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OT Barton's Pendulum

If you've built a Barton's Pendulum, your insight could help.

I'm looking for "how to construct" advice to build a Barton's Pendulum for teaching the kids. Where I have trouble - and the kids will too - is tying the strings such that they end up at the exact proper length. This is not an issue with most of the pendula, but you want the driver and the target as close to each other in length as possible to get resonance, or as close to resonance as possible.

I Googled, and there's lots of references to Barton's Pendulum, but I didn't find anything on construction method. I suppose if I was an expert on knots I could get the exact length, but I'm not. I have an as yet untried idea: I could use turnbuckles as the bobs and adjust the length that way to get it exact. I intend to try that next week when I can go to the hardware store.

In the meantime, I figured someone here may have done the same thing, and can offer advice from their experience. Also, is it easier to get an effective demo out of a larger system? At present, my longest pendulum is ~9 " and every one I saw on YouTube is a lot larger.

The bigger ones make transportation & setup more difficult - the space for the demo is somewhat limited. The small one I built works ok for me, but only ok, not great. But I already know what the thing demonstrates - the kids don't. For 8 to 11 year olds, you want a really obvious demo. And you want them to be able to build one for themselves, with some help.

Thanks, Ed

Reply to
ehsjr
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What's the matter with this:

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Reply to
Fred Bloggs

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The site does not tell me how to achieve equal lengths when I tie the string. As I stated: "Where I have trouble - and the kids will too - is tying the strings such that they end up at the exact proper length. This is not an issue with most of the pendula, but you want the driver and the target as close to each other in length as possible to get resonance, or as close to resonance as possible."

Ed

Reply to
ehsjr

The driver wants to be fairly massive so it stores plenty of energy. If you make it out of a stack of fender washers on a fully threaded eye bolt, between two nuts you can easily adjust the effective length by moving the whole stack up or down the bolt.

If you want an easy way to make fine adjustments, add an extra nut which needs to be a fairly well worn nylock (so it can be turned by fingers) with a gap between it and the washer stack nut, then adjust the nylock up or down to fine tune the center of mass and thus the length. It can be above or below the stack.

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Reply to
IanM

Can 8 to 11 yr olds use super glue, (probably not, they've taken away their dodge balls) Anyway, tie the strings to the the string, then thread a string through a cup, slide the cup to the proper measured position and put a drop of superglue on the cup/string connection. Mikek

Reply to
amdx

Harvards instructions are about as clear as they get.

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"Belmont springs" drinking cups are hard to find on this side of the pond but N identical objects of about the right mass should do. At this time of year identical Xmas baubles would be a nice touch if this is for a Christmas popular science lecture for children.

And you can make the length of the driver adjustable with a bit of cunning. Trapping the thread between a pair of washers on a bolt to adjust its length for instance.

I'd be inclined to carry it a a shoebox full of pendulums and a plank to hook the thing onto at the venue. Allow plenty of time for untangling spagetti and running repairs setting up!

Good luck.

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Regards,
Martin Brown
Reply to
Martin Brown

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Use a little constrictor knot

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, the Q of these little pendulums is Mw/R , w=3Dfreq M=3Dmass R=3D frictional loss, therefore the Q's of the cups is low, hardly any mass and lots of air friction, compared to driving pendulum, so a little bit of off resonance error due to length has negligible effect on the visual.

Reply to
Fred Bloggs

Thanks! You all have posted some great ideas. :-)

Ed

Reply to
ehsjr

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