When a rock is thrown vertically upward, at the top of its path, why the velocity is zero but acceleration is not?

Any object being acted on by gravity has constant acceleration,

and it's the same number for every object.

On Earth, the acceleration of gravity is 9.8 m/s² downward.

That means that an object moving down moves 9.8 m/s faster every

second, and an object moving up moves 9.8 m/s slower every second

(and eventually runs out of upward gas and starts moving downward).

At the top of its path, the rock's velocity changes from upward to downward.

If you change from upward velocity to downward velocity, the change

has to be smooth ... There's no way to jump from upward to downward

without going through every value in between. There has to be a split

second when you're not moving upward OR downward. At that split

second, your velocity is zero.