Arch wrote:
Will, be careful. I think you got it right, but you might get your nos=
e
tweaked for saying "precise" if you meant "accurate". I did once and =
I
still don't understand the difference. Or care! I think it has to do
with repeatability. Maybe somebody will set both of us 'cynics' straigh=
t
(ie.without curves or angles) G)
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Turn to Safety, Arch =20
Fortiter
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http://community.webtv.net/almcc/MacsMusings
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Arch:

I must apologize - I forgot the following..

When multiplying or dividing -- the percentage of error adds absolutely, =

when adding and subtracting, the absolute error adds absolutely.

That would make it clear why you have the rules about guard digits and=20
places of precision.

For example if you multiply two number together, (and you had been able=20
to make the measurements and obtain the two numbers to 5% accuracy...)=20
The resulting answer would be accurate to 10% -- so any digits beyond=20
the first decimal place would be guard digits or used for rounding...

So you could haver 5 decimal places of precision -- but the number would =

be accurate to one decimal place...

So in this situation if you had 1.115 (plus as many digits more as you=20
please since they are not relevant) in the calculator window after the=20
calculation -- the _answer_ would be 1.1 and be accurate to _one_=20
decimal place... No matter what your calculator said...

All kidding aside -- you should remember this when doing calculations=20
when you multiply or divide measured numbers.


Adding or subtracting..
1 1/2 (+- 1/16 ) + 1 1/4 (+- 1/16) is 2 3/4 (+- 2/16) or

2 2/4 (+- 1/8 ) in other words... Because it is the ABSOLUTE SUM of the=20
two estimated errors that is the error factor.


Now you made me think... That was terrible. But --- Not too bad for a=20
guy with a degree in Basket Weaving huh?

Anyone with eng, phys or math degrees - don't even hassle me about this=20
or I will pop a cork. LOL


You asked for it. You got it. Don't complain.


--=20
Will R.
Jewel Boxes and Wood Art
http://woodwork.pmccl.com
The power of accurate observation is commonly called cynicism by those=20
who have not got it.=94 George Bernard Shaw