Re: [Phys-l] Warped 2-metersticks....

["John Clement" <clement@hal-pc.org>]

It should be possible to unwarp a wooden stick and still have its length
fairly consistent.  This is because the expansion occurs mainly
perpendicular to the gain and they are very stable in the direction of the
grain.  Wooden meter sticks have the grain parallel to the length.  Good
cabinet makers know that you can not rigidly join extensive lengths of wood
with the grains crossed.  You have to allow for expansion perpendicular to
the grain.

Plywood takes advantage of this by bonding many crossed layers.  This
produces a product which is dimensionally stable in both directions.
However, numerous small cracks can happen in the surface.

I wonder if the wooden meter sticks were accurately made in the first place.
Since shrinkage occurs as the wood dries, one must start with properly dried
wood before making the stick.  But still most of the shrinkage should not
have a large effect on the length.

Alternately you could use the difference in lengths to have a lesson on
precision and accuracy.  Students can be easily lulled into thinking that
all measurements are exact when they always have what looks like exact
numbers.  Digital scales may be promoting this problem.

I have seen students look puzzled when asked to measure the circumference of
a circle with a sheet of paper and a ruler.  They don't seem to think of
wrapping the paper around the object.  A curved meter stick might be a good
clue.  And having nothing but 1m sticks might be better than 2m sticks
because it requires a bit more thought.  I have a picture in Minds on
Physics where they have to find the graph of position time from a motion
diagram of a ball falling.  The authors drew 2 meter sticks end to end.
Students would ask things like where do I start, at the top, or in the
middle.  When I say it is up to you they will sometimes say "oh, the scale
goes -80,-90,0,10,20...."  Hmmmmmm!!!!!!

Turn the problem into an advantage.  After all a bug can be turned into a
feature by advertising it.

John M. Clement
Houston, TX

> It is doubtful that a warped 2-meter stick can provide
> accurate readings within half a millimeter even after
> it has been carefully straightened out.
>
> A number of years ago The Physics Teacher magazine
> reported a comparative evaluation of wooden meter sticks.
> I recall seeing a photograph of a stack of these "meter"
> sticks that varied in length by as much as 2 or 3 millimeters.
>
> Herb Gottlieb from New York City
> (Where it is almost impossible to find a meter stick that is
> one meter long.
>
>
>
>
>
> On Sun, 19 Mar 2006 08:28:05 -0600 Brian Whatcott <betwys1@sbcglobal.net>
> writes:
> > At 07:31 PM 3/18/2006, Cherie Lehman, you wrote:
> > > Hi everyone,
> > >
> > > I was just wondering if any of the processes  suggested for
> > "un-warping" the
> > > warped 2-meter sticks would disturb the  spacing between the
> > hashmarks on the
> > > sticks. I'd be pretty nervous about turning  it into spaghetti for
> >
> > > fear that I
> > > would end up with a French curve. For that  matter, would the over
> > all length
> > > of the stick be changed?
> > >
> > > Just wondering.
> > > Cherie
> >
> > This is an opportune question for a person interested in measuring
> > something
> > with an accuracy of less than one half the smallest division mark
> > over the
> >   length of the rule (which would be the rational lower limit of
> > acceptable
> >   accuracy in a measuring stick). It is however, the sort of
> > question
> > only likely
> >   to be answered by someone in a physics lab or class-room.
> >   Other wood users can be content with slightly wider acceptable
> > tolerances.
> >     The cheapest one meter sticks I've seen go under $2. A suitable
> >
> > experimental
> >   prospect, possibly?