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Re: moving wall laser demo

I would like to try the wall-moving demo described below.
1. Is the aluminum rod held in a horizontal or a vertical position next
to the wall?
....or does the free end of the rod extend into the room?

2. is the flat piece of metal (or cube) attached to the wall or to the
top of a horizontal table?

3. Will it make any difference if the aluminum rod is much longer or much
shorter than 30 cm??

Thanks for the clarification.

Herb Gottlieb

On Wed, 28 Jun 2000 00:45:13 -0400 Hugh Haskell <hhaskell@MINDSPRING.COM>

danmac@nau.edu http://purcell.phy.nau.edu PHYS-L
list owner

Take a piece of 1/2" aluminum rod about 30 cm long or so. Attach
one
end to the wall with a glob of modeling clay. Rest it on a flat
piece
of metal (those 1" square cubes with hooks in them work great). A
dissecting pin from the biologers works great because its got that
little bent handle on one end that you can easily glue a piece of
mirror to. Put the pin between the rod and the metal support, so
that
when you push on the wall and the rod moves, the pin will roll. Set
up the laser to reflect off the mirror onto the far wall, and you're
in business. It works on most any kind of wall, even cinder block. I
have set it up on a reinforced concrete support post and got it to
move, although you have to push pretty hard to show the motion. But
if you have it on a reasonably flexible wall (check it out first),
then have the smallest person in class come and push on the wall
with
one finger, the wall moves! I really impresses the students. In
fact,
if you hit the wall, the spot will oscillate on the opposite wall,
indicating that the wall you hit is "ringing."

The to cap off the demo, you can do a rough calculation of how much
the wall moved by measuring the motion of the spot. It isn't hard to
show that the motion is only a few 10s of microns. The amplification
of this scheme is on the order of 10,000 or maybe 100,000, depending
on how skinny your needle is.

good luck.

Hugh

--

Hugh Haskell
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows
because they
have to..
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