Motion: a relative concept

Are you at rest right now?

You’re sitting on a moving train, reading this lesson on your tablet. Are you at rest or in motion?

The answer is: it depends on what you’re referring to!

• Relative to the seats on the train, you’re at rest (you’re not changing your position within the carriage).
• Relative to the trees and houses outside, you’re moving (you’re moving away from them at high speed).

This simple example illustrates a key concept in physics: movement does not exist in an absolute sense. An object is never ‘moving’ or ‘stationary’ on its own: you always have to specify relative to what you’re observing it.

An everyday example

Imagine a balloon resting on the back seat of a car travelling along the motorway.

Observer	The balloon is...
The passenger sitting next to the balloon	At rest
A pedestrian on the pavement	Moving (at 90 km/h)
The driver of another car travelling at the same speed, right next to it	Stationary (the distance between the two cars remains constant)

The balloon isn’t actually moving of its own accord, but its description changes depending on who is observing it.

Key takeaway

To describe motion, a single question is enough: “relative to what?”. Without an answer to this question, saying that an object “moves” has no precise meaning in physics. It is this observation that will lead us to the concept of a reference frame in the next lesson.