- #1

- 94

- 2

## Main Question or Discussion Point

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-v

, but instead I get:

(dm/m)-(dv/(1-v

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

What could be the problem? Any ideas?

Thank you so much.

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}))=0As you see, the solution is the same, except for one minus sign.

What could be the problem? Any ideas?

Thank you so much.