Lesson 4 of 4

Relative motion

Goal: Describe how speed depends on who is measuring it.

Puzzles for this lesson

How fast something seems to move depends on how the observer is moving. This is called relative motion.

Same direction: subtract the speeds. A car at 30 m/s overtakes a car at 20 m/s. From the slower car, it seems to move at only 10 m/s.
Opposite directions: add the speeds. Trains at 30 m/s and 20 m/s pass each other at a relative speed of 50 m/s.
Same speed and same direction: the relative speed is zero. The passenger in the seat next to you does not seem to move at all.

Your turn

This lesson’s activity starts here in a moment.

I can work out the relative speed of two cars moving the same way.I can do the same for cars moving in opposite ways.

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Speed How far something travels in a certain time

Relative motion How something seems to move to an observer who is moving too

Relative speed The speed of one object measured from another object

Observer The person who watches and measures

For teachers: AQA 3.1.1