\int_{S_0} \overrightarrow{T}(\overrightarrow{q}).\overrightarrow{e}_{i}dA+\sum\limits_{p=1}^{3} \int_{S_p}\overrightarrow{T}(-\overrightarrow{e}_p).(\overrightarrow{e}_{i})dA =\int \rho (\overrightarrow{\gamma}-\overrightarrow{f})dv