Forces and Motion

Movement and speed

Physics Narrative for 5-11 Supporting Physics Teaching

Changing distances, speed and relative movement

When asked about the speed limit on the motorway children will readily use the language of 70 miles per hour. Speeding motorists are recorded on police cameras and reported in the news. Speed records for the fastest bird or animal, car or plane hold children's attention.

But there are subtleties. What I (Charlie) record as passing at 13 metre / second, you (Alice) may record as stationary (so a speed of 0). The reason may be simple: Alice is driving along in her car, and Charlie is standing on an overhead footbridge. The recording is a record of the speed of Alice's handbag, on the front seat of the car.

Alice notices no movement from her point of view: the handbag is at all times the same number of metres away from her. That's why she records the speed as 0. If Alice says that's at rest, she means nothing more or less than it's just moving with her.

Charlie notices the handbag getting 13 metre closer to the shadow of the bridge on the road every couple of seconds. That's why he records the speed as 13 metre / second.

Now consider Bob, cycling along at half of the car's speed, and overtaken by Alice. What will he record for the speed of that handbag?

Bob won't record what Alice does, nor what Charlie does. Each individual with a unique point of view will record a different speed. These will only agree with each other if they are moving together. Duncan, cycling along with Bob, agrees with Bob, for example. Elizabeth, sitting in the rear seat of the car, agrees with Alice. Fayed, standing with Charlie, agrees with him.

Teacher Tip: Choose a point of view to describe moving things.

So far as we can tell, there is no absolute point of view. No absolute at rest. At rest means staying the same distance from me.

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