I'll write the equation first, and then state my problem with it.(adsbygoogle = window.adsbygoogle || []).push({});

mv=(m+dm)(v+dv)-dm(v-v')

The book says that in the term (m+dm), that dm is a negative quantity. If I believe this, then what about the -dm(v-v') term? Why is dm negative?

If I were to derive it I would have (m-dm) and +dm(v-v'), then I wouldn't have to say "dm" is a negative quantity. But if I use this method, then my result is different by a minus sign.

Will someone please explain this in a more intuitive sense. Thanks

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Rocket Equation Derivation

**Physics Forums | Science Articles, Homework Help, Discussion**