## eseidl 2 years ago what did I do wrong on this one?

1. Hr.Tboy

which one?

2. eseidl

Answer is $v_f-v_i=-u \ln (\frac{m_f}{m_i})$I have no idea where the negative sign comes from. Here's what I did:$m \frac{dv}{dt}=u \frac{dm}{dt}$where u is a constant. so,$dv=u \frac{dm}{m}$and integrating:$v_f-v_i=u \ln (\frac{m_f}{m_i})$I don't know where the negative sign come from....

3. AravindG

you seechange in velocity dv is positive quantity whereas change in mass is negative

4. eseidl

yeah the negative sign makes it work, and I agree it needs one. I just don't know how to justify it mathematically.

5. AravindG

6. AravindG

|dw:1356100792938:dw|

7. eseidl

ok....it's confusing the way the instructor wrote it. If there were vector signs on v and u then it would make sense to add the negative sign once the vector arrows were dropped since v and u are in opposite directions. The instructor didn't do this though. Ok, thanks!

8. AravindG

welcome!