---
title: "Projectile Motion as Two Independent Motions"
description: "The horizontal and vertical motions of a projectile are worked separately: constant velocity across, uniform acceleration down, sharing nothing but the time. A ball rolled off a bench becomes two one-"
canonical: https://lightmysky.com/learn/science/projectile-motion-as-two-independent-motions-mt_OBgL97RFbb
source: https://lightmysky.com/learn/science/projectile-motion-as-two-independent-motions-mt_OBgL97RFbb.md
retrieved: 2026-09-12
---

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# Projectile Motion as Two Independent Motions

The horizontal and vertical motions of a projectile are worked separately: constant velocity across, uniform acceleration down, sharing nothing but the time. A ball rolled off a bench becomes two one-dimensional problems joined by t.

Subject: Science · Area: Forces & Motion · Ages 16 to 17
Page: https://lightmysky.com/learn/science/projectile-motion-as-two-independent-motions-mt_OBgL97RFbb

## Ready when they can

- Keeps a separate list of u, v, a, s and t for the horizontal and the vertical direction
- Uses time as the only quantity the two directions share
- Predicts where a ball rolled off a bench lands and says why its mass does not appear

## Lesson: A projectile is two motions in one

Keep two lists, one per direction, sharing only the time. Sideways there is no acceleration, so the sideways speed never changes, since nothing pushes sideways. Vertical motion has g downward, so only the vertical speed grows downward. Mass appears in neither list.

Time is the only quantity both directions share: solve one direction for t, then carry it across. A ball rolled off a 1.25 m bench at 2 metres per second sideways falls for about half a second, since 1.25 is 4.9 t squared. Sideways distance is speed times time, so it lands about 1 m out.

**Example.** Mass never appears because gravity accelerates all masses equally, and no mass term enters the sideways motion either. A heavy ball and a light ball rolled off together at the same speed land together, side by side.

**Tip.** Never mix the lists: bench height belongs in the vertical list, sideways speed in the horizontal one. Write u, v, a, s and t twice, once per direction, and circle the shared t.

**Recap.** Work sideways and vertical separately, join them with time, and ignore mass.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Resolving Vectors into Components](https://lightmysky.com/learn/science/resolving-vectors-into-components-mt_tc698unGJG)
- [The Uniform Acceleration Equations](https://lightmysky.com/learn/science/the-uniform-acceleration-equations-mt_yJMJQ8_KqR)

## Opens up

- [Projectiles Launched at an Angle](https://lightmysky.com/learn/science/projectiles-launched-at-an-angle-mt_MMuacyCqb8)