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

Hi John,

I am a Physics teacher from Singapore, and i am interested in your
discussion of the liquid accelerometer. Could you please kindly send =
the
pictures to me?
Thank you for your professional sharing.

Julie Quah

On 1/28/06, 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 j=
ar.
It was a giveaway several years ago at a meeting of ISPP, one of ou=
r
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 bal=
l
moves forward. Ask "What was the direction of your acceleration?" H=
ave
the student walk, then stop abruptly - again the floating ball poin=
ts in
the direction of the acceleration. Then have the student rotate, st=
ill
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 th=
e
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 lis=
t:
"comparing SHM". Drill a hole for an upright stick on the turntabl=
e
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 pendul=
um
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 th=
e
correct length for the string or the correct mass for the SHM sprin=
g
(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 be=
ing
horizontal.

Quoting John Denker <jsd@AV8N.COM>:

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

For an additional application while you have the apparatus: sec=
ure 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 flo=
ats
from the bottom. Next hang a weight on a string from the top (u=
se 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 mo=
ve
outward and the floating ball will move inwardmimmicking the hel=
ium
ballon vs the air filled balloon in an accelerating and decelera=
ting
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-ta=
ped
to the turntable.


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