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Stepside
12-09-2010, 01:21 PM
The question has to do with what type of bearing if any at the suspension point. I am going to help a Junior High School teacher build one in his classroom.

Thanks
Pete

Evan
12-09-2010, 01:25 PM
Classic answer: It depends.

How long will the pendulum be? How much mass do you contemplate using? How do plan to drive the pendulum? What do you intend to suspend it with? Will it be protected from air currents?

Note on the mass, it must be non magnetic.

Stepside
12-09-2010, 02:52 PM
The ceiling beams are 16 to 20 feet above the floor. One of the sources suggested using an electro-magnet to power the set-up. This wouldn't work well with a non-magnetic bob/weight. Is the shape of the bob/weight an issue?

My goal would be 1) that it works 2) that the students take part in building and design 3) that the students both enjoy the process and understand the "Form follow restrictions" as opposed to "form follows function".

So what I am looking for is the "restrictions list" to present to the students.

Evan
12-09-2010, 03:32 PM
Excellent height for a pendulum.

The main mass of the pendulum needs to be non magnetic to avoid acting as a compass. A very small piece of magnetically soft iron on the bottom will serve for the drive. The drive may be a solenoid placed directly below the at rest position. It should be activated in a manner that acts isotropically so that no direction is preferred. That means the soft iron piece should be round and the drive Should have a current imposed on the solenoid when the bob is approaching. The pole piece on the bob will disturb the field which can be sensed by an ac coupled op amp to initiate a timer that is adjusted to cut off the current for a short time when the bob crosses centre. The current is then switched back on so it can sense the approaching bob again. All parts must be radially symmetrical.

The bob should have as much density as possible. Lead is preferred but is probably out as is depleted uranium. Barring that, brass is often used. Since this project undoubtedly has a tight budget I would recommend a basketball filled with sand. The shape of the bob is important since it must present the same aerodynamics in any orientation.

The suspension should be as rigid as possible and also non magnetic. No bearing is required for relatively short term use. Stainless steel downrigger wire line should do the job. To avoid it breaking at the point of suspension it is fed through a machined orifice that opens in an inverse parabolic curve. I can supply you with the CNC code for same if you have access to a machine or make one for you if you wish. The point of suspension must be rigidly mounted so that the forces imposed are equal in all directions.

A low wall around the pendulum pit is recommended to prevent air currents from disturbing the swing.

Come to think of it you could also use a short piece of stranded metal leader line for the first few inches of the suspension at the top. All it would need then to prevent fatigue is a nicely rounded hole from which to exit the mounting.

Stepside
12-09-2010, 04:14 PM
Evan

Thank you for the input. This will allow me to start with the students next week. I am sure there will be more questions.

Tomorrow I am taking a complete Street Clock works including pendulum, hands and transmission on a stand for their enjoyment.

Hopefully I can help them see that their Science class is really a tool for problem solving and that on occasion school could even be fun.

The Artful Bodger
12-09-2010, 05:06 PM
My opinions on this:-

I think a needle roller Hooke's type universal joint would be a suitable and thorougly confidence inspiring suspension point. Should be easy to get too from any machinery supplier or salvaged from a small car, garden tractor, etc etc.

rohart
12-09-2010, 05:40 PM
Just make sure there's enough room for the pendulum to turn 90 degrees during the night. Someone wrote a detective story - could have been a Sherlock Holmes add-on - about a Foucault's pendulum doing in a sleeping janitor when it got round a few hours later while he was sleeping it off.

The sand-in-ball solution sounds a bit low density to last very long without slowing down, if you don't use magnetic kicking. I'd try to come up with something more dense - like a bundle of short lengths of steel hung from the central length and wired/brazed together. Even a plastic bottle of water hanging from a home in its cap ? That's as safe as the sand, too.

I like Evan's few inches of thin wire rope at the top to give you the flex you need. The rest can be more rigid wire. Make sure no one's around when you string it up, as several yards of wire swinging free can do damage.

The pendulums they hang from the roof in cathedrals are very impressive, with periods of over ten seconds.

dp
12-09-2010, 07:30 PM
I built one in high school using a bowling ball suspended from a 30' phospher bronze wire. It worked great. I can't recall the details of the energizer - I think it was a many turns coil of wire on a wood dowel with the ends soldered together to form a short circuit (and electromagnetic field generator) buried in the ball and it was driven by an electromagnet mounted at the plumb spot in the center of swing and that was pulsed as the ball went by. I'm certain there was a proximity sensor in the mix but a lot of variations were tried over time. It was supposed to be a science fair project but they didn't have a place for hanging a 30' wire with a bowling ball on it. It was loosly modeled after the one in the San Francisco academy of sciences.

For such a simple thing it was fascinating to watch.

Edit:
I forgot to describe the bearing - I used used conical wheel bearing from my 1949 Chevy's left front wheel! I ground a shallow v-notch in the inner race and a T-bolt went through that. The T part of the bolt was a piece of steel I brazed to a bolt. I ground the lower edges to a knife edge and they fit into the v-notch. That allowed it to pivot with no drag when the wire was attached to the bolt. The conical bearing was free-rolling enough for the job. The pendulum never operated for more than a few hours to demonstrate how it could knock things over at 15 minute intervals, so any drag the bearing had never revealed itself.

jugs
12-09-2010, 07:38 PM
For such a simple thing it was fascinating to watch.

Simple things for simple minds ?? :D :D

john
:)

Stepside
12-10-2010, 10:29 AM
Would a woven string work as well as a wire? This would take care of thermal growth. There are some fishing lines that are woven which should keep the line from wanting to spin.
I think DP has a good idea for the bearing at the suspension point. (see his edit of an earlier post) I have a good selection of bearings, so the front wheel of my truck is safe.
I am open to suggestions as to both the size of the bob as well as the shape. Reasons for the size and shape that I can pass on to the students would be a great addition.

Thanks in advance for the help.
Pete

alanganes
12-10-2010, 11:07 AM
You may already be aware of this, but C.L. Stong's book "The Amateur Scientist" has a chapter on building a Foucault pendulum. The book goes back a ways, so some of the info on materials and electrical stuff may be dated, but it may be a good source of ideas and thoughts on potential trouble spots.

Cobbler
12-10-2010, 11:10 AM
When I was in college, I did an independent study in physics for a couple credits I was short. My project was building a full-scale ballistic pendulum capable of "catching" high power rifle rounds. I followed the design described by P.O. Ackley in one of his books.

On this project, the bob was suspended by wires connected to an eye loop. Because there were multiple wires, there were actually bearings at both ends of the supporting wires. The bracket I made to suspend the pendulum from the ceiling (of the garage) and the bob itself had knife edge surfaces that the eye loops fit over. To insure that all of the support wires had equal tension, I installed a turnbuckle one one end of each wire and "tuned" the wires to the same note.

Because of the low friction of the knife edge bearings, that thing would swing for a LOOOOOOOOONG time. I know the multiple supports traveling in a single direction of my experiment as opposed to the single support traveling in multiple directions for the one of the OP are different but the theory of the knife edge bearing might be able to transfer to some sort of double knife edge or a single pinpoint like you see in some of those desk-top motion toys. Long term, an automotive type of U-Joint might be the least hassle and still provide a pretty good pivot point.

A.K. Boomer
12-10-2010, 11:49 AM
What about magnetic suspension? that would be cool.

Cobbler
12-10-2010, 12:32 PM
Here's one with some nice notes on how it was built. A bit beyond simple but might provide some inspiration, especially regarding the pivot.

http://www.astro.louisville.edu/foucault/

dp
12-10-2010, 12:58 PM
I think DP has a good idea for the bearing at the suspension point. (see his edit of an earlier post) I have a good selection of bearings, so the front wheel of my truck is safe.

The knife edge seemed utterly brilliant at the time and it did work, but only if you took great care to align it perpendicular to the direction of initial motion of the bob. That required installing a pointer at the bearing to show the proper direction to deflect the bob to get it going. Something closer to a gimbal would have been preferable but I didn't have the idea or means to create that. I had been working on a separate project at the time and just stole the idea. I'd built a seismometer sensor that used a razor blade for a pivot.