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PHIL KRASICKY: Good afternoon, and welcome to Favorite Physics Demonstrations. I'm Dr. Phil Krasicky, and I teach a variety of the undergraduate physics courses here. And what I'm going to show you today are a collection of demonstrations that we use in teaching those courses.
And I'd also like to remind you that this is not magic. This is science. This is the way the world really works if you look at it the right way. So since we have the table all set, why don't we begin? Yo ho ho.
[APPLAUSE]
Inertia. Bodies at rest tend to stay at rest unless acted on by a force that can cause them to move. As long as this tablecloth is slippery enough and I pull quickly enough so that any force that acts on these table settings are small and act for just a short time, then they don't move very much. What happens if we do apply forces to objects?
Well, they don't just stay at rest. They accelerate. They start moving. And so here's an object that has some very important significance in our society these days. And if I want this object to accelerate downwards harder than it does normally with just gravity, I push on it. OK?
And so by pushing on it and making it fall harder than it normally would with just gravity, I can dribble the ball around and maintain contact and maintain some control. Here's another trick that some people are able to do. I'm not sure how well I'm able to do it, but. OK, see, if I move this around in a circle, I don't lose contact with it at the top as long as I swing it around rapidly enough. And I'm not palming it, because I can't palm a basketball.
I don't have monster hands. So when things are moving in circles, you often have to apply special forces to make sure that they maintain that circular motion. And so here's something that we can do. This is a glass. This is water. Whoops. We'll put some water in the glass. And then I'm going to-- it's not stirred or shaken but whirled. Stopping.
[LAUGHTER]
[APPLAUSE]
There are some other very interesting things you can do with objects that spin. For example, if any of you are ice skaters, figure skaters, maybe you've done this trick. Not in this way, but on the ice, where you start yourself spinning and then you pull in your arms and you go faster, or you go slower, or you go faster, or you go slower.
The reason that I changed my spin rate is because of what's called conservation of angular momentum. Angular momentum is a quantity that measures how objects spin. And in order to keep my angular momentum constant, when I pull the weights in, I have to spin faster. There are some other neat things you can do with angular momentum. For example, if I get on this turntable here and have Lauren and Andrew come out with this wheel, if I make sure that I'm not spinning and have no angular momentum, they can give me some angular momentum. OK?
So I now have some angular momentum. Well, the wheel has angular momentum. And now I can share that angular momentum with the wheel. OK? And I can give that angular-- when I'm done, I can give that angular momentum back to the wheel and give it back to Andrew and Lauren. Now, if they give me that angular momentum pointing in the wrong direction, I can still use it.
OK. So now we have angular momentum about an axis that's horizontal. But if I tip it up over my head, ooh. See? It's smart. It knows that it now has angular momentum spinning one way over my head, and so I have to go the other way. If I turn it like this, I come back. OK? So I can navigate around here. I can point over here and wave to those people over there, wave to all you folks there.
Wave to those of you in the center here. Yeah. Very interesting things you can do with angular momentum. If you want to come up and try some of these afterwards, please feel free. But make sure that you get one of our assistants to help you. Thank you. There is another quantity that's very important to us, not just as scientists, but in our world. It's called energy. And you often hear about conservation of energy.
One of the demonstrations that we use to demonstrate conservation of energy is this long pendulum here. If we pull this pendulum back to one side and let it go, what I've done when I've pulled it to the side is I've raised it up in height. I've increased its gravitational potential energy. And when I let it go, it'll go down in height and it'll pick up kinetic energy.
So some of that gravitational potential energy will be converted into kinetic energy and it'll start swinging. It'll go until it gets over to the other side, returns to the same height, and then it'll stop and turn around.
Now, one of the demos that any instructor that teaches physics is supposed to do, just to show their class that they really do believe in physics, is they're supposed to stand here against this bullseye with their head against it, and release this pendulum from rest. And if they believe hard enough in physics, it should go down and come back and, well, let's see what it does. OK, so here we go. Ready, set, go. Did I mention that in order to do--
[LAUGHTER]
Do we have a violation of the laws of physics here?
AUDIENCE: You pushed it.
PHIL KRASICKY: I pushed it. Yeah. I pushed it. Why would I push it? Well, what I did was-- yeah, I did, in fact, push it. I did some work on it while I let it go, and that added to its energy. And so when it got over there and then came back, it now had more energy than it started with, so it could get up to a higher height. Let's do this the right way.
Guys, maybe that's why we've been having-- going through so many bandages with this demonstration. Let's try it this way. OK? Now I'm going to do this. I'm just going to let it go gently. And I will stay here. I do believe in physics. I do believe in physics. I do believe in physics.
[APPLAUSE]
Another principle that's very important in mechanics is the principle of conservation of momentum. And it basically says something like, if you push something and start it moving that way, then something else has to be pushed that way and start moving that way.
So let's demonstrate this with a balloon. So I'm going to put some air into this balloon and then I'm going to let this balloon go. And I think you can guess what's going to happen. Air is going to rush out of the balloon downwards, be pushed downwards, and therefore, the balloon is going to be pushed upwards.
And that's the principle of rocket propulsion. We know that if you send gases out of the bottom end of a rocket fast enough, and enough of that gas comes out fast enough, you can actually push rockets into space, to the moon, to the planets, and further. I'm going to demonstrate for you something along those lines. I won't quite take off and go to the moon, but I'm going to show you what happens if you push gas out of a container fast enough. So here we go. Rocket propulsion.
One of the other topics that we talk about in physics is electricity and magnetism. So you probably know about some of the things you can do with electricity. Well, obviously, you can turn lights on, but where does all that come from? It comes about because of electric charge. We can separate electric charge by rubbing objects together.
For example, this wool cloth and this plastic rod. And then when I bring it near the soda can, it attracts the soda can. So this is electrostatic attraction. And it's interesting that this soda can is one of these soda cans that remembers NFL football teams. This one happens to be the Chargers.
We can also produce repulsive forces with electrostatics. If we have two objects that are charged with the same sign of electric charge, they repel one another. And so I'm going to charge up this aluminum plate here. And then I'm going to take a little piece of metal foil. Touch it to the plate. So it picks up the same charge as the plate and now, like charges repel.
We can also do some neat stuff if we pile up a lot more charge. One of our demonstrations that does that is this electrostatic generator over here. It's called a Van de Graaff generator. And if I get enough charge built up on this dome over here, it'll start jumping over to the other dome over there. So let's see if we can do that. So what we've produced here is artificial lightning. And the crack that you hear is artificial thunder.
Here's something else neat that you can do. If you have the right style of hair, then you can charge yourself up. And Val, would you come over here? We have a volunteer, one of our grad students, Val, and I'm going to charge up Val with this van de Graaff generator. And we're going to see if we can demonstrate the principle that charges repel. So let's give Val some charge here. Shake it up, Val.
AUDIENCE: Is it going?
PHIL KRASICKY: It's going. It's going. Keep it shaking. Keep it shaking. Oh, you're looking good. Face the crowd. Show them what you look like. Good. Good job. Let's discharge Val now. OK, give her a hand.
[APPLAUSE]
And one of the other neat things you can do with this is even on a hot day like this, you can actually make it snow.
[APPLAUSE]
Now, we know what magnets do. Magnets attract things like iron, steel, some other metals. But magnets can also be used to produce electricity. There's a principle called Faraday's law of magnetic induction that says that a changing magnetic field can produce induced electric currents.
And so if I take this magnet and I run it through this coil of wire over here, watch the light bulbs. Can you see them lighting? Yeah. So we can produce electricity. And this, in fact, is the way that most of our electricity is produced that we use everywhere in our buildings, our schools, our homes, our society. It's produced by magnetic induction using magnets.
So if you go to an electric power plant, you'll see a bunch of people standing around with magnets pushing them into and out of-- yeah. It's done with electric generators. It's done with coils of wire spinning in the magnetic field of permanent magnets. And we have a small electric generator over there that you can play with afterwards.
Now, if we have electricity that alternates in time, we can also produce electric effects. This is an electromagnet. I have coils of wire wrapped around here. And I'm going to run some electricity through here that alternates very rapidly 60 times a second. And that produces a magnetic field that alternates 60 times a second.
That changing magnetic field can produce electric current, can induce electric current in this coil of wire that I put over here. See how the light bulb lights? There's no direct connection between these. It's just the changing magnetic field produces an electric current. And this is the principle of what we call the electric transformer. These electric effects are, in fact, very real.
And to demonstrate that for you, I'm going to show you what happens if we now let our magnetic field oscillate not just 60 times a second, but a million times a second. We can light a light bulb, a fluorescent tube with no wires attached to it. So the changing magnetic field due to this generator is producing electric effects that cause the gas inside this tube to light up.
One of the other things that these induced electric currents can do is they can produce forces or they can experience forces. So what I'm going to do is take this ring of aluminum, and I'm going to put it here on top of this AC electromagnet and then turn on the power. And what happens is we'll induce an electric current in the aluminum, and that will act like an electromagnet, and it will interact with this other electromagnet and experience a force.
So let's see how this does. It's an electromagnetic ring launcher. One of the neat things about these induced electric currents is that you can make them bigger. If you can make the metal be a better conductor of electricity. One of the ways I can do that is by cooling it. So I'm going to cool this ring in liquid nitrogen.
This is basically liquid air that's cooled so low in temperature that it becomes-- it's air that's cooled so low in temperature that it becomes a liquid. OK, that should be good enough. So this should become a better electrical conductor. We should get a larger induced electric current and we should get this going up higher.
Now there are some-- there are some neat things that you can do with everyday stuff that is all around you, namely the air. Air is very interesting stuff. We usually take it for granted, but it does some very important things for us. One of the things it does is it pushes, it exerts pressure forces on us. And we say that air pressure is about 15 pounds per square inch of surface area. That means for every square inch of surface area, anywhere that's exposed to the air, it feels a force of about 15 pounds.
Now, if you think about how much that means for something like this can, if you add up the number of square inches on each side of this can, you'll come up with the idea that there should be several hundred pounds of force acting on each side of this can due to the air, and yet, the can maintains its shape.
Why is that? Well, there's also air inside pushing out. But if we can get rid of the air inside, then I can show you what happens when the pressure force is due to the air act on the can, pushing inwards only. Let's take the air out from inside the can, and we'll see what the air pressure force is pushing in on the can can do.
Those forces are big enough to crush the can. So we experience those forces every day. But our bodies are used to them and the human body has developed and evolved in an environment where it's used to having those high pressures. And so basically we cannot live without them. Let me show you another demo of this. And for this, I need two volunteers from the audience. Looks like our volunteers are over here. Here's one. Here's another.
I'm going to ask our two volunteers to play tug of war. And they're going to do it with this hollow metal sphere that comes apart into halves. And there are handles on it. So grab your handles, grab your handles. And wait, wait. Wait till I say go. When I say go, you pull. OK? Ready, set, go. OK. So they were able to pull these two halves of the sphere apart.
Now don't go away. Don't go away. Stay here. Now, what I'm going to do is I'm going to take the air out from inside the sphere. OK? We're taking the air out from inside. That's probably enough. And now we're going to ask them to play tug of war once again. All right? You all set here? Grab your handles. Ready, set. Pull. Pull. Pull. No, I mean really pull. Pull, pull, pull, pull. Not so easy anymore, is it?
AUDIENCE: Yeah.
PHIL KRASICKY: OK, now pull gently, and I'm going to let the air back in and-- pull again. Pull gently. I'll let the air back in. And it comes apart. Give them a big hand.
[APPLAUSE]
And you shouldn't feel bad, because the first time this demonstration was done, it was done by the person that invented the vacuum pump, and he had two halves of a sphere that were about this big. And instead of having two people on either side, he had a team of eight horses over here and eight horses over there. When he pumped out the air from inside, those two teams of eight horses couldn't pull them apart.
So that's the magnitude of pressure forces that are provided by the air around us. Now, there's some interesting things you can do with moving air. Moving air can keep us cool when it's hot. No strings attached. Thank you, Grace.
Well, it's probably no surprise that when you blow the air on the ball this way, the ball gets pushed up. But what's more interesting is what happens when you tip the airstream at an angle. The ball doesn't just fall down. And why is that? Well, the ball tries to fall, so it drops down below the middle of the airstream. And now you have air moving faster over the top than over the bottom. And what that does is it makes this air on the top be deflected downwards.
And if you remember the principle of conservation of momentum, if air is deflected and pushed downwards like that towards that side, then the ball is pushed back up. So it always moves back towards the middle of that airstream. If we blow air fast enough, we can actually reduce the pressure in that air. And so it turns out that the air is moving fast enough at this nozzle, that the pressure in the moving air is actually lower than the air pressure outside.
This is what's called Bernoulli's principle, and it's part of why airplanes fly and why other sorts of interesting effects occur with moving air and moving fluids. And what I'm going to do here is I'm going to blow air over the top of this tube. And so what I'm going to try to do is produce low pressure over the top, higher pressure, the atmospheric pressure down below.
And I'm going to put this bottom end of the tube into this dish of ping pong balls and see how high we might be able to pull these ping pong balls up in this tube. OK, so you have to watch carefully. Here we go. Moving air, low pressure.
[APPLAUSE]
I had to watch this. How high did they go? Pretty high, huh? Yeah. So if you want to try playing around with this later, you can do it. But again, make sure that we have someone up here helping you with it.
AUDIENCE: Are you blowing air or sucking air?
PHIL KRASICKY: Blowing air. We're blowing air. It's a leaf blower, right? I have one other thing I'd like to show you with moving air. And for this one, I'll need another volunteer from our audience. You want to come up and help? Come on. OK. What I'd like you to do is hold this candle. So stand right about here and hold that candle. Now, what I'd like all the rest of you to do is to blow out the candle. I'll give you the signal. 1, 2, 3. 1, 2, 3.
[BLOWING AIR]
Oh, come on. Really try this time. 1, 2, 3.
[BLOWING AIR]
Ah. OK. I'm going to blow out this candle, and I'm going to do it using this candle blower-outer. It's a box, it's a hollow box that has a hole on it. And the back is a rubber membrane, like a drum head. And so I'm going to give this box, this drum head a whack. And let's see, we've got to get it at the same height as the hole. So right about there. And we're going to see if we can blow out the candle. Let's get this aimed correctly. 2, 3. Go. Boom.
[APPLAUSE]
Thank you. So the trick is, we don't want to use a narrow focused stream of air because that spreads out too much. Rather, we want to use a puff of air that comes out of something like this. Now, what do those puffs of air look like? Well, it's a little hard to see air. But fortunately, we have a way that we can make air visible. We can make some fog.
So I'm going to take some of this liquid nitrogen that we had over here. I'm going to pour it into a basin of water that's inside there, and that's going to make fog. So we got some nice fog coming out of there. And now you'll be able to see what these puffs of air look like.
AUDIENCE: Whoa.
AUDIENCE: That is awesome.
[LAUGHTER]
PHIL KRASICKY: They're like smoke rings. They are ring vortices, we call them. And the air in those vortices are swirling around like this in the donut, and they're basically swimming their way through the surrounding air. And they can travel for large distances before they disappear. So if you'd like to try this later on as well, please feel free to come up and ask one of our helpers to help you with it. Let's see, what do we have remaining here? Good.
One of my other favorite topics in teaching physics is vibrations and sound. There are a variety of ways that we can make things vibrate. This is a tuning fork, and if I whack it, the prongs vibrate. It's a little tough to hear it. You can hear it if I put it on a piece of wood.
Or you can hear it if I put it up to an empty tube that happens to be the right length. This empty tube has a resonant frequency that matches the frequency of this tuning fork. And so when I put them together, the sound is amplified by the tube. Speaking of tubes, this is called a whirly tube. And if I whirl this around,
[WHIRLING]
It produces sound.
[WHIRLING]
And it can produce a variety of different sounds. So you can actually try playing some music on this.
[WHIRLING]
Well, it's a little tough to do, but you get the idea.
[APPLAUSE]
Here's another one of my favorites. The aluminum rod. This is just a piece of aluminum. And if I take my fingers and stroke them against it, I can make it sing.
[RINGING]
What you're hearing is sound produced by the ends of this rod vibrating in and out. When I pull my fingers along them, I'm actually stretching out the aluminum. And then it goes into vibration, where its length is stretching and compressing this way, and the vibration of the end produces the sound that you hear. Touch the end.
AUDIENCE: OK.
AUDIENCE: [INAUDIBLE].
[RINGING]
PHIL KRASICKY: Very end, very end, very end. There you go.
[RINGING]
So, yeah, I can stretch aluminum with my bare hands. Someone a long time ago took a bunch of wood and carefully shaved them to be the right sizes. And if you take these pieces of wood and you drop them onto this stone block, you can actually hear sounds. And see if you can recognize these sounds.
["HAPPY BIRTHDAY" PLAYING]
Anybody have a birthday today?
AUDIENCE: I do.
PHIL KRASICKY: It's all for you. Happy birthday. One of my other favorites for making sound is the wine glass. And what's neat about the wine glass is that you can tune it. By pouring water or wine into it, you can change the pitch. So I've changed the pitch of the wine glass. I've made it go down by adding water. If I take the water back out.
The other neat thing about wine glasses is that you can make them sing the same way I did with the aluminum rod, by stroking my finger across it by using what's called slip stick friction, the kind of idea that's used by violin players when they play-- or any stringed instrument players when they draw a bow across a string of a stringed instrument. You ultimately get slipping, sticking, slipping, sticking, and that causes a vibration. And so if I do that with a wine glass.
Who's tried this one? This is fun. Well, wouldn't it be neat if we had a whole collection of glasses? Just think about what we could do with them. Wow, look at this. It's a wine glass harmonica. 22 wine glasses. Different sizes, with different amounts of water in them.
And by adjusting the water level, you can adjust the pitch of each note. So here's a note. And here is my tuner. Just adjust the water level to adjust the pitch. This is my coarse tuner, for some of the glasses in here. This is the fine tuner. And then for the real small ones, the extra fine tuner. Well, since we've got this, let's see if we can have some fun.
[WINE GLASS MELODY]
[APPLAUSE]
So on that note, we will bring our Favorite Physics Demonstration session to a close. We hope you enjoyed it. If you'd like to stick around and ask questions about anything that you've seen here, or about anything out there in the hallway, please feel free to come up and see me or one of our helpers, Val, Lauren, Grace, and Andrew. Thank you very much. We also have refreshments out in the lobby way. Thank you very much.
[APPLAUSE]
Phil Krasicky, senior lecturer in physics, has spent more than 20 years collecting hands-on ways to explain physics, and he shared many of them with a Reunion audience June 11 in Schwartz Auditorium.
Krasicky teaches a variety of undergraduate physics courses at Cornell.
