1Compared with what?
Every speed is measured relative to something. A passenger in their seat isn't moving compared with the train, but is moving fast compared with the ground.
relative
Lesson 18.4 notes
Relative motion
How fast things seem to move depends on what you compare them with: trains and cars passing.
2Overtaking
When two vehicles move the same way, subtract their speeds. A car at 30 m/s overtakes a lorry at 25 m/s at a relative speed of 5 m/s.
relative speed
3Passing in opposite directions
When two vehicles move in opposite directions, add their speeds. Trains at 20 m/s and 15 m/s pass each other at 35 m/s.
4Side by side
Two trains at the same speed in the same direction look still to each other. Their relative speed is zero, even though both are moving fast.
5Walking on a train
Walk forwards at 1 m/s on a train going at 30 m/s, and your speed compared with the ground is 31 m/s. Walk towards the back, and it is 29 m/s.
6Which train is moving?
In a train at a station, you may feel you're moving when the train beside you pulls away. Look at the platform to tell which train is really moving.
7Everything is moving
Even sitting still, you move with the Earth: it spins, and it carries you round the Sun at about 30 km/s. So we usually measure speeds relative to the ground.
Explore: Train watcher
Set the speed of each train, then choose where to watch from: the platform, train A or train B.
Common mistake: There's no single true speed
A speed only means something when you say what it is compared with. Usually that's the ground, but a passenger sitting on a train has a speed of zero compared with the train.
Remember
- Every speed is measured relative to something, usually the ground.
- Moving the same way: subtract the speeds. Moving in opposite directions: add them.
- Two objects at the same speed in the same direction look still to each other.
Going further at GCSE
GCSE: Distance, speed and velocity → GCSE: Forces revision →