0228: "Resonance"

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0228: "Resonance"

Postby saxmaniac1987 » Mon Feb 26, 2007 5:10 am UTC

http://xkcd.com/c228.html

I definitely am guilty of doing this constantly. It drives the kids who sit in the desk in front of me in classes nuts...
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Postby thefiddler » Mon Feb 26, 2007 5:12 am UTC

One of my friends at the lunch table last semester did this all the time and he sat right next to me. It drove me crazy. :(

But I do it, too, just to annoy people.

...It would be awesome to be able to make people cups fall over and whatnot, though. :D

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Postby spatulated » Mon Feb 26, 2007 5:12 am UTC

me and a friend where trying to find the resonance frequency of the elivator in our dorm. we quickly realized this was a very bad idea
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Postby saxmaniac1987 » Mon Feb 26, 2007 5:14 am UTC

dorm elevators rarely give one a sense of security...
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Postby spatulated » Mon Feb 26, 2007 5:26 am UTC

ours keeps moving a little when the doors are opening and closing, has broken 3 times with sutdent in it this year alone. also it always stops at the 3rd floor and if the doors are stopped from closing at any point you have to force them all the way open manually or they wont do anything
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Postby CaptainJuan » Mon Feb 26, 2007 5:36 am UTC

Good lord. Brilliant. I've always wanted to shake something to pieces. I almost managed to tip over my psych teacher's stool in the front of the room once my senior year, but I got poked in the head and lost my rhythm.
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Postby skeptical scientist » Mon Feb 26, 2007 5:36 am UTC

Aaah, people do this all the time when I'm in class, and it annoys the hell out of me! However, I'll try to remember this comic, so the next time I'm talking about periodic functions in a class I can ask if anyone is having their seat jiggled. :P
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Postby Benevolent Lion » Mon Feb 26, 2007 5:40 am UTC

I remember one time I was really out of it during class, and all of a sudden a large tremor like movement rattled my desk for a good 10 seconds. I was, woah was that an earthquake? Did anybody feel that?

Everyone wondered what the hell was I talking about. I soon learned it was the kid behind me moving his legs
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Postby Gordon » Mon Feb 26, 2007 6:00 am UTC

saxmaniac1987 wrote:dorm elevators rarely give one a sense of security...

qft, I still have nightmares about mine


My first encounter with resonance came at a younge age when me and the guy two seats over realised we could make the desk between us rattle very violently if we matched our rates of banging our legs up and down. I believe this was also my first encounter with getting detention.
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Postby Ronfar » Mon Feb 26, 2007 6:18 am UTC

I do this, and my father complains that it makes his laptop screen shake.
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Postby SpitValve » Mon Feb 26, 2007 6:36 am UTC

It is really hard to change the frequency accurately... you can double or halve it alright but to increase it by just a little bit is quite hard. I guess your leg has a natural resonance :)

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Postby ohki » Mon Feb 26, 2007 9:13 am UTC

500 watt speakers + cement dorm construction + frequency sweep. I'm really glad they never realized it was me :P

the THX sound was always fun too
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Postby Gordon » Mon Feb 26, 2007 9:25 am UTC

haha, I remember the RA's doing their rounds at night "oh those are YOUR speakers we heard on 5 (I lived on 8 )... turn it down"


edit: I didn't live on sunglasses dude, I lived on floor 8
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RealGrouchy wrote:I still remember the time when Gordon left. I still wake up in the middle of the night crying and screaming his name.
I do that too, but for an entirely different reason.
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Gordon wrote:How long have I been asleep?!
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Postby Carl T » Mon Feb 26, 2007 9:30 am UTC

In my experience nothing much happens unless the vibrating person is in contact with the desk/table/thingy in question. Wouldn't the floor have to be horribly unsteady for the phenomenon pictured in the comic to work? (Like in a trailer or a shed or somesuch.)

Oh, and since this is my frits psot here: <handwaving>

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Postby tylerni7 » Mon Feb 26, 2007 1:54 pm UTC

Well hello there Carl T! Welcome to xkcd!

And yea this happens to me all the time. Having enough control over leg tapping would be really sweet. I don't know if there is enough force in the leg to knock something over though, is there? Even if there were two people working at the resonant frequency of something I'm not sure...

Oh and I just got some 500 watt speakers too. I spent a half hour trying to find something that I could get the resonant frequency of and they try to shatter it, but then gave up after about an hour and just blasted music...

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Rattlesnakes

Postby Quake » Mon Feb 26, 2007 1:57 pm UTC

Rattlesnake neighbour ;

It's not hard to imagine how awesome such behaviour isn't to a close neighbour :?
(sort of double negations here).

A semaphoring, Hummer type, close elephant can be easier to stand sometimes. Just don't let it use the elevator to join up. :shock:
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Postby Dustbin » Mon Feb 26, 2007 4:13 pm UTC

I do the leg shaking thing too. Usually not on purpose, but I have managed to find a resonant frequency of something on purpose a couple of times. Also singing bass while sitting in a wooden bench is apparently uncomfortable for those sitting next to you :P

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Postby Mike Graham » Mon Feb 26, 2007 4:57 pm UTC

*papers of dynamics calculations strewn about*

What's the best way to find out the effective stiffness of my officemate's desk?

*ponders dragging it downstairs to set it up in a displacement-controlled actuator.*

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Singing Bass on Benches

Postby TheEldest » Mon Feb 26, 2007 5:00 pm UTC

For holidays, I go to my Grandma's with the rest of my family, and we all go to church together.

My cousins (ages 5-10) all like to sit in the same pew as me because they think it's funny that the pew vibrates when I sing.


I also do the leg tappy thingy. Most of the time, I don't even know that I'm doing it.

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Postby Mathmagic » Mon Feb 26, 2007 5:25 pm UTC

I'm so glad we're doing SHM in physics right now...otherwise I probably wouldn't have got this comic! :D
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Postby eclipse » Mon Feb 26, 2007 5:47 pm UTC

i usually shake my leg as well, but most of the time its me using both legs pretending im playing double kick drumming. i wonder how to figure out the combined frequency?

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Postby Aoeniac » Mon Feb 26, 2007 7:33 pm UTC

I had a nervous leg when I was younger.

In fourth grade there was a tall cabinet in the classroom with potted plants on top of it.

Every time my leg started shaking, the cabinet would start clattering.

Eventually one of the potted plants just up and fell over and rolled off the cabinet. Everybody was startled, and I stopped twitching.
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Postby Purple » Mon Feb 26, 2007 7:59 pm UTC

Remember those movies they showed in in high school Physics of the bridge swaying back and forth because the wind hit it at just the right frequency? That wasn't the wind; I also have the bad habit of bouncing my leg. My bouncing leg is like Bruce Schneier's roundhouse kick.

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Postby Tractor » Mon Feb 26, 2007 8:35 pm UTC

Purple wrote:Remember those movies they showed in in high school Physics of the bridge swaying back and forth because the wind hit it at just the right frequency?


Yes, Tacoma narrows.

And now every time I notice my leg doing the bounce thing, I'm gonna be thinking of this comic...
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Postby william » Mon Feb 26, 2007 9:34 pm UTC

Is it just me or did xkcd start using a new pen in this comic?
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Postby Emmy » Mon Feb 26, 2007 9:41 pm UTC

Looks a lot like a new pen to me. Thinner.

This leg-shaky thing works in the car, too, the driver tends to think there's some kind of fault in the engine.

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Postby SpitValve » Mon Feb 26, 2007 11:04 pm UTC

Mike Graham wrote:*papers of dynamics calculations strewn about*

What's the best way to find out the effective stiffness of my officemate's desk?

*ponders dragging it downstairs to set it up in a displacement-controlled actuator.*


Best way to figure out the resonance is to just hook up a loudspeaker and a frequency generator and play around with it until you can feel the whole thing hum.

They tried this on Mythbusters - a little rod weighing a couple of kgs bouncing back and forth at the right frequency made an entire bridge shake :)

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Postby iw » Tue Feb 27, 2007 12:38 am UTC

My God, I didn't think hardly anyone did this. I actually picked the habit up from copying my father's bad habit (grumble...)

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Postby Toeofdoom » Tue Feb 27, 2007 1:24 am UTC

Bah, that stuff is awesome. I have to make one of those mythbuster contraptions too. Its theoretically possible to destroy buildings with them. Must try...
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Postby Drooling_Sheep » Tue Feb 27, 2007 1:28 am UTC

Restless Leg Syndrome!
You all need treatment. It's not a condition made up to sell prescription drugs...I swear...

http://www.rls.org/NetCommunity/Page.as ... 8&srcid=-2

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Postby tylerni7 » Tue Feb 27, 2007 1:59 am UTC

So today I had a snowday, and I spent all day finding the resonant frequencies of things and then making them shake (besides a crystal glass, nothing worked very well).

It's way too pathetic that a web comic can influence how I spend 8ish hours in a day. I would spend more, but my head is starting to hurt from all the annoying frequencies.

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Postby Aoeniac » Tue Feb 27, 2007 7:28 am UTC

Drooling_Sheep wrote:Restless Leg Syndrome!
You all need treatment. It's not a condition made up to sell prescription drugs...I swear...

http://www.rls.org/NetCommunity/Page.as ... 8&srcid=-2


Restless leg syndrome is when you move your legs because you feel pain or tingling.

I'm sure most of us feel no such sensations and move our legs as a nervous or bored habit.
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Postby Mike Graham » Tue Feb 27, 2007 3:40 pm UTC

SpitValve wrote:
Mike Graham wrote:*papers of dynamics calculations strewn about*

What's the best way to find out the effective stiffness of my officemate's desk?

*ponders dragging it downstairs to set it up in a displacement-controlled actuator.*


Best way to figure out the resonance is to just hook up a loudspeaker and a frequency generator and play around with it until you can feel the whole thing hum.

They tried this on Mythbusters - a little rod weighing a couple of kgs bouncing back and forth at the right frequency made an entire bridge shake :)


I was just trying to be silly, as though I was going to (or had the mad dynamics skillz to) analytically/numerically determine the forcing function required to hit resonance with a desk, like the dude on the left did.

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Postby fjafjan » Tue Feb 27, 2007 4:25 pm UTC

I mostly do this when I am "sitting on my toes" in a certain position, then it's almost uncontrollable.
Otherwise it's not something I do that often, I used to in 9th grade or so, could be quite frustrating for others :D
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Postby Grincement » Tue Feb 27, 2007 4:52 pm UTC

Apparently it's a sign of sexual frustration...

It only happens to me when, like fjafjan, in a certain position...
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Postby Belial » Tue Feb 27, 2007 4:55 pm UTC

Apparently it's a sign of sexual frustration...


I find this hard to believe based on totally anecdotal evidence.

Namely, that I am still doing the leg-shaky/bouncy/whatever thing today.
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Postby LE4dGOLEM » Tue Feb 27, 2007 5:49 pm UTC

Belial wrote:
Apparently it's a sign of sexual frustration...


I find this hard to believe based on totally anecdotal evidence.

Namely, that I am still doing the leg-shaky/bouncy/whatever thing today.


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Postby Belial » Tue Feb 27, 2007 6:27 pm UTC

Hey, I didn't say it disproved anything, just that my own experience causes me to be skeptical.
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Postby elminster » Tue Feb 27, 2007 7:16 pm UTC

I do that with my leg sometimes, it sort of becomes a comfortably numb and uniform sensation. I havnt noticed a link with anything at any one time though.
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Postby genewitch » Tue Feb 27, 2007 7:55 pm UTC

eclipse wrote:i usually shake my leg as well, but most of the time its me using both legs pretending im playing double kick drumming. i wonder how to figure out the combined frequency?


heh, since the frequency is so low you wouldn't have any harmonic frequencies being created.

I got a kick out of this comic, and we here in southern california call this phenomenon "Junk Food Syndrome"

My buddy has this reaaaal bad... whenever he's doing anything that requires concentration it's taptaptaptaptaptaptapSTOPIT.

I know about the resonance sweeps... 1600 watts, here. Whenever i am upset with my landlord i make stuff fall out of cabinets on the other side of this raised foundation house. (do you have a good one as MP3 format? i lost my really good 1-200hz sweep)

Ok it's 1600 watts peak, 1300 before the limiter kicks on to save my subs. :-D

Oh and someone mentioned how hard it would be to change the frequency by a little bit... I write electronic music as a hobby... so i could probably go "i'm tapping at about 120BPM, i'll just switch to 100BPM" since i know how many seconds it takes to do a measure in most BPMs that are divisible by ten from listening to the stuff nonstop for 15 years. :-)


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