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



SSHS KPHOX wrote:

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?

I don't know what "tools" this refers to.

In my toolbox I have a spreadsheet that does a full-blown Monte Carlo.
For me this is as easy as pi. I just take the existing spreadsheet
http://www.av8n.com/physics/mg-digs.xls
and replace the three raw data points (and their uncertainties) and
change the formula in one place, fill down, and press F9 (recalculate).

Bottom line: I get 2.67 +- 0.28
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