wunder beta🚆 Relative Motion
Are you sitting still right now — or hurtling through space? Both can be true. Discover how motion depends entirely on who is watching.
What you’ll learn
- Motion Depends on Where You StandDefine relative motion and explain that motion depends on the reference point chosen.To judge whether something moves, you compare it to a reference point, so motion is relative. The chosen reference point is a frame of reference. The same object can be still to one observer and moving to another — a train passenger is still relative to their seat but moving relative to the platform.
- The Train WindowCompare how different observers describe the same motion using different reference points.The same journey looks different from different frames of reference. To someone on the platform, a train passenger flashes past; to the seat beside them, the passenger is still. Choosing the wrong reference point — like a neighbouring moving train — can even trick your senses about who is really moving.
- Adding Up Relative SpeedsCalculate relative speed for objects moving in the same or opposite directions.Relative speed depends on direction. For objects moving the same way, subtract their speeds: a car at 60 km/h pulls away from one at 50 km/h at just 10 km/h. For objects moving in opposite directions, add their speeds: two trains each at 50 km/h pass at 100 km/h. Motion, and speed, is always relative to a reference point.
Questions this course answers
To decide whether something is moving, you must compare it to a...
Motion is judged by comparing an object to a chosen reference point, which is what makes it relative.
You sit on a train going 100 km/h. Compared with the seat beside you, you are...
Relative to the seat, which travels with you, you are not moving at all.
Why can the same passenger be 'still' and 'moving' at the same time?
Each observer measures motion against their own reference point, so their answers differ for the same passenger.
You feel like you're moving, but it's the train next to you sliding away. What went wrong?
Judging your motion against the other moving train gives the wrong answer; the still platform is the reliable reference point.
Two trains head straight toward each other, each at 50 km/h. Their relative speed is...
For objects moving in opposite directions you add the speeds: 50 + 50 = 100 km/h.
A car at 60 km/h overtakes one going 50 km/h the same way. How fast does it pull away, relative to the slower car?
For the same direction you subtract the speeds: 60 − 50 = 10 km/h.
Grounded in trusted sources
- BBC Bitesize
- Britannica
- The Physics Classroom
- NASA
Every Wunder lesson is built from real, reputable sources — never invented.
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