Here's why:
* Gravity is constant: Gravity acts on all objects with mass, pulling them towards the center of the Earth. This force doesn't disappear just because the ball has stopped moving upwards.
* Zero velocity, not zero force: When the ball reaches the peak of its trajectory, its velocity is zero. This means it's not moving up or down at that instant. However, its acceleration due to gravity is still present, meaning its velocity is about to change.
* Change in direction: Even though the ball is momentarily stopped, gravity is causing it to start accelerating downwards, which will cause it to change direction and begin falling back to the ground.
In short, the ball is in a state of momentary equilibrium at the top of its path, but gravity is still acting on it, preparing it to fall back down.
