Derive (1.44) from the momentum balance (1.40). Hint: Dot multiply (1.40) by the material velocity $\mathbf{v}$. Show that $\mathbf{v} \cdot \operatorname{Div} \mathbf{P}=\operatorname{Div}\left(\mathbf{P}^i \mathbf{v}\right)-\mathbf{P} \cdot \nabla \mathbf{v}$, where $\nabla$ is the gradient with respect to $\mathbf{x}$, and that $\nabla \mathbf{v}=\dot{\mathbf{F}}$. Integrate over $\pi \subset \kappa$ and invoke the divergence theorem.