Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

Kelistrikan dan Kemagnetan

Topics covered:

Resonance
Destructive Resonance
Electromagnetic Waves
Speed of Light
Radio - TV
Distance Determinations using Radar and Lasers

Instructor/speaker: Prof. Walter Lewin

Information about the Tacoma Narrows Bridge Collapse:
http://www.pbs.org/wgbh/nova/bridge/tacoma3.html
http://www.ketchum.org/bridgecollapse.html

Free Downloads

Video

• iTunes U (MP4 - 105MB)
• Internet Archive (MP4 - 208MB)

  » Download this transcript (PDF)

  Before we're going to dive into electromagnetic waves, I would like to discuss a few more mechanical resonances with you.

  Last Friday, we discussed the resonances of string instruments and wind instruments.

  But there are several that you see around you quite often -- without realizing it, perhaps -- that you're looking at a resonance frequency.

  You may have noticed that traffic signs have the tendency, sometimes, to do this, and at certain wind speeds, they go like this.

  Enormously strong amplitude, that's a form of resonance.

  Undoubtedly, you have been motels or at homes where you open a faucet, and then all of a sudden, when the water's running in a certain way, you hear an incredible noise, a terrible noise.

  You close the faucet a little, or you open it a little further, and that noise goes away.

  That's clearly an example of resonance.

  You drive your car, or you're in someone else's car, and at a certain speed, something begins to rattle.

  Very annoying.

  You go a little faster, it stops.

  You go a little slower, it stops.

  Or, if you go a little faster, something else begins to rattle, there's some other resonance of something else in the car.

  And of course, there are cars whereby something rattles at any speed.

  But in any case, there's this idea, then, of resonance, which is all around us.

  I remember when I was in a student, and when we had an after-dinner speaker which we didn't like, we would very quickly empty our wine glasses -- in those days, we were still allowed to drink, by the way -- and what we would do is the following, something extremely annoying.

  We would generate the fundamental of our wine glasses.

  You take your finger, you make it wet, and you rub it like this.

  Listen.

  [Rubs glass]

  Believe me, if 100 students do that, it's very annoying.

  But it's also extremely effective.

  Speaker -- speaker gets the message very quickly.

  [Rubs glass].

  What the glass is doing, it's the fundamental of the glass, it's the lowest frequency, the glass is actually doing this.

  And there are rumors that people can break glasses by singing.

  And we'll talk about that in a minute.

  Um, I remember a, um, commercial, Memorex.

  Memorex is an audio tape.

  And they bragged about breaking glasses -- some of you may actually have seen that commercial.

  There was a, uh, a picture that I can show you that goes with the commercial, and then a very dramatic story.

  The story is that someone goes to a concert.

  And there is a woman singer, puts a glass on the table, raises her voice, hits the resonance frequency of the glass, [pshew!], and there goes the glass.

  And this gentleman was recording it, of course, on his Memorex tape.

  So let's, um, see this, uh, this slide.

  So if we get the slide -- yes! You see this, um, this glass, maybe you can focus a little better John, thank you.

  Memorex.

  So the story then goes that the guy goes home and tells his wife about this.

  Well, she is smart enough not to believe this story.

  But he plays back his tape.

  And at the moment that this glass breaks at the concert, he has some wineglasses himself at home, and lo and behold, they also break.

  And so then the idea is, that is the commercial -- that's the great pitch of Memorex -- that the reason why they break at home is because of the enormous quality of this tape which is made of very special material.

  And the material, as you could have read on the box, is a very special chemical compound, it is MRX2.

  Two atoms of X, one of R, and one of M, and then you make it oxide, and then you have the best tape that you can imagine in the world.

  Well, they overlook a small detail, and that is that, um, for one thing, a tape recorder would never generate enough volume to break a glass in the first place.

  But in the second place, the glasses that this guy had at home, obviously didn't have exactly the same resonance frequency as the glass at the concert.

  So this could never have happened.

  But like with all commercials, you know that you're being swindled, and this, of course, no exception.

  I've always questioned whether it is actually possible that a person, without the aid of strong amplification, and without the aid of huge sound volumes which you can generate with loudspeakers, whether you can actually break a glass.

  I've always wondered about that.

  People say it can be done.

  Caruso, famous singer, was known for being able to do that.

  He put the glass there, he would rub it with his finger so that he knew the resonance frequency [kllk], and there he would go, and [poit] bingo.

  Frankly speaking, I don't believe it.

  I don't believe it can be done by a human being without the aid of amplifiers and speakers.

  And when I lectured 8.01 several years ago, together with Professor Feld here at MIT, we discussed the -- the possibility of designing something that actually would be able to break a glass.

  And -- and he actually deserved a lot of credit for that, he worked with a graduate student, and he managed to design a setup that works most of the time.

  But don't put your hopes too high, it doesn't work all the time.

  So here is a wineglass, the same series as that one.

  By the time -- when -- when he got it to work, we bought 500 of those glasses -- we got a good discount, by the way, because we wanted to be sure that we can do it for years to come.

  So here's the wine glass, and here is the loudspeaker, and we are going to generate sound very close to the resonance frequency of this glass, which we have already determined before you came in, 488 Hertz.

  You're going to see the glass there, and to make you see, actually, this wonderful motion of the glass, we will strobe it with light at a frequency slightly different from the frequency of the sound so you see the glass move very slowly.

  And then we will increase the volume of the speaker, and then with some luck, if we are right on resonance, [poit], the glass may actually break.

  I think this is the sound that you're going to hear at low volume.

  [tone].

  And I think I turned on the, um, the strobe light now.

  [tone] So I'm going to go make it dark.

  [tone] And I want to warn you that the sound level is going to be quite high.

  [tone] I will have to protect my ears, [tone] and you actually may have to do the same.

  [tone] I will first increase the volume of the sound to see whether I'm close enough to resonance.

  [tone] So this slow motion that you see [tone] is the result [tone] of the strobe, [tone] which is not exactly at the same frequency as the glass.

  I can change that a little.

  [tone] All right.

  So we are very close to resonance.

  [tone] The glass is clearly responding to the sound, [tone] and now I will [tone] cover my ears [tone] and slowly increase the [tone] sound volume.

  [tone] I can't go any louder.

  [tone].

  It's tough glass.

  [tone] [glass breaking] [tone] It was a tough glass.

  [applause].

  I think you will probably agree with me now that for a person to do that without electronic help is just not so believable.

  The most dramatic example of destructive resonance is the collapse of the bridge in Tacoma in 1940.

  Many of you may have seen that dramatic movie, but some of you may not have seen it.

  And even if you have seen it, it's worth seeing it again.

  There's a little bit of wind, there's a little bit more wind, and just like with these wind instruments, you're dumping a whole spectrum of frequencies onto a wind instrument, and it picks out the resonance frequency.

  And this bridge, as you're going to see, picks out its own resonance frequencies.

  And the consequences are quite dramatic.

  So if you can start, Marcos, with this movie.

  [sniffles].

  It was 1940, and at this, uh, in Washington State.

  Movie: On the First of July, 1940, a delegation of citizens met in Washington State.

  Movie: The weather was beautiful, the occasion historic, and the speech-making and fanfare altogether appropriate.

  Movie: This was the grand opening of the Tacoma Narrows Bridge.

  Movie: From the beginning, the bridge, which spanned Puget Sound between Seattle and Tacoma, was traveled in style.

  Movie: As well it should have been.

  The Tacoma Narrows Bridge was one of the longer suspension bridges on Earth.

  Movie: And, if somebody hadn't overlooked something, it probably would have remained one of the longer suspension bridges on Earth.

  Movie: The problem wasn't that, right from the beginning, a lot of people didn't pay a lot of attention to details.

  They did.

  Movie: But somewhere along the line -- and this was obvious in the end -- it looks as if someone forgot -- Look at those cables over -- the significance Movie: of resonance.

  Movie: Among other things, the Tacoma Narrows Bridge was the most spectacular Aeolian harp in history.

  Movie: Unfortunately, its first performance was destined to run only about four months.

  Movie: In the meantime, she was a beautiful bridge.

  Movie: Beautiful, but a little strange.

  Movie: Even before construction was completed, people observed its peculiar behavior.

  Movie: That was because, even in a light breeze, ripples ran along the bridge.

  After a while, one of the local humorists called her Galloping Gertie.

  Movie: And for fairly obvious reasons, the name stuck, at least until the seventh of November, 1940.

  Movie: Then as now, Seattle and Tacoma were sports-minded cities.

  For four months, a regional sport was to drive across the bridge on a windy day.

  Movie: While some claimed it was like riding a roller coaster, others found it a little disconcerting to see the car in front disappear.

  [laughter] Movie: How popular this bridge sport was, or to what extent it might have spread across the country, is anybody's guess.

  Movie: On November seventh, 1940, the winds were relatively moderate, about 40 miles per hour.

  Movie: A mew mode appeared.

  Rather than ripple, the bridge began to twist.

  Movie: A wind of 40 miles per hour is not too strong, but it was strong enough to start the bridge twisting violently.

  I contacted the physics teacher of the local high school, and we'll see him very shortly.

  Thought he might be able to fix it.

  There he comes.

  I've known his son, who told me that it was his father.

  No other example of re- destructive resonance is more impressive than this one.

  All right.

Pengembangan Perkuliahan

1. Buatlah sebuah Esai mengenai materi perkuliahan ini

2. Buatlah sebuah kelompok berjumlah 5 orang untuk menganalisis materi perkuliahan ini

3. Lakukan Penelitian Sederhana dengan kelompok tersebut

4. Hasilkan sebuah produk yang dapat digunakan oleh masyarakat

5. Kembangkan produk tersebut dengan senantiasa meningkatkan kualitasnya

Ucapan Terima Kasih Kepada:

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

(http://web.mit.edu/physics/people/faculty/lewin_walter.html)

b. Prof. Bernd Surrow, Ph.D.

(http://web.mit.edu/physics/people/faculty/surrow_bernd.html)

Staff

Visualizations:
Prof. John Belcher

Instructors:
Dr. Peter Dourmashkin
Prof. Bruce Knuteson
Prof. Gunther Roland
Prof. Bolek Wyslouch
Dr. Brian Wecht
Prof. Eric Katsavounidis
Prof. Robert Simcoe
Prof. Joseph Formaggio

Course Co-Administrators:
Dr. Peter Dourmashkin
Prof. Robert Redwine

Technical Instructors:
Andy Neely
Matthew Strafuss

Course Material:
Dr. Peter Dourmashkin
Prof. Eric Hudson
Dr. Sen-Ben Liao

Acknowledgements

The TEAL project is supported by The Alex and Brit d'Arbeloff Fund for Excellence in MIT Education, MIT iCampus, the Davis Educational Foundation, the National Science Foundation, the Class of 1960 Endowment for Innovation in Education, the Class of 1951 Fund for Excellence in Education, the Class of 1955 Fund for Excellence in Teaching, and the Helena Foundation. Many people have contributed to the development of the course materials. (PDF)

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.