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

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

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

eseidl
 2 years 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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