| (1) | \frac{D {\bf u}}{Dt} = \frac{1}{\rho} \nabla {\bf \sigma} + {\bf g} |
where
( t, {\bf r} ) | time and spatial variables |
{\bf u}(t, {\bf r}) | velocity of matter |
\rho(t, {\bf r}) | velocity of matter |
{\bf g}(t, {\bf r}) | sum of all body forces |
\frac{D}{Dt} | Material derivative of the matter |