  Misinterpretations of Newton's second law for variable mass systems found in
the literature are addressed. In particular, it is shown that Newton's second
law in the form $\vec{F} = \dot{\vec{p}}$ is valid for variable mass systems in
general, contrary to the claims by some authors that it is not applicable to
such systems in general. In addition, Newton's second law in the form $\vec{F}
= m \vec{v}$ -- commonly regarded as valid only for constant mass systems -- is
shown to be valid also for variable mass systems. Furthermore, it is argued
that $\vec{F} = m \vec{v}$ may be considered as the fundamental law,
alternatively to $\vec{F} = \dot{\vec{p}}$. The present work should be of
direct relevance to both instructors and students who are engaged in teaching
and/or learning classical mechanics in general and variable mass systems in
particular at a middle- to upper-level university physics.
