that's loud sound. you know how tomorrow's jackhammers are gonna be, gang? tomorrow's jackhammers-- you know the sound they make, yeah? i see people out here in campus last semester doing those things with no earplugs. "yeah, i don't need any earplugs, man. i don't need any"-- you do need earplugs, honey. my cousin's deaf. but the sound, the sound, the sound--shaking. tomorrow's, gang, you know what tomorrow's jackhammers will do? they're developing them right now. tomorrow's jackhammers broadcast the sound to your ears. but exactly out of phase. so when a high pressure hits your ear, you get broadcast a low pressure. and guess what one does to the other? - cancel. - cancels it out. so the guy with a jackhammer tomorrow, you're going down the street and you hear this noise, and you see these two guys like this just chatting with each other. you know what? everything--all the jackhammer noise cuts out,

but everything else doesn't. they don't hear it. ain't that neat? interference of sound. they'll interfere that, and it's nice for the health too. the eardrum is not going like that anymore either. see? it's come to hand. nice, nice. interference. so interference is a property of waves and refraction and what other things? oh, reflection, of course. reflection, like echoes. yeah? light can be reflected. yeah? so all this properties that that we're talking about now, vibrations and waves, will, of course, hold with light or any kind of wave. when we--you see these tuning forks on top these boards, these are called sounding boards. when i hit the fork, the fork vibrates. but the fork is attached to the wood, and it makes the wood vibrate as well. and that makes the sound more intense. and then you can see, when i put it towards you-- the sound will go up. i'll pass it by the microphone in my chest. watch. you can tell-- all hear that more, did you? no, not from your frame of reference, okay? all right, now?

so it's like the megaphone we have in here. the sound is coming out like that, huh? here's a little tune-- here's a little music box. [music] you hear it? mm-hmm. want to hear it better? [music] a little better, gang? you like? what we're doing is we're forcing the wood to vibrate. that's a forced vibration. could you guys remember that? when you force something to vibrate, that's an example of? forced vibration. ain't that neat, huh, gang?

but, sometimes, you can make things vibrate in the direction in which they would-- how do i cut this off? we'll have a little vibrations as we continue, gang. sometimes, you can get the vibrations matched to an object's natural frequency. because it turns out that everything-- everything will vibrate at its own frequency. and once in a while, you can set something vibrating without touching it. and when that happens, we don't call it refraction. guess what we call it, gang? it begins with an r, nonetheless. - resonance. - resonance. let me just show you an example of resonance with a couple of tuning forks. i've got these tuning forks so that they're the same frequency. i think they're 256. yeah, 256, musical note c. yeah? okay. what i'm gonna do is i'm gonna strike one fork, and you listen.

hear that? now, you guys think, "oh, hewitt struck that one too." no, i didn't do it. why don't we try it again? watch. no, no, no. okay? i can resist horsing around. you get the--i strike this from what? either hands. ain't that neat, huh? the wonders of science, yay. all right? we call that resonance, gang. resonance. now, hc. how come this resonate? we can understand resonance if we think small. this is something you can really understand. watch. i'm gonna strike this. this thing is gonna-- flap back and forth, yeah. flap back and forth. isn't it like that ping pong ball, hitting the balls, right? those things like-- back and forth to you. and also we have--over here.

yep. do you believe that little pitter-patters of molecules could bend hard steel? yep. how many say, "oh, no, that could never happen"? come on. i can set this thing vibrating those little molecules, yeah? yep. okay, here we go. i hit this. boom. now, let's look at-- let's slow it down. high pressure hits this prong, yeah? bends it, right? okay. now, what follows the high pressure? lower pressure. so what it does? it swings back to where it started, over shoots over here and right to here. and what happens right then? right then, right then, right then. bam. you see why the timing is important? at that particular instance, the next train comes in. but now, it's already moving. yeah. so it goes a little farther. now, what does this swinging do, gang? the vacuous region, right? the rarefaction. and when it gets right here, right here, right here, right then what happens? one more time.

boom. you see the next one. so it's not how hard you push. it's the rhythm with which you push that counts. do you see that? and if these are both of the same frequency-- you ever hear people talk about, "oh, someone is always on my wavelength"? they're kind of talking about that. see? if you have the same wave, same frequencies, then this one will set this into resonance. a couple of years ago about-- hey, must be five years ago now. i was skating to golden gate park. i roller skate. now, i'm skating through golden gate park and there's a kid-- saturday morning. this kid's sitting on a swing all by himself. and the kid says, "hey, mister, give me a push." i look and i see all the wet, soggy grass between me and him, i gotta clank through all the grass, get my ball bearings all wet and everything. you know, i said, "what you say, kid?" "you heard me. come over here and give me a push." you know how some of the kids act today?

the kid didn't say, "i know you're busy "and i'm busy, one human being to another. "i'd be ever so delighted if you would come over "and take a little time off and give me a little push, interaction between human types, huh, brother?" there was no tone of that in his voice at all. and i said to that little kid, "you know, son, i'll give you a push," and i clang through the grass. and here's the little kid sitting there on a swing like that, right? and he sees with a look of terror in my eye, and the little kid, "oh, don't push me too hard." i said, "i won't push you too hard, you son of a -- --okay." [laughter] so again, it's not how hard you push, it's what? it's the rhythm with which you push. if you push right in rhythm, you can set up a very, very nice amplitude. see? a large amplitude and that's what resonance is all about, pushes at the right time. i used to have an old car. my god, it's a '54 plymouth. and that '54 plymouth had a loose front end. oh. and it turned out at 30 miles an hour, gang,

the frequency of the wheels lobbing at 30 miles an hour happened to correspond to the frequency of the whole front end. - bye-bye. - so you're driving the car 28-- 29-- 30-- and 31 is okay. and so how come the one speed-- how come that one speed? a magic--it's no magic speed. did you guys see it? see why? when you get the frequencies matched. one time, we're walking on a store. i see these glass shelves and all these radios on them. and you know happens is when the notes on the radio had get to a certain pitch, the whole thing would vibrate and go down and the whole thing vibrate. you guys know what's going on? - yeah. - resonance, see? i was matching the frequency of the shelf 'cause everything will vibrate at some frequency. and when that frequency is matched, you can get things to resonate. this happen tragically some time ago. it was on dallas, texas, some big hotel. all the people on the shop are going, "yay. yay. yay," and all bouncing up and down. and guess what's the resonant frequency of the structure was? yay, yay, yay. down it comes. yeah. yeah.

you know when people cross bridges, military types, going across the bridge? before they hit the bridge, the honchos going, "hop, hop, hep, hop, hut, 2, 3, 4" or "step, step, step, step--" but when they get to the bridge, they--break, stop. and then--like that. and everyone crosses the bridge out of step. why? 'cause the bridge-- what if the resonant frequency of the bridge is--hut, 2, 3, 4. you know what i'm saying? in fact, this happened in manchester, england some years ago. all the troops are following the honcho and the honchos is counting-- so he goes across-- and he's ahead and he keeps doing-- he's on the other side of that bridge yelling-- and the people are-- wipe out city. you guys know about the golden gate bridge in new york, i guess.

how the golden gate bridge is-- get the same resonance. yeah, they really screwed up in this, man. talk about the disaster of the earthquakes. the golden gate--i mean, the george washington bridge, george washington. [laughter] you guys been known the george washington bridge has a resonant frequency that's equal to that of cat's trot? you guys know about that. cat, yeah. medium-sized cat. who's from new york city? they can back me up on this. you're gonna cross the-- walk across the george washington bridge. they got a little cat guard, little sign, "no cats." now, a lot of people think that's cute and that's a joke. but come on, you're physics type. we know what, right? you know i do. you know how a cat runs, by the way. you got a cat running. 20 minutes later. beautiful timing. how about a dog? but the cats, honey. guess what they found out that the natural of frequency of the george washington bridge is?

so what would happen if you let a cat run across that bridge? you know, you guys, you notice when i hit this that these air molecules bent the steel. you had evidence of that. you heard it. right? so oh, well, maybe little air molecules can bend steel but not a cat. [laughter] how about it, gang? you know what happened to that cat when it crossed that bridge. in world war ii, that's why they had-- they're up there all the time looking for cats. they might be-- might be some spy. little, little cat going-- take our whole bridge down, man. can't they do it--or something? oh, no, there's no way to protect against the cat's fall. no. we're at the victim of cats. i'm not gonna-- i think, you know--i woke up on a dream recently. i woke up on a dream. and i dreamed that i was gonna cross that bridge and i had, you know, that stones on the radio and everything, and everything going. and all the sudden, i looked down on my dream

and i saw cat running right beside me. i mean, talk about waking up on a cold sweat. could you imagine? what could be a more horrible thing to be right in the middle of the george washington bridge and seeing a cat running beside you, right? i'm talking about those calico jobs. the three-calico jobs. you know what i'm talking? okay. bring me that whole bridge right down. would you like to see a movie of a cat that brings the bridge down? yes. some of you guys thinking, "oh, you're horsing around. you're not being serious." let me ask you a question. am i a type of person who would not be serious with you? yeah. [laughter] would you like to see a movie of the catastrophic collapse of such a bridge by a cat? yes. nine-pound, calico-colored cat. okay. well, let's see that movie right now. here's the similar bridge in the state of construction. and that-- it's the tacoma narrow bridge in the state of washington. well, look at the size of those girders. very strong steel. and here we are on opening day. what were you guys doing in 1940?

and here we have dignitaries coming across the bridge, opening day. look at the strength of that bridge. big steel girdles, concrete pavement, a mighty bridge, a very strong bridge. but one thing, very elastic. and here you see some of the evidence of that elasticity. turns out the bridge undulated. we called it galloping gertie. especially in the wind, it would kind of a move around a little bit. look at these vibrations, evidence of the elasticity of steel and even the concrete pavement. people coming over the bridge, some were reluctant to do that and they took alternate routes. galloping gertie. whoop, let's look at this. look at this. in a slight gale one day-- whoop, i shouldn't say gale.

when the cat was released on this bridge one day, look what happens here. there is an evidence of resonance. see those bushes moving around a little bit? a little bit of wind there, but let's stick to our cat theme. and there it goes. look at that. this is in real time, gang, scout's honor. this is not time lapse so speed it up. these are the real vibrations, real time. that bridge is resonating. and what's the cause of that resonance? that calico-colored cat. hmm, hmm, hmm. they sent to follow out there to kick the cat off the bridge, but the animal rights activist said, "no, let the cat "do its own thing. every creature has a right to do its own thing." uh, uh, uh. you look very carefully.

in the middle of the picture, you can see the cat. cat's purring, making its paws go up and down, generating this terrible, terrible event. turns out this bridge was inadvertently uninsured. turns out the insurance premiums were unpaid by a city official, and that turned out to be a disaster financially for the city as we'll see very soon. you really believe that a cat's generating this, gang? there's a catch. you can see it on the side there. a little close-up view, but better. take a look at your book. maybe the truth is in there. oh, there it goes. there it goes. disaster, disaster. elastic things have their breaking point. and there we see the collapse of the bridge itself. [music]

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