In more general cases, the last equation comes from the first law of the thermal physics,

(A.21) |

(A.22) |

Since the total energy per unit volume is expressed using the basic physical quantities as

(A.24) |

**Problem**:
From the continuity equation (A.11) and
the equation of motion (A.7) or
the equation for momentum density (A.15),
show that the equation for the kinetic enerygy density
is expressed as follows

(A.25) |

Kohji Tomisaka 2009-12-10