The vertical velocity decreases to zero at the highest point of an object's trajectory when it's thrown upwards.

Here's why:

* Gravity's Pull: When an object is thrown upwards, gravity acts on it constantly, pulling it back down. This force causes the object's upward velocity to decrease.

* Zero Velocity: As the object rises, its upward velocity slows down until it reaches a point where its upward velocity is completely countered by gravity. At this point, the object momentarily stops moving upwards, and its vertical velocity becomes zero.

* Changing Direction: After this point, gravity continues to act, causing the object to accelerate downwards, and its vertical velocity starts increasing in the downward direction.

Let me know if you have any other questions!