# 5.3.1: Non Deformable Control Volume


When the control volume is fixed with time, the derivative in equation (??) can enter the integral since the boundaries are fixed in time and hence,

Continuity with Fixed b.c.

$\label{mass:eq:intSrho} \int_{V_{c.v.}} \dfrac{d\,\rho}{dt} dV = -\int_{S_{c.v.}} \rho\,U_{rn} \, dA$

Equation (??) is simpler than equation (??).