Cabal màssic: diferència entre les revisions

In elementary classical mechanics, mass flow rate is encountered when dealing with [[variable-mass system|objects of variable mass]], such as a rocket ejecting spent fuel. Often, descriptions of such objects erroneously<ref name="Halliday">{{cite book|last=Halliday|coauthors=Resnick|title=Physics|volume=1|pages=199|quote=It is important to note that we ''cannot'' derive a general expression for Newton's second law for variable mass systems by treating the mass in '''F''' = ''d'''''P'''/''dt'' = ''d''(''M'''''v''') as a ''variable''. [...] We ''can'' use '''F''' = ''d'''''P'''/''dt'' to analyze variable mass systems ''only'' if we apply it to an ''entire system of constant mass'' having parts among which there is an interchange of mass.|isbn=0-471-03710-9}} [Emphasis as in the original]</ref> invoke [[Newton's second law]] '''F'''&nbsp;=d(''m'''''v''')/d''t'' by treating both the mass ''m'' and the velocity '''v''' as time-dependent and then applying the derivative product rule. A correct description of such an object requires the application of Newton's second law to the entire, constant-mass system consisting of both the object and its ejected mass.<ref name="Halliday" />

 

 

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