Introduction The applicability of Newton’s second law in the oft-quoted “general form”  \begin{align}\frac{d\mathbf{P}}{dt}=\mathbf{F}_{\text{ext}}\end{align} was an issue in a recent thread (see post #4) in cases of systems with variable mass.  The following example illustrates the kind of confusion that could arise from the (mis)application of Equation (1): A rocket is hovering in place above ground...