| Celestial Mechanics and Dynamical Astronomy | Cambridge UK: Cambridge University Press |
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| Any single force is only one aspect of a mutual interaction between two bodies | Quoting Newton in the Principia: It is not one action by which the Sun attracts Jupiter, and another by which Jupiter attracts the Sun; but it is one action by which the Sun and Jupiter mutually endeavour to come nearer together |
] Consequently, the mass of the system can not change during the time of interest.
10| When the mass varies due to accretion or ablation, [an alternate equation explicitly accounting for the changing mass] should be used | I Bernard Cohen Peter M |
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| I]t is essential to deal with the same set of particles throughout the time interval[ | Hannah, J, Hillier, M J, Applied Mechanics, p221, Pitman Paperbacks, 1971• Cambridge UK: Cambridge University Press |
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