Problem 2
Consider a velocity field \(\mathbf{u}(x, y, z, t)\) From conservation of mass and a spaceindependent density show that it follows that \(\operatorname{div} \mathbf{u}=0\).
Problem 18
Prove that for a fluid at rest the stress tensor \(\sigma_{i j}\) is isotropic \(; \sigma_{i j}=\delta_{i j} \sigma_{k k} / 3\).
