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  GR8677 #62
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Verbatim question for GR8677 #62
Mechanics}Separable Differential Equations

Solve m\frac{dv}{dt}=-u\frac{dm}{dt}. Separate variables to get \par
\int u\frac{dm}{m}=\int -dv\Rightarrow u \ln (m_0/m)=v(t). None of the answer choices such a ln relation for v, and thus the answer is (E), none of the above.

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Comments
lattes
2008-08-06 13:19:11
Another way to solve is considering m=m_{0} on the choices. None of them gives v=0 so the answer must be (E)
chrisfizzix
2008-10-03 13:21:18
This man speaks the truth. Solving a DEQ no matter how simple is almost always not the way to go about solving an ETS problem. Boundary conditions are definitely the way to go here.
shak
2010-07-31 13:03:10
in this problem, boundary condition works very good, right:) and also we can always check solution by initial and boundary condition, and eliminate wrong answers
NEC
Blake7
2007-07-22 20:55:16
Why would someone attempt a "None of the above" problem on a timed record exam? I see that and think "BYPASS" immediately.

Rune
2007-10-14 22:51:00
Because in this case it is very simple as long as you know the \int \frac{1}{x}dx, in which case it takes all of 15 seconds to read and solve.
gliese876d
2008-10-17 11:26:40
Or because, crazy lunatics like me actually had the eq. for this memorized, so it took 2 seconds to realize the right answer wasn't there :)
neon37
2010-11-03 10:18:31
None of the above in this was pretty easy to figure but, it was very hard to be quite confident. I must remember use boundary conditions!! It is the way to go for ETS DEQs.
llama
2013-10-16 18:47:14
Or these days, because you play kerbal space program and have an interest in physics so you know the rocket equation
NEC

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