## Limitations on Newton's 2nd LawOne of the best known relationships in physics is Newton's 2nd Law but, though extremely useful, it is not a fundamental principle like the conservation laws. F must be the net external force, and even then a more fundamental relationship is The net force should be defined as the rate of change of momentum; this becomes only if the mass is constant. Since the mass changes as the speed approaches the speed of light, F=ma is seen to be strictly a non-relativistic relationship which applies to the acceleration of constant mass objects. Despite these limitations, it is extremely useful for the prediction of motion under these constraints. | Index Newton's laws concepts | ||

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## Limitations on Newton's 2nd LawOne of the best known relationships in physics is Newton's 2nd Law but, though extremely useful, it is not a fundamental principle like the conservation laws. F must be the net external force, and even then a more fundamental relationship is The net force should be defined as the rate of change of momentum; this becomes | Index Newton's laws concepts | ||

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## Variable Mass ApplicationsThe generalization of Newton's 2nd Law to apply to variable mass systems takes the form The term involving the derivative of the mass is responsible for the thrust in rocket propulsion and must be included in any problem where the mass changes. | Index | ||

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