* Momentum is a vector quantity: This means it has both magnitude (how much) and direction.

* At the highest point, the ball momentarily stops: Before falling back down, the ball's upward velocity reaches zero.

* Zero velocity means zero momentum: Since momentum is mass times velocity (p = mv), if the velocity is zero, the momentum is also zero.

Important Note: While the ball's momentum is zero at the highest point, its potential energy is at its maximum.