OT - Primer of Simple Harmonic Motion

No metal content, but a pretty good refresher on why things start shaking with slight impulses, and then keep on doing so...

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Happy New Year,

Jeff

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

Reply to
Jeff Wisnia
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It rings a bell ;)

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs

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Ontario

Reply to
Boris Mohar

Thanks for sharing, Jeff. ('tho I suspect the objects in question are having additional forces applied, hence no longer "simple" harmonic motion.)

Nice demo.

(another) Jeff

Reply to
A.Gent

Well, I went to that link and immediately thought of the physics lab at the Univ of Oregon, where my son works. He recently took me on a behind-the scenes tour and showed me the demo with the highest "simplicity versus coolness factor" that they have. It's just a chunk of metal shaped like a yoke from a car u-joint (like a small pipe cross except solid metal) hanging from a spring. Pull it straight down, let go and it bounces up and down, up and down... and pretty soon the "yoke" begins to rotate back and forth a little while bouncing up and down, then a little more, and soon the vertical motion completely stops, and this little yoke is winding up and unwinding that spring like the mechanism in an old clock, then the vertical motion begins, and gets bigger and the spinning slows... It was entrancing, and I thought it was the best ever exhibit!

Until today... boy could they wake up the sleepyheads in physics 101 with that demo you linked to!

Rex lurkin when I shoulda been a-workin

Reply to
Rex

OK, any references to that experimental item that we can go to? Sounds so neat that I want to make my own.

Reply to
The Tagge's

Me too!

Here's another one: on my car ('95 Ford Contour), if I pluck the radio aerial, it swings back and forth in a plane at first, then the end starts describing an elliptical motion until it's eventually vibrating in a plane perpendicular to the one it started in, then the process reverses.

I *think* what's going on is that the aerial might be slightly elliptical so it has slightly different resonance frequencies in the different directions,

Gotta find a spring and try that yoke trick.......

Reply to
Don Foreman

Nope. Coupled resonances. Gauranteed to happen. Don't know if you saved it but we had that discussion back in May '02. I set it up in APL and sent you the graphs. I'll send it agian if you like. If you want to duplicate it in MathCAD (that's the one you have isn't it) just set up a Runge-Kutta solution for two identical SHM's and run it with initial conditions to start one but not the other. This is to convince you that there is no coupling yet. Now add a _small_ amount of the displacement of each to the other and plot again. You will see the described phenomenum.

Then when you make the physical one, you'll know how to explain it! :-)

Ted

Reply to
Ted Edwards

My favorite example is coupled pendulums. A pendulum exhibits approximately SHO behavior.

Take a string and stretch it horizontally taught. Suspend two weights from strings attached to the first string making two pendulums. Start one pendulum swinging and watch the energy transfer to the other pendulum and back. The pendulum that starts swinging will be a quarter cycle ahead of the other pendulum. When the first pendulum comes to rest, it pauses to let the other pendulum get a quarter cycle ahead.

Bob S

Reply to
Bob Summers

I haven't found a web reference yet for the oscillating yoke/cross, but I'll have my son take pictures, then I'll post them in the drop box after MLK day.

Or, were you referr>OK, any references to that experimental item that we can go to?

Reply to
Rex

Coupled pendulums are nice, and so are chaotic pendulums. Here are directions for a small chaotic pendulum; scale up as desired.

From .5"x.2" aluminum bar, cut a 4" piece and a 6.5" piece. At A and B (see diagram) drill holes for press fit of .250"-diameter roller bearing; at C, drill for press fit of a shaft. A B C --------------------------- -------------- |o o| |o | --------------------------- -------------- Make an 11"-high stand and attach a shaft through bearing at A. Attach a shaft from C through bearing B.

Now give the coupled bar a push. As the long bar spins around A and the short bar around B, from time to time the short bar will do strange flips, reverse direction, start and stop, etc.

The bearings I used came out of old disk drives. They have short stub shafts tightly pressed into the inner race, and work ok for this thing without modification.

-jiw

Reply to
James Waldby

Yup, I do recall the discussion and the Runge-Kutta sim.

Reply to
Don Foreman

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