Lesson 13 - Archimedes' Principle - Demonstrations in Physics

how do you do ladies and gentlemen and boys and girls I'm Julia Sumner Miller and physics is my business and our business today is the classic adventure of Archimedes about the 3rd century BC with some wondrous things to report first consider a body a lead weight which I hang on a spring scale the scale reads so much I am NOT interested in what it reads but it reads the weight of the body now let me submerge this body in a vessel of water and we see a remarkable thing a very remarkable thing as I submerge it the scale reads less and less and less and I urge you to see that the weight is not resting on the bottom of the vessel of water accordingly when I submerge the body the water pushed up on it provided a buoyant force as we say which Archimedes discovered in this classical bathtub experiment Eureka Eureka I have found it now how much is the push-up of the water on a body submerged Archimedes said it this way when a body is submerged in a fluid that's either a gas or a liquid we're talking liquids here when a body is submerged in the liquid it is buoyed up by a force equal to the weight of the liquid displaced now I can prove that here I have a vessel which has an overflow spout and I'm going to put a little water in it to be sure the water is up to the level of the spout there it is it's coming out there somewhat and now I'll wait until it stops the water is up to the level of the spout in this chamber I'm going to get rid of that water now what am I going to do I'm going to submerge this body in that chamber of water and as anyone in his right mind would recognize and attend we'll not some water be pushed out indeed and I'm going to push some water out and catch it in that little bucket watch it now and of course again the scale would read less but I'm not interested in that this time I'm interested in the water that gets pushed out watch it now and I'll wait till all its gonna be pushed out is pushed out that's good enough now what would I do with these adventures well first I would have noticed how much less the scale read when the body was submerged in the liquid suppose it read less by 10 grams then I weigh this water in this bucket and what would the water in this bucket weigh 10 grams meaning that the body was buoyed up by a force equal to the weight of the water this body displaced Archimedes principle fantastic now I said that the water pushes up I'm going to prove that indeed you should recognize it already from your adventures by the sea when you walk into the ocean do you not discover that the deeper you go the less push your feet have on the earth below because the water is buying you up more and more the deeper submerged your body becomes here I have a glass and a hole in the very bottom and I'm going to push the glass down the evidence that the water pushes up will be witnessed by the water coming up into that hole watch it there it is there it is and I say the water pushes up I'm going to prove that in another way here is a glass tube and a plate on a string and I'm going to put the string through this glass tube and now I'm holding the string in hand and obviously since the plate has weight if I let go of the string the plate will fall down watch it well of course as anyone who's watching doesn't that Professor know that if you let's go of the plate it will fall out course it'll fall down now I'm going to prove again that the water pushes up Wow I'm going to put this into the water and instantly I make connection with the water I'm going to let go of the string when I let go of the string out here the plate fell off the plate will not fall off in there watch it there it is and you see water rushing into the inner cylinder climbing higher and higher and everybody says well and when it gets the same height inside the vessel as outside the vessel then the plate will fall off and I'm going to leave that for you to think about but now my new adventure Archimedes bucket and cylinder I have here a little bucket very nicely formed and a solid cylinder which fits it exactly which fits it exactly and here's what I'm going to do I'm going to balance this bucket I'm going to balance that bucket with the cylinder below let me see if I can represent that here is the cylinder below and remember that solid cylinder fits that bucket exactly and I'm going to balance this with some weights over here put the beam balance in equilibrium then what am I going to do I'm going to come up underneath with a vessel of water in this fashion vessel of water so that I submerge the solid cylinder the equilibrium in the balance will be destroyed as we might say because why the water here is pushing the cylinder up how much is it pushing the cylinder up it is pushing the cylinder up with a force equal to the weight of that water displaced and how much water is displaced the volume of the cylinder and how much is the volume of the cylinder the volume is a cup accordingly with the equilibrium destroyed because there is a lift of buoyant force Here I am now going to pour some on it into this little bucket water water water water water water water oho when this bucket gets full what will we see an absolutely enchanting thing we will see the equilibrium restored now I have such a bucket in cylinder there's the cylinder there's the bucket and I have put it in equilibrium on the scale now I'm going to bring up this water and submerge the lower solid cylinder I hope you I hope you see that the equilibrium has been destroyed the balance is no longer in balance oh I came up too high now I am going to fill the little bucket and having a little trouble here being steady I'm going to fill the little bucket and the equilibrium which does not now exist in the scale reading will be restored watch me now oh I filled it over much in this a very high meniscus so Archimedes bucket and cylinder the proof is very beautiful a body is buoyed up by a force equal to the weight of the water displaced now regarding buoyant forces and bodies that sink and bodies that float here is a vessel of water and here's a piece of wood floats here is another piece of wood wood wood water huh heavier than water lignum vitae here is a piece of wood heavier than water so there are woods that sink in woods that float their densities being less or more than that of water now regarding body submerged in liquids I have here something absolutely enchanting here is a vessel of water in which I'm going to put a wooden ball there it sinks about oh I don't know five eighths or something like that now here is a vessel of mercury that's not mercury I'm going to the ball in the vessel of mercury watch it it doesn't sink it all hardly hardly it rides on the very surface but that's wood very light how about steel oho you know that the specific gravity of steel is about seven or eight the specific gravity of water is one you know that steel sinks in water watch it sinks to the bottom by the way I say steel sinks in water on an earlier program on surface tension in the strange behavior of the free surfaces of liquids I showed in a demonstration where I float a steel needle on water so steel can be made to float on water but how about the steel ball in the mercury watch it oh this is something there it is there it is the steel floating on the mercury this is a marvelous thing let me use a larger steel sphere because this is enchanting to see there it is still floating in mercury now talking about Archimedes leads me to show you a classic mosaic you remember that Archimedes defended the city of Syracuse when it was attacked by the Romans under the general Marcellus and then Archimedes was put to death by a Roman soldier who ran upon him while he was doing a demonstration in the sand so here is that mosaic which I hope you will see there is the Roman soldier urging Archimedes come to my general Marcellus and Archimedes beg him let me finish first my demonstration and here is another monument to Archimedes which shows him holding a mirror this is a little glossy and hard to see holding a mirror with which you remember he set fire to the enemy ships far from shore so what have we discovered here remarkable principle of Archimedes that a body submerged in a liquid is buoyed up by a force equal to the weight of the liquid displaced now Archimedes as you know apart from being a natural philosopher of the first magnitude was also a mathematician and I want to tell you quickly about some things he discovered of a mathematical nature because they are enchanting and here they are here I have a right circular cylinder and here I have a right circular cone and here I have a sphere they are the same altitude and the same radius and what did Archimedes discover he discovered by playing with these filling them now in turn with sand that the volumes were uniquely related in the ratio of 1 to 2 to 3 I'll leave it to you to discover how that ratio goes with respect to the bodies now in his later years having made many discoveries of great moment and significance he was asked what he viewed as his greatest and he referred to this the ratio of the volumes of the cylinder to the cone to the sphere and asked that when he dies such a matter be put atop his tomb which shows a cylinder correction a sphere circumscribed by a cylinder so here is a sphere about which his circumscribed a cylinder and this was put on top of Archimedes tomb buried near Syracuse where are we then in this dialogue Archimedes 3rd century BC whom are certainly ranked with Newton of the 17 and Einstein of the 20th so next time you go to the sea to bathe remember Archimedes and when indeed you take a bath in your bathtub and you submerge yourself in the water and the water rises remember the affairs here which revealed that beautiful fact which we use abundantly because after all this is why ships can sail on the sea and submarines under the sea and airplanes above the sea and I thank you for your attention you

Channel: Professor Julius Sumner Miller

Views: 268,624

Rating: 4.9311662 out of 5

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

Id: uIbX4TSguTI

Length: 14min 25sec (865 seconds)

Published: Mon Feb 03 2014

Reddit Comments

[u/[deleted]]

Another good JSM video: https://www.youtube.com/watch?v=LIfxV7uNZXQ -- he's actually getting pretty damn hardcore with the science in this video

Also, from my own childhood, The Curiosity Show: https://www.youtube.com/watch?v=VROfEsWz6WY

Sadly, the days of science education shows for kids presented by daggy old men are gone.

[u/earwig20]

This isn't Dr. Karl

[u/come-on-now-please]

HOLY SHIT!! I remember watching this series as a kid on PBS using bunny ears! I never remembered what the show was called though so I could never google it to find out more, you just solved like a 20 year old mystery for me dude! Thank you so much!

[u/jasohemm]

Julius Sumner Miller ftw. I have my 9 year old watch these . She thoroughly enjoys it.

[u/IkonikK]

Bill! Bill! Bill! Bill! Bill Nye the Australian Science Guyyyyy 🎵

[u/mizu__luffy]

We always used to watch this guy in school. I dunno if our school had something going on but we never watched Bill Nye in school.

[u/Nautilus1000]

I remember seeing him on Frightenstiens Castle. Ahh the good old days.

[u/blackandyellowandred]

What I always loved about JSM was how formal he was (at least compared to some of the other educational programming).

Adults trying to be 'fellow kids' (looking at you Behind The News) gave me too hard a cringe to actually listen.

This and his asides into latin, history or whatever made his lessons feel grown-up and important.

[u/nubosis]

More like the Australian Mr Wizard. C'mon guys