Projectile Motion, Broken Into Two Simple Parts
Updated 2026-07-12
Projectile motion is the motion of an object launched into the air and moving under gravity alone. Its secret is that the horizontal and vertical motions are independent: horizontal velocity stays constant, while vertical motion accelerates downward due to gravity.
Projectile problems look hard until you split them. Horizontal and vertical motion happen at the same time but do not affect each other, so you solve each one separately with simple equations, then combine the answers.
Two independent directions
Horizontally there is no force (ignoring air), so the horizontal velocity never changes. Vertically, gravity pulls down at a constant acceleration, so the vertical motion is ordinary free fall. Time is the one thing the two directions share.
Using time as the link
Because both motions take the same amount of time, you usually find the time from the vertical motion, then feed that time into the horizontal motion to get distance.
Worked example
A ball is thrown horizontally at 5 m/s from a 20 m high ledge. How far does it land from the base?
- Vertical: find the fall time from 20 = (1/2)(9.8)t^2.
- Solve: t^2 = 40 / 9.8, so t is about 2.0 seconds.
- Horizontal: distance = horizontal speed times time = 5 times 2.0.
Answer: About 10 meters from the base.
The common mistake
Wrong: Giving the horizontal motion an acceleration, or mixing the two directions together.
Fix: Horizontal velocity is constant, there is no horizontal acceleration. Keep the two directions separate and use time as the only shared quantity.
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Frequently asked questions
What is projectile motion?
It is the motion of an object thrown into the air and acted on only by gravity. The horizontal and vertical parts of the motion are independent of each other.
Why do you split projectile motion into horizontal and vertical?
Because the two directions do not affect each other. Horizontal velocity stays constant while vertical motion is free fall, so splitting them turns one hard problem into two easy ones.