About the trajectory of an idealized projectile:

It said that "its vertical velocity changes by equal amounts during equal time intervals".

What factors caused the vertical velocity to change?

Is the change of the vertical velocity affected by downward acceleration?

3.17 If air resistance is negligible, the trajectory of a projectile is a combination of horizontal motion with constant velocity and vertical motion with constant acceleration. At the top of the trajectory, the projectile has zero vertical velocity (v, = 0), but its vertical acceleration is still -g. y Vly V3x Vertically, the projectile Vly is in constant-acceleration U3y motion in response to the U3y earth's gravitational pull. Thus its vertical velocity changes by equal amounts during equal time intervals. a, = -g vo voy Voy V3x Vor Horizontally, the projectile is in constant-velocity motion: Its horizontal acceleration is zero, so it moves equal x-distances in equal time intervals.