[Phys-L] Re: infinite sig. figs.

[SSHS KPHOX <kphox@CHERRYCREEKSCHOOLS.ORG>]

Forum for Physics Educators <PHYS-L@list1.ucc.nau.edu> on Thursday,
September 22, 2005 at 12:34 PM -0700 wrote:
> First of all, I'm going to flame any sentence that uses the
> term "sig figs."  People who care about their data do not follow
> the "sig figs rules" in any form.  Instead, they talk about the
> _uncertainty_ in their data.  They represent the uncertainty
> separately and explicitly.

First off I am in this camp philosophically but have a question as I teach
high school introductory level and the "evil" words are part of the
curriculum.

In determining a density of a rectangular soilid I might, with my cm ruler
get this data:
l = 3.75 +- .05 cm
w = 1.32 +- .05 cm
h = 0.54 +- .05 cm

all the tools I hav been taught to do this gives the volume as:
2.7 cc +- 0.4 cc  (14%)

I do not like this very much as the 0.54 has the same certainty as the
others.
I would prefer:
2.67 =- 0.39 cc
but not
 2.673 +- 0.388 cc

I have tried to take the approach that we are multiplying 3.7x by 1.32x by
0.5x  (where the x represents an uncetain number). When I do this I get
2.4xxx  which I think supports my choice of 2.67 cc.

I wonder if I am misleading my students with what I call a reasonable
approach?

Ken Fox
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