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eseidl
 one year ago
Best ResponseYou've already chosen the best response.0Answer is \[v_fv_i=u \ln (\frac{m_f}{m_i})\]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:\[v_fv_i=u \ln (\frac{m_f}{m_i})\]I don't know where the negative sign come from....

AravindG
 one year ago
Best ResponseYou've already chosen the best response.1you seechange in velocity dv is positive quantity whereas change in mass is negative

eseidl
 one year ago
Best ResponseYou've already chosen the best response.0yeah the negative sign makes it work, and I agree it needs one. I just don't know how to justify it mathematically.

AravindG
 one year ago
Best ResponseYou've already chosen the best response.1ots because u should have had ve sign in rhs from the first equation onwards!!

AravindG
 one year ago
Best ResponseYou've already chosen the best response.1dw:1356100792938:dw

eseidl
 one year ago
Best ResponseYou've already chosen the best response.0ok....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!
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