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[Phys-L] Re: rotating stuff (was: telescope mirrors)



Thanks for the picture offer. Could you e-mail me your pictures?
Thanks in advance,
-Tony


On 1/27/06 6:23 PM, "Milton, John" <JMILTON@DEPAUL.EDU> wrote:

I tried to send a reply that was rejected - I forgot that one can't
attach pictures to a Phys-L posting. Here is my original message,
without the pictures.

We use a "liquid accelerometer" made from a (clean) peanut butter jar.
It was a giveaway several years ago at a meeting of ISPP, one of our
local teacher alliances. A gasket is made from a plastic coffee can
cover. (Pictures 1 and 2) Have a student hold the accelerometer at arm's
length (Picture 3), then step forward. (Picture 4) The floating ball
moves forward. Ask "What was the direction of your acceleration?" Have
the student walk, then stop abruptly - again the floating ball points in
the direction of the acceleration. Then have the student rotate, still
holding the accelerometer with arms extended, and all will see the ball
pointing inward. (Picture 5, with the device on a Pasco rotating
platform) Physics Cinema Classics, in the section on rotation, has some
footage of a similar device: "Acceleration of air table puck and
automobile."

I will be glad to send pictures to anyone interested. Contact me by
e-mail.

John Milton
De Paul University
Chicago, IL

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of Karl Trappe
Sent: Friday, January 27, 2006 2:26 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: rotating stuff (was: telescope mirrors)

Hey, Denker...you ought to know how *cheap* I am!

I use the string to tie the beaker down so it doesn't fly off, but
then the string slips down while in motion, so I duct tape it to the
beaker...Your annulus rig is much cleaner. It does need to be thick
enough that a shift in CofM doesn't let the beaker tip.

Good point about how cheap turntables are nowdays. Add to your list:
"comparing SHM". Drill a hole for an upright stick on the turntable
or make a magnetic support if the turntable is out of steel.. Add a
ball on top of the stick for visibility. Set the turntable to one of
its RPM values. Mount above this either a ring stand with a pendulum
adjusted to have the corresponding frequency to the revolution or a
SHM spring with the correct mass. Voila, direct observation of
equivalence of rotation with spring with pendulum. You can shadow
project with a bright point light source and observe the coincident
location of the shadows on the wall. Have the students calculate the
correct length for the string or the correct mass for the SHM spring
(find k first).

If you have the option, getting a turntable that will run while on its
side will make the SHM of spring comparison easier. Alas, most are
driven with an idler wheel, that relies on the turntable platter being
horizontal.

Quoting John Denker <jsd@AV8N.COM>:

Karl Trappe wrote:
Tie a beaker to a lazy susan turntable (or on your rotating stool
apparatus) using string and duct tape. Partially fill it with water,
spin it and observe. If you move it off center, the results will not
be symmetric, of course.

For an additional application while you have the apparatus: secure a
string to the bottom center of the beaker (epoxy a swivel to the
bottom). tie a cork/styrofoam ball to the string so that it floats
from the bottom. Next hang a weight on a string from the top (use a
strp of thin aluminum with a hole in it).Set this beaker far off
center and rotate the lazy susan slowly. The heavy ball will move
outward and the floating ball will move inwardmimmicking the helium
ballon vs the air filled balloon in an accelerating and decelerating
car.

Very nice. Thanks.

Some tangential remarks and refimements:

lazy susan turntable

You can still get _record player_ turntables ($10 on ebay) (also
garage sales sometimes) which are absolutely ideal for this demo ...
and for many other demos, including e.g. bouncing a laser off a
tuning fork (or loudspeaker) to draw the acoustic waveform graph on
the wall......

using string and duct tape

I don't understand how "string" enters into it.

If you really want to get fancy, make an _annulus_ out of light
gauge plywood. The hole in the middle is just the right size to
hold the beaker. The outer rim of the annulus is easily duct-taped
to the turntable.


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