Hello all. 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-v2))=0 , but instead I get: (dm/m)-(dv/(1-v2))=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.