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One of the criteria for similarity of the motion of a liquid or gas, applicable in cases when the influence of gravitational forces is significant. The Froude number characterizes the relationship between the inertial and gravitational forces acting on an elementary volume of the liquid or gas. The Froude number is

$$\mathrm{Fr}=\frac{v^2}{g \ell},$$

where $v$ is the speed of the flow (or the speed of the moving body), $g$ is the gravitational acceleration and $\ell$ is the typical length of the flow or the body.

The Froude number was introduced by W. Froude (1870).

====References====
<table>
<TR><TD valign="top">[a1]</TD> <TD valign="top"> L.I. Sedov, "Similarity and dimensional methods in mechanics", Infosearch  (1959)  (Translated from Russian)</TD></TR>
<TR><TD valign="top">[a2]</TD> <TD valign="top"> G. Birkhoff, "Hydrodynamics, a study in logic, fact and similitude", Princeton Univ. Press  (1960)</TD></TR>
</table>