# Archimedes number facts for kids

The **Archimedes number** is named after the Greek Archimedes.

In viscous fluid dynamics, the Archimedes number (**Ar**) is used when the motion of fluids is affected by density differences. It is a dimensionless number, the ratio of gravitational forces to viscous forces.

The relationship and has the form: :

where:

- is the local external field (for example gravitational acceleration), m/s
^{2}, - is the density of the fluid, kg/m
^{3}, - is the density of the body, kg/m
^{3}, - is the dynamic viscosity, kg/ms,
- is the characteristic length of body, m.

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