Date: Tue, 30 Sep 2003 14:06:07 -0700

Author: Bill Alexander

Subject: Re: Inertia Ball

Post:

We use a 1 kg mass hung by thread from a horizontal bar, w/ a piece of

>OK, this is a classic. There is nothing critical about the apparatus,
>so you can make one from whatever is easy to get.
>
>Our set-up uses a disk of lead, left over from something. Two
>eyebolts are screwed radially into the edge of the disks at opposite
>ends of a diameter. The frame consists of two sturdy lab stands
>holding up a cross bar.
>
>A string is looped through one of the eyebolts, around the bar, and
>tied into a loop. The weight now hangs from a loop of string. Another
>loop is formed through the bottom eyebolt. The "operator" puts a heavy
>steel rod (from another lab stand) through the lower loop and strikes
>down hard to break the bottom loop. If done quickly the bottom loop
>breaks before the lead mass displaces significantly, so the top string
>does not break. We usually repeat with a double loop of string through
>the bottom eyebolt. For effect, you can put a big pad under the
>apparatus "to avoid damaging the table".
>
>As a demo, it is in the same class as pulling a tablecloth out from
>under a bunch of dishes. It shows that a=F/m, sort of,
>qualitatively. I'm not sure how much students learn, but maybe it
>helps them realize that big accelerations require big forces. They
>certainly like these demos.
>
>Stan
>
>On Tue, 30 Sep 2003, George M. Caplan wrote:
>
>> Sam writes:
>>
>> Can you describe the demo?
>>
>> Thanks,
>>
>> Sam
>> *********
>> A large cylindrical mass is supported by a string.
>> Another similar string is tied to the bottom of the mass.
>> The whole thing is in a "frame".
>> The "operator" pulls *with two hands" on a rod tied to the lower string.
>>
>> George
>>
>>

Bill Alexander Excuse me if I ramble,
Physics Dept. but I drive a Nash.
Humboldt State University
Arcata, CA 95521

phone: (707) 826-3212
e-mail: wca1@humboldt.edu
http://sorrel.humboldt.edu/~phyx/faculty/alexander.html

From grahamaj@appstate.edu Tue Sep 30 14:50:52 2003
Date: Tue, 30 Sep 2003 16:06:05 -0400
From: Andrew Graham
Subject: Strobed pulsed water stream.
To: tap-l@appstate.edu
Message-id: <3F79E22F.E844A612@appstate.edu>
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Greetings,

Thanks for all the requests for information on the "water dropper". The
original design for this apparatus came from Dr. Harold Edgerton at MIT,
famous for his work with strobes and high speed photography. I am
really in a time crunch, so I'll suggest to any of you with access to
back issues of The Physics Teacher to look at the November 1987 issue
page 513 (and cover). All the specifications are given there, and
construction hints. I'll try to make some notes and photos available on
the web very soon.

As for the pump, it must be solenoid driven (oscillating) not
reciprocating.
For an example see

http://gongol.net/literature/gr/ots/105-106.pdf

GRI 17000-002 S
Polypropylene body
0.5" suction, 0.5" discharge
115 VAC

The price seems a bit step. I purchased mine from Cole-Parmer, but I
didn't find them in their latest catalog.

Tubing: I highly recommend getting Fisher Brand C-Flex. It's soft and
will carry the pulse without dampening.

Nozzle: needs to be tapered. I use brass, about 3 cm long, 1.3 cm
outside diameter, inside tapered from 1.3 down to .8 cm.

Power the pump from a Variac to control the pressure and make good
stable water drops.

More later.

andy
From alistair.lightfoot@canterbury.ac.nz Tue Sep 30 16:52:10 2003
Date: Wed, 01 Oct 2003 11:35:13 +1200
From: Alistair Lightfoot
Subject: Electroscopes
To: tap-l@listproc.appstate.edu
Message-id:

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Greetings, all,

Here at University of Canterbury Physics & Astronomy Dept I have an
occasional need for electroscopes in lecture demonstrations and
presently have a selection of old and run down ones, one of which has

I am looking at the Sargent Welch Projection Electroscope, model WL1971C
for the projection tasks, and the metal cased WL1963A for demonstrations
where projection is not required.

In the past I have had only mediocre performance from electroscopes in
cans and flasks (the like of which Sargent Welch also offer).

I would appreciate comments re the effectiveness, sensitivity,
robustness etc. of the two I've mentioned, especially the projection
one.

Thanks

Alistair Lightfoot
Teaching Support Technician
Physics & Astronomy Dept.
University of Canterbury
Christchurch
New Zealand
From wreitz@neo.rr.com Tue Sep 30 16:54:45 2003
Message-ID: <007001c387ab\$eb6aaca0\$92c2d2cc@toshibauser>
From: "Bill Reitz"
To:
References: <3F79EEEE.1030609@physics.syr.edu>
Subject: Re: Inertia Ball
Date: Tue, 30 Sep 2003 19:38:14 -0400
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I've seen several people do this demo horizontally. I usually do the demo
vertically first using cotton kite string above & below a 1 kg mass ( a
fugitive of an Ohaus hooked weight set) Then we go horizontal. When I've
done it I've used a PSSC lab cart (2x4 wooden body with roller skate wheels)
piled with around 10 or more kg masses. The rear of the cart is tied to a
fixed support rod with light string or thread while the front is tied to
cotton clothes line connected to a steel rod. I swing the steel rod like a
baseball bat away from the cart & snap the clothesline while the thread
"holds" the cart from moving. Students seem impressed - especially sometimes
when I rip the metal front off the cart or when I used the clothesline with
the hidden polyester center by mistake & nearly popped my elbow.

Bill Reitz,
suffering from occasional inertia elbow

---- Original Message -----
From: "sampere"
To:
Sent: Tuesday, September 30, 2003 5:00 PM
Subject: Re: Inertia Ball

> George,
>
> I do the demo with 1 kg masses and cotton string, like from a bakery. I
> have a 50 year supply of the stuff. Choose your string and masses so
> that the string breaks when about 1.5 ~ 2x the mass breaks the string.
>
> So, you found some string and want to try this. Set up some clamps and
> rods so you have a secure horizontal bar above the table Tie the string
> to the horizontal rod and add mass until the string breaks. Divide this
> mass by 2 and that gives you value of one of the single masses. Always
> use the same string and this will never fail.
>
> Always use the rod below. Keeping your hands away from the falling mass
> is important! Put a board on the table for protecion from the falling
mass.
>
> To make this easy, thread some hooks into your mass. Cenco used to sell
> large blue balls (ok, take it easy) with the hooks already attached. SW
> may still sell them.
>
> If you want to make it, buy some large steel balls from McMaster-Carr or
> use a shot put (a girls high school shot is lighter). Drill and tap the
> mass and thread in some hooks.
>
> Sam
>
> George M. Caplan wrote:
>
> >Sam writes:
> >
> >Can you describe the demo?
> >
> >Thanks,
> >
> >Sam
> >*********
> >A large cylindrical mass is supported by a string.
> > Another similar string is tied to the bottom of the mass.
> >The whole thing is in a "frame".
> >The "operator" pulls *with two hands" on a rod tied to the lower string.
> >
> >George
> >
> >
>
>

From wreitz@neo.rr.com Tue Sep 30 16:58:33 2003
Message-ID: <009b01c387ac\$7757e700\$92c2d2cc@toshibauser>
From: "Bill Reitz"
To: ,
References: <3F79E22F.E844A612@appstate.edu>
Subject: Re: Strobed pulsed water stream.
Date: Tue, 30 Sep 2003 19:42:09 -0400
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I use an old "water-pik" to produce the pulse and return the water to the
water-pik's
reservoir for continuous operation.

Bill Reitz

----- Original Message -----
From: "Andrew Graham"
To:
Sent: Tuesday, September 30, 2003 4:06 PM
Subject: Strobed pulsed water stream.

> Greetings,
>
> Thanks for all the requests for information on the "water dropper". The
> original design for this apparatus came from Dr. Harold Edgerton at MIT,
> famous for his work with strobes and high speed photography. I am
> really in a time crunch, so I'll suggest to any of you with access to
> back issues of The Physics Teacher to look at the November 1987 issue
> page 513 (and cover). All the specifications are given there, and
> construction hints. I'll try to make some notes and photos available on
> the web very soon.
>
> As for the pump, it must be solenoid driven (oscillating) not
> reciprocating.
> For an example see
>
> http://gongol.net/literature/gr/ots/105-106.pdf
>
> GRI 17000-002 S
> Polypropylene body
> 0.5" suction, 0.5" discharge
> 115 VAC
>
> The price seems a bit step. I purchased mine from Cole-Parmer, but I
> didn't find them in their latest catalog.
>
> Tubing: I highly recommend getting Fisher Brand C-Flex. It's soft and
> will carry the pulse without dampening.
>
> Nozzle: needs to be tapered. I use brass, about 3 cm long, 1.3 cm
> outside diameter, inside tapered from 1.3 down to .8 cm.
>
> Power the pump from a Variac to control the pressure and make good
> stable water drops.
>
> More later.
>
> andy
>
>

From gcaplan@wellesley.edu Tue Sep 30 19:23:06 2003
Message-id:
Date: Tue, 30 Sep 2003 22:06:40 -0400
Subject: Re: Inertia Ball
To: tap-l@listproc.appstate.edu
From: "George M. Caplan"
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I think I need to explain why I am asking about the
inertia ball demo seen on the video.
I do an "inertia ball" demo which works well as a demo.
Some of my students are considering doing a video analysis
of this kind of demo. (See the paper by Mark Heald and me in the Nov. '96
TPT.)
When we play back the video frame by frame (Or is it field by field? I'm
not sure.),
we get a pretty good picture of the difference between the two cases.
I think with a device like the one in the video we can make our own
videos, or movies,
or strobe pictures and get some quantitative data.

From gcaplan@wellesley.edu Tue Sep 30 19:27:59 2003
Message-id:
Date: Tue, 30 Sep 2003 22:11:31 -0400
Subject: Re: Inertia Ball
To: tap-l@listproc.appstate.edu
From: "George M. Caplan"
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Just to clarify.
I wrote:
When we play back the video frame by frame

I meant when we play back the video on the laser disc in the Video
Encyclopedia...
From billb@eskimo.com Tue Sep 30 20:11:32 2003
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Date: Tue, 30 Sep 2003 19:55:03 -0700 (PDT)
From: William Beaty
To: tap-l@listproc.appstate.edu
Subject: Re: Strobed pulsed water stream.
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On Tue, 30 Sep 2003, Bill Reitz wrote:

> I use an old "water-pik" to produce the pulse and return the water to the
> water-pik's
> reservoir for continuous operation.
>

How about \$22.50? C and H Sales, oscillating pump.
http://www.aaaim.com/cgi-local/shop991/shop.pl/SID=304127607293/page=REGS.htm#PC2051

(customer must supply bracket, connects to 4 springs at corners.)

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://amasci.com
EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci

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