- #1
GBA13
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- 0
Homework Statement
Hi Guys,
I am a bit confused concerning one part of this topic. Specifically when trying to find non dimensional groups. My problem is a small thing in the finding of the indices.
So, for example, your are trying to find a pi group when finding the drag on a car.
so you have D,A,V,rho.
then you find indices by doing [M][L][T]^-2 ([L]^2)^a ...
and then do [L]: 0 = 1 + 2a...
but sometimes I have seen in solution people doing
[L]: 1 = 2a...
This obviously gives a different answer to the one above. as a = 1/2 for the bottom one and -1/2 for the other. I am confused about which one we are supposed to use for what
Homework Equations
The Attempt at a Solution
One thing I am finding is that often when it is like
[L]: 1= 2a.. that is because the pi group has been labelled D = kA^aV^b rho^c or something like that while then it looks like
[L]: 0 = 1 + 2a... it is normall labeled DA^aV^b rho^c. I am not sure what the difference is but the two methods give different numbers and I'm not sure when one should be used or not. I'm not sure why k is in the group above and isn't in the one below etc..
All I am tyring to do is form a functional relationship with things.
Thanks, I'm sorry if this is a bit disjointed I am struggling to explain it!
I would really appreciate someone setting me straight.