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It's only Physics...But I'm Confused
Old 07-28-2011, 06:52 PM   #1
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It's only Physics...But I'm Confused

I read the words and saw the video and then read the words again, but I am still stumped as to how this all occurs. (I am assuming that this has not been photo shopped.)

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What it shows: Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce visual traveling waves, standing waves, beating, and random motion. One might call this kinetic art and the choreography of the dance of the pendulums is stunning! Aliasing and quantum revival can also be shown.



- Traveling Waves Harvard Natural Sciences Lecture Demonstrations
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Old 07-28-2011, 07:02 PM   #2
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cool beans!
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Old 07-28-2011, 08:09 PM   #3
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What's "quantum revival", and what does it look like?
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Old 07-28-2011, 08:20 PM   #4
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They all have slightly different, progressively lower frequencies as the string lengths increase. They all start at the same place at the same time. the longer strings lag the shorter strings. Watch the start again and you can see this clearly. We are seeing how patterns emerge as their phases are sometimes close and sometimes not, but always changing.

Dunno if that helps.

"Quantum revival"? A single word from Jimmy Swaggart?

It is pretty.
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Old 07-28-2011, 08:21 PM   #5
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I sure would not want to do the math to 'splain it.

Watching each weight, is neat as well. The speed of the pendulum is strictly a function of the suspension length, given equal weights. They each swing at a specific speed. I'm sure they spent a lot of time figuring how long the suspension strings need to be, to produce the visual effect.

Neat experiment.
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Old 07-28-2011, 08:25 PM   #6
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Originally Posted by mickeyd View Post
(I am assuming that this has not been photo shopped.)
Hard to tell. If you look closely at the longest pendulum, you would see that there is a pause/hesitation before the pattern changes. Probably just my eyes.
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Old 07-28-2011, 08:53 PM   #7
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Thirty years after engineering school, I still have occasional nightmares. In my dream, a professor teaching differential equations is standing at a 40 foot long blackboard, back to the class, speaking in a barely audible monotone...

Pendulum (mathematics) - Wikipedia, the free encyclopedia


Actually, I do remember fondly the pendulum demonstration from freshman physics. It was held in a cavernous lecture hall with exposed ceiling beams. When we arrived for class, there was a bowling ball suspended on a 30 ft. rope, laying at rest just above the lectern at the center of the stage.

Two tall ladders were set up - stage left and stage right. I don't remember all any of the details of the physics, but midway through the lecture the prof climbed up one of the ladders and continued to talk. A short time later, the grad student grabbed a crook of some sort and pulled the bowling ball toward the other ladder.

You can guess the rest...professor makes his key point about conservation of energy or whatever just as the bowling ball is released and begins a looonnng, slow sweep across the stage toward his head. He didn't flinch at all, of course. He just kept right on going into his next point about the motion of the pendulum.

This was early in the semester. After a round or two of this prof's tests, the demonstration might have been too risky due to the possibility of the students moving the ladders or paying off the grad student.
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Old 07-28-2011, 09:59 PM   #8
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If you look at the front longest ball...all the way thru it is simply swinging back and forth. Also ..the balls seem to be attached with 2 strings and it looks like it is possible the ball may be sharing an attachment mechanism with the ball beside it. maybe? or maybe it is just my eyes.
Pretty cool...
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Old 07-28-2011, 11:40 PM   #9
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Also ..the balls seem to be attached with 2 strings and it looks like it is possible the ball may be sharing an attachment mechanism with the ball beside it. maybe? or maybe it is just my eyes.
Pretty cool...
It is cool. Ed_The_Gypsy explained it well. I think they used two strings to keep the ball swinging in a straight path perpendicular to the beam, so they don't swing sideways and hit each other.

Things moving slightly out of time with each other will have all sorts of interesting phase effects. It is similar to the spokes on a wheel in a movie, as the wheel slows down or speeds up, the strobe effect changes from fast to slow to stopping, and all different effects at different multiples of the frame rate.

-ERD50
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Old 07-29-2011, 12:21 AM   #10
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Originally Posted by Htown Harry View Post
Thirty years after engineering school, I still have occasional nightmares. In my dream, a professor teaching differential equations is standing at a 40 foot long blackboard, back to the class, speaking in a barely audible monotone...
I think I had that same professor!

"... Unt den ve do an inferse Fourier transform"...
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Old 07-29-2011, 07:11 AM   #11
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This has nothing to do with physics at all. It is all done with a special, trick pine 1x4 board. Note the flick of the wrist before it all starts.
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Old 07-29-2011, 07:38 AM   #12
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This explains why I'm not an engineer. It gave me a headache.
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Old 07-29-2011, 08:02 AM   #13
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Quantum Revival = new religion of science?
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Old 07-29-2011, 08:13 AM   #14
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In essence, this is no different from Christmas tree lights where there seems to be a pattern chasing up or down the chain. In fact, given 15 sufficiently long and dense strings of LEDs, you could do the same thing in electronics. The physical realisation is cool (and probably took a lot of trial and error), but the mathematics needed to make it work is trivial and can be done in Excel in a few clicks.
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Old 07-29-2011, 08:20 AM   #15
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"...the mathematics needed to make it work is trivial..."
Flashbacks! Flashbacks! Help me. I'm wide awake and now I'm having flashbacks!
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Old 07-29-2011, 09:32 AM   #16
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What's "quantum revival", and what does it look like?
God particles handling snakes?
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Old 07-29-2011, 10:37 AM   #17
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Flashbacks! Flashbacks! Help me. I'm wide awake and now I'm having flashbacks!
Well, since it's trivial, it'll be left as an exercise for the student...
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Old 07-29-2011, 10:43 AM   #18
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Thirty years after engineering school, I still have occasional nightmares. In my dream, a professor teaching differential equations is standing at a 40 foot long blackboard, back to the class, speaking in a barely audible monotone...
<snip>
Post triggered by the word monotone: Some of us formed a little calculus study group. One of the guys said "There is something a little odd about Dr. Temple. I can't quite put my finger on it.", to which everybody agreed. That was the start of "what's the deal with Dr Temple?". After a few more class meetings somebody offered "I believe I have figured it out. Do you notice how he starts at the left side of the board writing with his right hand, and at least every ten minutes or so he glances over his right shoulder to see if anybody is still there?" Yep. "And do you notice that by the end of class he is way over on the right side of the board, writing with his left hand, glancing over his left shoulder?" Yep, That's it!
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Old 07-29-2011, 02:57 PM   #19
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Back in physics class, anytime we didn't understand something (quantum ANYTHING, for instance) we called it "science fiction". One whole physics class went by that term. Every student referred to it in that way instead of its class number.

My favorite (intro) physics demo was a conservation of momentum experiment put on by the prof. (a guy of about 65 or 70 - heck he could have been 40). He would push a heavy cart and do "tricks" with it. The best one was when his assistant would push the cart at him while he ran (well, sort of) toward the oncoming cart. He'd jump onto the cart and then off the other side. The cart would continue as if nothing had happened. Now, I can (more or less) explain it. At the time, it was pretty cool - especially seeing an old guy doing all this physical stuff. He would be panting and puffing by the time the 4 minute demo was through.

Htown Harry, my school nightmares were always about missing a test or being late with an assignment. Probably some psychological explanation could be attached to that. I actually had a "school" dream within the last couple of weeks (almost 45 years after the fact!)
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Old 07-29-2011, 04:27 PM   #20
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Well, since it's trivial, it'll be left as an exercise for the student...
Famous last words.
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