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In Morin's classical mechanics book, chapter 12 (relativistic dynamics), in the axample about the relativistic rocket, we have a rocket that propels itself converting mass into photons and firing them back. Here Morin takes dm as a negative quantity, so the instantaneous mass of the rocket goes from m to m+dm.

I have tried to solve the same example taking dm as a positive quantity, and assuming that the instantaneous mass goes from m to m-dm, but I do not get the same answer. The correct rocket motion equation should be:

(dm/m)+(dv/(1-v^{2}))=0

, but instead I get:

(dm/m)-(dv/(1-v^{2}))=0

As you see, the solution is the same, except for one minus sign.

What could be the problem? Any ideas?

Thank you so much.

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# I Morin relativistic rocket example (chapter 12, page 606)

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