During the motion of a projectile, when is the vertical velocity zero?

At the maximum height. During the motion of a projectile, the vertical velocity becomes zero at the maximum height it reaches. This is because, as the object travels upward, gravity exerts a constant downward force on it, causing it to slow down and eventually come to an instantaneous stop before it begins its descent. At the moment when the object reaches its maximum height, the upward force imparted by the initial launch exactly balances the downward force of gravity, resulting in a vertical velocity of zero. Once this point is reached, the object's velocity in the upward direction decreases, and it starts to fall back towards the ground due to gravity's influence.