Explain the statement from Albert Einstein “Motion is relative”

Question

Explain the statement from Albert Einstein “Motion is relative”. Give an example

Answer 1

Formally "Velocity is relative", in contrast "Acceleration is absolute".

A typical example is being on a train surrounded by other trains on both sides,

so you cannot see any other surrounding.

You may observe that a train on the next track is moving, but you cannot tell

whether it is actually the other train that is moving, or whether your

train is moving, or both trains are moving at different speeds.

This is not just an optical illusion -- it is impossible to perform an experiment

that would answer the question.

Every experiment will have the same outcome on your train as well as on the surrounding trains.

Therefore you cannot talk about your speed in any absolute sense -- only your speed

relative to one or the other trains, or your speed relative to surface of the Earth,

or your speed relative to the Sun, ...

In contrast, acceleration is absolute; you can measure it independently of your surrounding.

For example, you could measure it by how deeply you are being pushed into your seat.

Or more accurately, you could remove your shoe and hang it by its shoelace.

If the train is accelerating then the shoe will not hang vertically, and its angle

against the vertical is a measure of your acceleration.