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I need a bit of help...quick please! - HSN forum

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I need a bit of help...quick please!


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#1 AM4R

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Posted 21 April 2007 - 09:56 AM

Say i have two different uncertainties of different units. Say one is of measurement (metres) and one is of frequency. How do I combine these to get a total uncertainty which has both in them??

Any help would be appreciated.
Thanks
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#2 ad absurdum

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Posted 21 April 2007 - 12:48 PM

You have to combine them as percentage uncertainties.
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#3 AM4R

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Posted 21 April 2007 - 09:49 PM

any idea how i do that?

Thanks
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#4 ad absurdum

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Posted 22 April 2007 - 12:21 PM

QUOTE(AM4R @ Apr 21 2007, 10:49 PM) View Post
any idea how i do that?

Thanks
To get the uncertainties as percentage uncertainties, you need to express the uncertainty as a percentage of what you were measuring. For example, say you were measuring the length of something, and it came out to be 40cm, and there was a scale reading uncertainty of \pm 0.1cm in this. Then your percentage uncertainty would be \pm (\frac{0.1}{40} \times 100)%  = \pm 0.25%.

You would get your other uncertainty(or ies) in terms of a percentage in that quantity, and then you can combine them in the normal way.
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#5 ice_illusion

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Posted 22 April 2007 - 01:19 PM

What is the normal way?
I was wondering for my chem invest and just ended up adding them.

#6 John

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Posted 22 April 2007 - 02:34 PM

I think it is just adding them, but tbh Uncertainties are barely used in Higher Physics

#7 ad absurdum

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Posted 22 April 2007 - 04:52 PM

I was hoping that wouldn't come up tongue.gif I can't remember how it was done in higher physics, but adding is probably fine. At physics advanced higher you would get everything as percentage errors, square them, add them together and then take the square root. So if your percentage errors were a,b and c, then your total percentage error P would be P = \sqrt{a^2 + b^2 + c^2} (however, if any errors are less than a third of your largest error you would disregard them). That's not part of the chemistry course though, so I don't think they would expect you to use that.
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