Every ball-flight story starts with the simplest case: a ball moving through empty space, with gravity as the only force. The key idea is that the two directions do not disturb each other.

• Horizontally nothing pushes the ball, so its horizontal speed never changes.
• Vertically gravity changes the vertical speed by 9.81 m/s every second (g ≈ 9.81 m/s²): it slows the ball on the way up and speeds it up on the way down.

Join the two motions and the path is a parabola.