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O potencial Newtoniano ou potencial da atração gravitacional $U(\bf r)$ obedece à equação diferencial de Laplace para pontos fora das massas.  Então o divergente do gradiente de $U(\bf r)$ é:  \begin{equation}   \bf {\nabla} \cdot \nabla {U(\bf r)} = \nabla^2 {U(\bf r)} = \frac{\partial^2 {U(\bf r)} }{\partial^2 x} + \frac{\partial^2 {U(\bf r)} }{\partial^2 y} + \frac{\partial^2 {U(\bf r)} }{\partial^2 z} =  0 \end{equation}  \par