Physics · KS3 Y9 · Forces & Motion

Relative motion

Relative motionFree4:04Heddy Score 3 of 7, below the standard

What you need to know

  • A velocity depends on who is measuring it.
  • A ball thrown forward at 10 m/s from a car moving at 20 m/s travels at 30 m/s relative to someone standing by the road.
  • Someone running towards the ball sees it approaching faster.
  • Someone moving in the same direction as the ball sees it moving more slowly.
  • Relative velocity is found by subtracting the observer's velocity, taking direction into account.

In this clip

Runs 4:04, from 0:12 to 4:16 of the full video.

Heddy Score3/7, Below the standard
See the seven stars
  • Trusted source · passed. The creator's own upload, live and playable here.
  • On the map · not judged yet. A teacher watched it: it mainly teaches this map topic.
  • Spec-true · not judged yet. Accurate, and right for the current UK specification.
  • Focused · passed. One idea, short enough to watch in one go.
  • Accessible · passed. Good captions, clear sound and pictures.
  • Safe for children · not judged yet. Right for 11 to 16 year olds, with no sales pitch.
  • Proven · not judged yet. Earned from our students: they learn from it, find it helpful and watch it to the end.
★ passed · ☆ pending · ✕ failed
Our students

Not enough students yet

Our own students' answers. These are what move ★7 Proven.
On YouTube

187K views · 1.17K likes · Michel van Biezen 1.21M subscribers

As reported by YouTube on 2026-11-03. Not part of the Heddy Score.
How the Heddy Score works

Why we picked it

A clear worked example with three observers that shows how the measured speed depends on the observer's motion.

Michel van Biezen on YouTubechecked 2026-10-22

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