Projectile Motion

A projectile is any object that has been launched and then moves only under the influence of gravity. A football kicked into the air, a bullet fired from a gun, a stone thrown off a cliff — all are projectiles once they leave the launcher. Understanding projectile motion is a key skill for Edexcel A-Level Physics and combines vector resolution with SUVAT equations.

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The Key Principle: Independence of Horizontal and Vertical Motion

The most important idea in projectile motion is that horizontal and vertical motions are independent of each other. This means:

• The horizontal velocity has no effect on the vertical velocity, and vice versa.
• Gravity acts only in the vertical direction, so it affects only the vertical component of motion.
• The horizontal component of velocity remains constant (assuming no air resistance).
• The vertical component of velocity changes at a rate of g = 9.81 m/s² downward.

This allows us to treat the two directions completely separately, solving each with SUVAT equations independently, then combining the results.

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Resolving the Initial Velocity

If a projectile is launched at speed u at angle theta to the horizontal: