variable-mass systems
... that Newton's second law is not valid for variable-mass systems, such as rockets?
... that <a class="mw-redirect" href="./Newton's_second_law" id="mwDe4" rel="mw:WikiLink" title="Newton's second law">Newton's second law</a> is not valid for <b id="mwDe8"><a href="./Variable-mass_system" id="mwDfA" rel="mw:WikiLink" title="Variable-mass system">variable-mass systems</a></b>, such as <a href="./Rocket" id="mwDfE" rel="mw:WikiLink" title="Rocket">rockets</a>?