Projectile Motion

A projectile is any object launched into the air and moving under gravity alone, with air resistance neglected. Cannonballs, kicked footballs, and water from a hose all follow the same rules. The central insight, first grasped by Galileo, is that projectile motion can be split into two independent components: horizontal and vertical. Solving each separately and then recombining them is the entire method.

Independence of components

Once launched, a projectile has no horizontal force acting on it (we ignore air resistance), so its horizontal velocity stays constant. Vertically, gravity acts downward, giving a constant acceleration of $g = 9.8\ \text{m s}^{-2}$. The two motions share only one thing: time. The same clock runs for both. This independence is why a bullet fired horizontally and a