  We have developed an alternative formulation based on ${\bf F} = M {\bf a}$
rather than ${\bf F} = d{\bf P}/dt$ for studying variable mass systems. It is
shown that ${\bf F} = M {\bf a}$ can be particularly useful in this context, as
illustrated by various examples involving chains and ropes. The method implies
the division of the whole system into two parts, which are considered
separately, allowing to explore certain aspects as constraint forces.
