# Formula Length Contraction Length / Distance    Rest length    Relative velocity

## Length / Distance

Unit
Length of the body (e.g. a rod) measured by an observer moving relative to this body with velocity $$v$$. But it can also simply be the spatial distance between two objects.

## Rest length

Unit
Length at rest is the length of a body measured by an observer when the observer is in the body's rest frame. That is, from the observer's point of view, the body is not moving.

## Relative velocity

Unit
Relative velocity between the observer and the body.

## Speed of light

Unit
Speed of light is a physical constant and has in vacuum the value: $$c = 299 \, 792 \, 458 \, \frac{\mathrm m}{\mathrm s}$$.