Lesson 34 - Resonance - Forced Vibrations - Demonstrations in Physics

how do you do ladies and gentlemen and boys and girls and teachers I'm Julia Sumner Miller in physics is my business and my business today making more noise I suppose our sound or somesuch more exactly this business of resonance and forced vibrations consider the following which I tried on an earlier lesson to do but with not too much success for some reason about this I should say more you know already my philosophy experiments demonstrations never fail for nature to do what you want her to do you must make some very special meet some very special requirements if they are not met absolutely precisely Nietzsche will not show what you want her to show she would do something else so if the experiment did not succeed in an earlier case we'll try it again but we are forced to think of why didn't it work before two forks middle C middle C we hope they're identical hollow chambers resonating chambers I'm going to strike this one quiet it and we will hear that one why this emits 256 pulses per second which are fed into this one this picks them up and is set into oscillation listen now there it is clear as the nose on my face I'm going to do it again in the reverse order listen there it is now how far apart could this be made to happen all great distances because this is exactly what your ear does and you could hear me at some great distance I am sure resonance resonance consider now the following I have here two cardboard tube one of which slips inside the other so that I can change the total effective length I am going to strike a tuning fork and hold it in front of this open pipe which is so long which has a certain natural frequency I'm going to change the length by this sliding tube and hope to find a resonant length for this fork listen now yeah there is a place there is a place at which this length is resonant to this frequency that's an open pipe this end is open that's open here is a closed one now it would be an interesting exercise to compare the lengths of open and closed pipes for the same fork but I leave that as an exercise for you this is closed because it is closed on the bottom at the table listen uh huh let me use another fork this wok happens to be what five twelve that's an octave above middle C oh I can't get a length that there it is there is somewhere in there let me try this open one very clearly resident very clearly resonant or as I might do it consider here here is a glass cylinder with some water in it and a lucite tube and this is a closed end so the air column here constitutes a closed tube let me show you resonance there it is now what can one do with this by making certain measurements we can determine the velocity of sound in air here is another tube another fork oh very clearly resonant I hope if the tube is long enough to find another resonant length listen yeah pretty nearly I couldn't quite make it with the lengths available to me resonance resonance I want to show it to you in a more delightful fashion here is something really enchanting a wooden frame which is hollow so like the belly of a piano or of a fiddle a tightly drawn string with two bridges one here and one here so that I have cut off a certain length of string this long the string therefore has such a length is under such attention and is so fat now that string has a certain frequency listen if therefore has a certain pitch it is resonant to a certain fork proof I've put a little paper writer on there because if the string is vibrating you can't see it and I'm going to strike a fork and apply the end of the fork to one of the bridges and we hope that the energy of the vibrating fork is communicated through its stem to the string and if the string is vibrating we would hope to see the little flag there quiver watch it now oh yeah I like that let's get tight on that again boys because I like that and do you see people if I like something well I might do it again because I like it and that's good enough reason Macchio I suppose I have done this demonstration four thousand seven hundred and eighty two times and it never fails to stir my soul because it is pretty and dramatic and spirited watch it now watch now there it is now the question is will this string respond to some other fork that would happen to be B 480 let me try 384 which is G listen watch no no that string can vibrate 480 times per second in a natural ordinary comfortable happy way it will not vibrate 384 times per second how about for 26.6 oh yeah that reminds me I find a fork on an earlier lesson I suggested that I said that a in America the orchestral a was 440 vibrations per second and then I said that I thought that a in Europe was 426 I find it for twenty six point six now listen watch no no but 480 yes there it is and so we have that kind of behavior of vibrating strings consider now the following let me get this out of the way a fork I strike it I deliver the the stem of the fork to the table what's happening the table is being put into oscillation at the same frequency as the fork which is for 26.6 but the music it emits the sound that emerges has a little different quality to the ear sure a when played on the piano sounds different from a played on the fiddle so this has a different quality but the table is forced into vibration that many times per second let's take 480 listen let me try this table back here which hasn't has more stuff on it may have higher or lower a pitch I don't know very interesting now something quite enchanting consider the human voice which results from the vibration of the vocal cords the hollow cavity called the mouth where the tongue is where the teeth are how the lips a hole held that chamber the vocal chamber has a resonant frequency and I have observed over the years that when I try resonance at the open mouth of boys the response is to a fork of lower frequency than that for girls let me show you what I mean listen listen well let me try another fork uh-huh uh-huh and and sure enough this is for 26 and that's for 80 let me try one lower 384 maybe I'm boom boom boom in pitch huh let me try a higher one 512 notice I have a strange local system resonance force vibration in certain places in the country in New England there are some wooden bridges still with signs for Nath that say no trotting of horses on this bridge no trotting why the periodic delivery of forces from the horse's hoofs might set the bridge into vibration and render it apart or consider soldiers in marching crossing a bridge they break up their stride now where do we have some ancient wonderful commentary of the same sort if you read Joshua in the Old Testament Joshua 6:20 it says something like this the priests blew their trumpets and the walls fell down flat and I've often wondered why the sackers of cities didn't just go around the world the then-known world with bands and musicians playing music in order to to fell the walls of the cities instead of attacking them with manpower all right resonance forced vibrations consider this which is quite enchanting to look at here I represent the towers that carry the big heavy cables in transmission of electric current across the country this is a cable it might be viewed as hanging like this the shape in which it hangs is very special incidentally it's called a catenary and if you look up what the word catenary in a in a in a dictionary you will find that means chain now when the wind is very high this cable could be set into oscillation which is dangerous for the mechanical system as well as for the electrical Affairs problem we need to damp out that mechanical motion and so what is done if you look closely next time you see these transmission lines you will observe that there hangs that there hangs that there hang on the wires some system that looks like this called a damper now what happens when that cable is set into vast oscillation with large amplitude by a stout wind the cable having flexibility as I will show you takes up the oscillation and damps out the mechanical motion of the wire here it is here here is a cable cable dampe the wire goes through there and now you see these are flexible and this takes up the energy of the oscillating wire so I say this business of residents forced vibrations mechanical damping and the like very enchanting to contemplate and I would invite you to think more about it so more on resonance more on forced vibrations Macchio Macchio all this evidence that the natural world presents to us is gotten by one of our senses namely the auditory and all governed by the reception that the eardrum gives to the variations in pulses and I thank you for watching you

Channel: Professor Julius Sumner Miller

Views: 425,814

Keywords: science, julius, sumner, miller, dramatic, demonstrations, physics, professor, Demonstrations In Physics (TV Program)

Id: C-Bn70PpbrM

Length: 14min 33sec (873 seconds)

Published: Mon Feb 03 2014

Reddit Comments

[u/Daoist_Pure_Light]

Love this video :)

Everyone's going to have their own perspective coming from their own individual frame of reality and take away their own ideas that resonates with them according to their own "frequency" in life.

It's sort of like the original rorschach test (life) that gives the energetic signal on what people are aligning with and reminds me of the discordian tale about the two guys at the beach:

One man looks down into the sandy grains of a beach and pulls out a single grain becoming enlightened on the immensity of scale between himself and the universe.

The second guy with him picks up a sea weed and puts it on top of his head saying, "hey, look at me, I'm seaweed man!"