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>> Thank you for having us here tonight. Like Tom said, my name is Rachel. I'm from the Wonders of Physics program. And we are hosting this week, David Willey, who is a teacher at the University of Pittsburgh-Johnstown, where he has a traveling show of his own that goes out to schools, similar to what we do. So he agreed to come and put on his show for you all tonight. He just informed me he won the Presidential Teaching Award last year. ( applause ) And he also has some very entertaining stories about how he has been involved in the world's hottest fire walk and the world's longest fire walk, actually taking some data to try to figure out what fire walking is all about. ( inaudible ) There you go. So without further adieu, David Willey. Oh, he's also the Mad Scientist from the Tonight Show with Jay Leno. So you may see him on TV. ( applause ) So, there you go. >> Welcome. I've got to inform you of two things before I start. One, I'm not using my own equipment, which will throw me off a little. And two, I don't have my wife here with me. She usually helps with the show. And as every woman in the audience knows, without your wife, you're usually totally lost. So if I stand up here looking totally lost, that's why, because she's not with me. I've got some sheets here that will tell me what's going on. The very first thing I've got to do is give you a little bit of a warning. Often, what we do at the start of a show is we have a great big pendulum rigged up. It's just a bowling ball on the end of a piece of chain. And I pull it back to my face and I let it go. And it shows conservation of energy, because of course, it doesn't come any higher than I let it go from. And that demo always works. except for two times it didn't work. Now one, don't push the ball. ( laughter ) Okay? That's not a good idea at all. I've never been so stupid as to push the ball. The other is, don't lean forward after you let it go. ( laughter ) Once, I was stupid enough to do that. Usually, you stand with your head against a wall, swearing that it's there so you that you can't possibly move your head backwards, when what it's really there for is so that you know you haven't moved your head forwards. So, we were at a Catholic school in Pennsylvania, and I let go of the bowling ball. I leaned forwards to keep my balance, only about two inches. That was about an inch and a half too far. ( laughter ) The bowling ball comes back up, hits me in the nose. I've got to admit, it didn't really hurt all that much, but boy did it make my nose bleed. So there I am, standing on stage, 200 little Catholic kids in front of me, blood streaming down the front. They're all, "Yeah! Great demo!" And I'm like, "No! I'm not supposed to be bleeding up here!" So, but we don't have the bowling ball, so I can't show you the one that hurt me. But I can show you one that just embarrassed the heck out of me one time. Just to give a warning. I'm at a physics meeting in Pennsylvania, and I'm listening to an admittedly pretty young teacher explaining a classic demo. And the classic demo is where you take a piece of paper and you set it on fire. And then, once it's going nicely, you put it inside a cylinder, and it's sticky. Whoa! Do that again. Sticks it to your hand. Or at least, it should've stuck it to my hand. Let's see if we can get it to stick to my hand. There we go. It sticks. Anyway, you've got to imagine that it's stuck to my hand, right. And the guy says, "The reason this is sticking to my hand is because all the oxygen was used up." I said, "Yeah, but there was an equal amount of carbon dioxide formed. What's actually happened is that you've heated up the air, it expanded." Let's see if we can get this to work again. "It expanded, and then when it cooled down, it tried to contract again, but of course, it's in a fixed volume there. So what happened was the pressure went down. And that's the partial vacuum is what stuck it to your hand." Well, he didn't like me telling him that he was wrong. So he pulls up his shirt, revealing a belly that makes it obvious that he likes beer a lot, and sticks this to his stomach, and says, "I can do it better than you." Well, I've got an ego that basically just about fits in this room. So I figured well, "I've got a better demo than you do, because I've got apparatus you don't have." ( laughter ) So I stick this to my head. And I go around and go, "What do you think of this?" It stuck there firmly. This is a little too small to do it. "You're really weird, David." "No, I'm not. What do you think? I look like a unicorn, huh?" Well, after doing this for a while, I figured we ought to get on with what we were doing. And "pop!" I popped this off the top of my head. Everybody bursts out laughing. I've no idea why. I haven't said anything funny that I know of. My wife is on the side of the stage. She reaches in her handbag, takes out a compact mirror and says, "Look at the top of your head." There is this huge hickey. ( laughter ) Two weeks I have this big, red spot on the top of my head. I looked like an idiot in front of all of my classes. So, when you go home, you do not try these things on your little brothers and sisters. Let's start actually warning over more with where the laws of physics usually start. Newton's laws-- We actually have a Newton in the audience, right? A beautiful name. I usually promise the kids in the schools that I do my show that if they behave themselves during the show, they will get Newton's birthday off as a holiday. And hence they all agree to this. And I see some people laughing, so you obviously know that Newton's birthday is December 25. ( laughter ) But it keeps them quiet for about an hour. Anyway, Newton's first law-- He had what, three of them. Newton's first law says a body-- I hate this, but ow! A body at rest stays at rest unless a force acts on it. So unless I push or pull on that piece of red modeling clay, it isn't going to do anything but sit there. And neither is the-- What color is it? Oh, it's blue. Okay, neither is the blue in the back. You see, I can move around. Now if I do set it in motion, it then what? Carries on in motion until what? I exert another force on it and take it off my head. I'll need this later. I'll keep this bit and look at another thing. Newton's law says the body doesn't want to move unless we put a force on it. And the second law says that if you do put a force on it, the bigger the body is, the more mass it's got, the smaller the acceleration is going to be. So I've got a nice, big mass here. A lot of inertia to it. It really doesn't want to move. Now if I put a force on it, of course, I can start it into motion. Anybody notice I didn't pull very hard on that, because I wasn't sure just how much friction I've got on the table. I was kind of hoping not much. Because what I want to do now is see, can I give it a big acceleration by putting a big force on it. Well, there's a built-in safety guard here. And that is, I can't put a big force on it, because the paper towel will tear if a big force goes on it. What I'm hoping, of course, is yep, I can pull that out from underneath the bottle. And the bottle stays virtually in place. It moved a little. While I'm doing this at a school a while ago, one of the kids afterwards came up to me and said, "Oh, that's the same as the magic trick where the magician pulls a table cloth away from underneath..." Crockery, or dishes, as you call them. And I try to explain to kids that when I'm in the schools, that I'm not a magician, I'm a scientist. There's a big difference between a magician and a scientist. A magician will do something and they will not tell you how it works. A scientist will do something and they won't shut up about it for the next three weeks with regards to how it works. So anyway, I figured I've got to incorporate this in the show. So we have some crockery, some dishes, and a table cloth. Ooh, plastic, must be for a picnic. And with any luck, if I do my bit right, yep, mother nature does her bit right. And I can get the table cloth out from underneath the crockery. Now I want to show you my all-time favorite physics demonstration. This is the greatest demonstration I know. Are you watching carefully? ( ball clunks on table ) That was it. No, honest. That is the greatest demo I know. The reason? It's never gone wrong. ( laughter ) Every time I let go, it falls down. Yeah, everything else in my life that I've done, I have messed up at least once. If my second wife was with me tonight, she would tell you that that's the truth. ( laughter ) Oh, yeah, I messed up everything really badly once. But this, if you let go, it will fall down. In fact, I've got a big enough ego, I want to hear this come from you. I'll do the "If you let go", you do the "It will fall down." Okay? If you let go. >> It will fall down. >> That was pathetic. ( laughter ) If you let go! >> It will fall down! >> Okay, but there's going to be a quiz on this later, because of that first response. Now down is what? Down is towards the center of the earth. I've never actually tried it with this particular hoop before. And usually, this is the most difficult thing I do all evening. It doesn't look it, but I've got to manage to balance this piece of chalk. That's not going to work. How about we try this pen cap. Everything's a piece of physics apparatus if you look at it right. There we go. We'll balance that on top. And now what we're going to try and do now is see if I can get this hoop out of the way. And in which case, if I can, what? The pen top ought to end up in the bottle. So let's see if we can do it. Yep! Every time. ( applause ) Gotta love that! We said Newton's second law says what? Force equals mass times acceleration. I try and convince people that this has relevance to their every day life, especially when they're driving. We're going to do a demo now where we're going to use some models. This is a model. I'm going to get it to the right shape. This is a model of your head. Yeah, I know, it looks more like my head. It's a model of your head. Your head, going along about 40 miles an hour, 50 miles an hour. I can't run that fast, but what? You could certainly be in a vehicle going that fast. I really hope this never, ever happens, but God forbid, that vehicle is in a head-on collision with an 18-wheeler truck. Bam! The truck hits the vehicle really hard, stops it really quickly. Because that's what Newton's second law says, right? If you want something to stop really quickly, you've got to hit it really hard. And the vehicle stops. You don't necessarily stop. Right, if you don't have a seatbelt on, or you don't have an airbag in your car, you keep going until what? Until you hit the windshield. Now, this is not-- It doesn't look like a windshield, but it'll act like a windshield. Watch how fast-- That stopped the head. I would not want that happening to my head. You can tell is must have been a pretty big force, because what? It put a pretty big dent in this piece of modeling clay. Now, if that's the windshield and this is a head, we also need is we need a model of an airbag. I'm going to get these two good gents here to give me a hand if you would. I'm going to hold this up. This is our airbag. Let me just show you how to hold it. I'm afraid I'm going to have to block your view a little. It's a fitted sheet. If you hold it by the corners, it'll hang in a kind of a "J" shape. That way, when I throw this against it, it won't roll away, it'll just drop in the bottom. Now, this has got some give to it. This makes the collision take longer. So this is the equivalent of the airbag. And in a collision, these are your alternatives. Hit the windshield. Head stops fast. Big force. Hit the airbag. Has some give. Collision takes longer. It doesn't have to be as big a force. Watch. You don't seem impressed. ( laughter ) This is very good. I did not want you to be impressed. Because right now, I'm not doing good science. In science, I said what? If you're going to use a model, it doesn't have to look like the real thing, but it does have to work like the real thing. This, throw me your head a minute, mate. ( laughter ) This doesn't work, right? I think we'd have road pizza. I don't think we'd have just a dented head there. So if we're going to do good science, we need a better model of a head. It just so happens that I've got a much better model of a head here. Now if you decide to do this at home, do it outside. Make sure that the head does not have a cracked skull. The shell of the egg is the skull. What's the squishy stuff on the inside? That's the brains, yeah. And make sure it's not a cracked egg by doing this. Hold the egg by the top and the bottom. Okay, it's not cracked. I really am squeezing that as hard as I can. Oh, show your friends. Look, you can't crack an egg by squeezing it, and you hold it like that. The reason, of course, you can't crack it is what? It's a symmetrical arch. That's why we build bridges this shape. That's why that looks like a tiny model of a sports dome. It's symmetrical. Sure, you can't break it by squeezing it that way. Then hand it to your friend this way. ( laughter ) They'll take it off you. We don't build bridges that shape, do we? Yeah, you know what happens. Okay, so let's think. You're just pulling out of your driveway, five miles an hour. We still good? Yep, we're good. Now in case anybody thinks you don't need a seatbelt at five miles an hour-- I think in this crowd, nobody's going to be that silly. But I'm in school sometimes and I get the sort of, "Oh, you don't need an airbag. You don't need seatbelts if you're only driving five miles an hour..." This is usually from about, oh, 14 or 15 year-old totally immortal little boys. And I tell them, look, if you don't think you need a seatbelt and airbags at five miles an hour, stand up. Two or three of them stand up. Put your hands behind your back. Put your chin proudly out. Walk face first into a brick wall. Now that's going to break your nose, right? And that's only three miles an hour. Five miles an hour, almost double the speed. Four times the kinetic energy. Oh, you're definitely going to do damage to your nose. Twenty miles an hour? Still good? Yep, we're still good. Definitely need a seatbelt. Speed limit in Wisconsin is? 75?! ( laughter ) No, I didn't ask what do you personally drive at. What is it? 65? Okay, I can throw this egg at 65 miles an hour. I've got to be honest. I can't throw much faster. If I could, I'd be in baseball making real money instead of teaching. No, I couldn't take the steroids. But at 60 miles an hour. Let's see, gents. Did we break the head? No, we didn't. Give these two gentlemen a big hand for helping me. Thank you very much. ( applause ) And just in case anybody's out there thinking that was a hard-boiled egg, or that wasn't a real egg. Oh, yes, that was a real egg. We make sure that we put our seatbelts on. We have airbags in our car. It's a really good idea. I want to talk about another motion for a minute. Right then, I was talking about translational motion, cars moving from one place to another. Another type of motion is what? Rotational motion. But it still follows Newton's laws. A body at rest stays at rest. A body in motion stays in motion. But this body wants to stay in motion in a particular way. An in fact, if I can get that young man there to come and help me. Yeah? You look pretty strong. Here's what I want you to do. Hold this in front of you. Okay. And twist it, oh, he knows, back and forth like that as fast as you can. That's pretty good. He's doing it pretty fast. Now, let me give it some motion. Let me get it to rotate in a vertical plane, an up/down plane. It wants to stay that way. It doesn't want to change. Now, when he tries to move it, as fast as you can. Go on. ( laughter ) >> Whoa! >> I love the sound effects! Thank you. Yes, whoa, it's tough, isn't it? I'll tell you what, give it back. Thank you. These people are going to give you a big round of applause for that. ( applause ) Thank you very much. It wants to keep moving the same way. What I've got here is a turntable with some pretty good bearings in it. If I make this go this way around while I'm still standing on the floor, I can turn it over. It took quite a bit of effort. Now it's rotating in the opposite direction. Newton's law says if we've got an isolated system-- I'm going to stand on here-- And something's rotating in this direction, something is going to have to keep rotating in that direction. If it isn't the bicycle wheel, it has to be me. Quite controllable. I can stop. I can start again. I don't go as fast as the bike wheel, because why? I'm a lot heavier than the bike wheel. But it wants to keep going. Now, I'm not sure if we can do the next one, but we'll see. If you let go? >> It will fall. >> Okay. If you let go of just one side. Let's make sure we see which side is which. If I let go with my one gloved hand-- I used to make Michael Jackson jokes at this point. I don't do that in schools any more. ( laughter ) Oh, no! If I let go with just my gloved hand, will that side fall down? How many people say yes, it will fall down? How many people say no, it won't. Ah, you're wrong. ( laughter ) Let's see. Actually, what they're thinking of, I hope, is this. If you get it going fast and you support it by just one side-- Let me see if I can do it. Yes, check that out. It stays supported by just that one side. We've got what? We've got ourselves a gyroscope going here. A means of steering rockets when you're away from a magnetic field that'll work. Okay. Another quick look here. Oh, I know. I want to have a race. I need a young lady to come and help me. Yes, would you? Cool. These good people who let you out. What's your name? Shannon, I'm David. Pleased to meet you. You come with me. We're going to have a race. We're going to see who can empty their bottle the fastest. This is just colored water. Newton saw me put water and food color in here, so that's all that is, is colored water. It's just to make them different. Which do you want? You take the blue-ish looking one. I'll take the red one. On the count of three, start drinking. >> No! ( laughter ) >> One, two... No? Okay, I guess what we'll do is we'll just empty them in here, okay? But I'm going to win. There's a good scientific principle behind why I'm going to win. I'm going to cheat. All you're allowed to do is turn the bottle upside down and hold it as steady as you can upside down. Can you do that for me? >> Sure. >> You promise? >> Okay. >> See, this is why you ask little girls to do it and not boys, because girls keep their promises. ( laughter ) Okay, you turn it upside down and hold it as steady as you can. All the way. You start first. I start second. I put a swirl on my bottle to cheat. Now, regardless of the fact that you started first I win every time, which wasn't really fair, was it, Shannon? But I tell you, these folks will give you a nice, big hand. ( applause ) Why could I know I was going to win every time? Why? Because I knew what was going to happen. Air, light stuff, was going to try to get up into the bottle. And water, heavier stuff, was going to try to get out of the bottle. I knew they were going to get in the way in the neck of your bottle, and it was going to make the water in there gurgle. And it just wasn't going to come out very fast. What I did was what? I put a spin on it. And the water tried to move in a straight line. It couldn't, because of the walls of the bottle. It effectively got thrown to the sides of the bottle. Now, if you think about what was happening with Shannon's bottle-- As I say, air trying to get up, water trying to get down, they gurgled. That's the same thing that happens with a flame. So now, we don't say a flame gurgles, of course. Uh-oh. I told you I was lost without my wife. We say a flame flickers. Here the same thing is happening. Light stuff, hot air, is trying to get up. Cold stuff, cold air, heavier, is trying to get down. They're getting in each other's way. The flame flickers. We don't say it gurgles. That's trying to get down on there. Now, if I take this and I put it on top, and we put a spin on this. I'm hoping we can get the air to spin. Now the light stuff ought to stand a better chance of getting up the inside. Let's see if we can do that. And indeed we can. We get quite a nice little tornado going. Before it gets too hot for me to do it, just in case there are geography majors in the audience, wait for it. Southern hemisphere. ( laughter ) Hey, a knowledgeable audience! I usually get kids going, "Uh...?" Okay, now, stop that. Pull it off. And we're going to kill the flame. How can I do that? Put that cap on top. Three things needed for something to burn, right? Heat, fuel... What's the other thing? Okay, I heard some people say air. Some people said oxygen. You're both right, because air is about 20% oxygen, about 80% nitrogen. There is some other stuff in there, but they're the two basic components. Things need air to burn. A friend showed me a really cool demo a little while ago that he'd learned about things burning in air. He'd spent a little time down at the space camp in Florida. They showed him a demo as to a model of a rocket engine. He couldn't wait to show me this. He said what you need is a five-gallon water bottle. Which one is this? Oops, wrong one. I had to make sure we get the right one here. You need a five-gallon water bottle. You need some ethyl alcohol, which I've got here. You pour some of that in the water bottle. And then, you've got to mix it up. Because we said what? To burn the fuel, it's got to be mixed with air. In your car, it'll be the carburetor doing this, or the fuel injector. Here, it's got to be me doing this. But now, we ought to have a model of a rocket. I can't send it off, because the ceiling is way too close. But I'm hoping that we can still have it work pretty much as a rocket. ( loud whooshing sound ) Let's see if it does. Sure enough, it does. If we cap off the top, notice what happens. As it gets cool, it tries to shrink. I don't want to take it too far, otherwise I'll put some nasty creases in the bottle, which is what happened to this one, which is why I didn't use it again for that demo. After he showed me that, I thought well, cool. If you can do that with alcohol from the chem lab. Maybe I can do it with alcohol from my bathroom. Isopropyl or rubbing alcohol. So we tried it. And guess what, you can. So let's mix some isopropyl alcohol in here. And again, I've got to be the one that acts almost like a carburetor. I've got to mix the air with the fuel. And this, because the alcohol has got much more water in it this time, it won't burn as fast. Sometimes, it doesn't burn at all. But I'm really hoping this time it burns. Let's see if we've got it. ( whooshing sound ) Whoa! My guess is there was some ethyl left in there from the last show, because that went a lot quicker than it usually does. Now if you can demonstrate rocket engines with that, maybe we could make a model of a single cylinder piston engine. Now it's not a scale model, because this is my cylinder and this is my piston. So it's a little undersized, I've got to admit. But if I take the ethyl alcohol again-- Actually, let's use this one. A little of that in here. Put this cup in top. Again, I've got to mix the fuel with the air. So I'll mix the fuel with the air. This time, I've got to make sure there's no excess liquid fuel in there. Because when this paper cup comes out-- Let's make it a double paper cup-- I don't want flaming alcohol to come out with it, again. Yeah, that was a bad show when it did. Let's see if we can get the paper cup to come out of there. ( popping sound ) And sure enough, we can. Don't look up, you're going to get covered in glitter. Oh, yeah, and I'm going to put the flame out. ( light laughter ) I did this demonstration for a bunch of teachers about a month ago. And a guy that I had covered in glitter, like you, came up after the show. I thought he's going to tell me how good a show it was and that he enjoyed it. He was furious with me. He says, "My wife will never believe I wasn't back in that strip club." ( laughter ) "Hey, not my fault, fella." So, yeah. Now, on the subject of things burning in air. I've got here some coffee creamer. Coffee creamer does not usually burn. If I had a teaspoon, I would put some of this in it, put this torch underneath, and we would see it doesn't burn. Oh, I see Rachel rushing from the room. She must be going for a teaspoon. Should we wait for her or should we disappoint her? Let's wait for her. Let's see what else we've got here. Oh, I know what we've got. I need to see how much of this we've got. I need some liquid nitrogen. This is one of my favorite all-time things. This is really cold. This is close to about 200 degrees below the freezing point of water. This looks like water, but it's not really. This is nitrogen. This is the other four-fifths of the air to oxygen. And we'll come back to burning that. What I want to show you here is what happens when we get things nice and cold. Now, if you notice, we're getting a little bit of fog coming off here. There's moisture in the air. You're all breathing out moisture. Hollywood makes fog by putting this into warm water. Fog is just clouds on the ground. There'd be a cloud up there right now if it was cold enough up there right now. It isn't, which is amazing, here. ( laughter ) I've never been so cold in my life as yesterday. But let's see if we can get our own personal cloud. There we go. Just for a little while, we get our own personal cloud. Relative to the liquid nitrogen, the air that I just put in there is very, very warm. I'm going to cool it down. What do things usually do when you cool them down? They shrink, yes. That man remembers the last time he went swimming. ( laughter ) Things shrink. Let's see if we can get this balloon to shrink without losing too much of my liquid nitrogen. That shrunk down pretty well. I'll toss that out there. I've got another balloon here I want to shrink. Let me see if we can shrink this one. Here we go. Now, the observant may notice that this one really doesn't want to shrink as much as the first one did. I can get it to shrink down some, but then if I toss it that way, it expands. Come back to me, I need you again. Okay, thank you. Not air in this one, right? What's in this one? Helium, right. There's really only one thing you can do if you've got a balloon full of helium. As long as, now I just learned this. This is really important, tell your kids this. When you breath the helium from the balloon, the helium is safe to breathe. The powder that they put in the balloons to stop the sides of the balloon from sticking together isn't good to breathe. So don't let kids just breathe from party balloons you've bought in a store. That'll have the powder in it. We took these balloons and we washed the inside of them out very well, so that I... ( in very high voice ) can indeed breath in the helium and make my voice go up. Now the reason my voice goes up is because sound travels very fast in helium. And as I talk to you more and more, what? The helium is coming out of my lungs, rising up, because it is less dense than air. I've got another gas in this balloon. It isn't air. It isn't helium. It's sulfur hexafluoride. Very, very dense. Notice how fast the balloon drops. It turns out that sulfur hexafluoride, mostly used to insulate switch gear and electrical apparatus. But sulfur hexafluoride, besides being very, very dense is also another gas that you can breathe safely. Now if helium, being low density makes my voice go up, what's something that's very high density going to do to it? ( in very low voice ) Yes indeed, it does make my voice go down. ( laughter ) I can sound like Darth Vader. Now there is a problem with this stuff. I've got to keep talking here to make sure that I get the gas exchange going. Because if I just stay quiet, this gas isn't going to come out of my lungs, because of it being heavier than air. In fact, I'm going to do that a couple of times. Because what? I'm going to suffocate if I don't get the stuff out of my lungs. So make sure that that comes out really well. There, let's get back to this. I don't see Rachel. Oh, I thought you'd gone to the bar a couple doors down. But no, it's okay. Coffee creamer. You'll believe me, this doesn't usually burn. But if I mix it with air-- If a little is good, more is better. Then I think I can make it burn. Let's see how well we can make this burn. Yep! Why? Because when I blew it through there, big surface area, lots of air could get to it and it could burn well. When I go to schools and do the liquid nitrogen demos, I often get kids coming up afterwards and saying to me, "That is really cool stuff!" And I have to tell them, "No, it's not cool, it's very, very cold." Let's make some other things very, very cold, like that banana. But I always get the question, "Where can I buy that stuff?" Oh, thank you. And I tell kids where they can buy it. But I also tell them if you do, you've got to have one of these special dewars. And I'm betting that this little dewar here-- Is there any left in there? A little. Costs probably on the order of $200 or $300. They say to me, "I can't afford $200 or $300. I'll just go down there and I'll get them to put some liquid nitrogen in a Pepsi bottle for me. And I'll take it to school for show and tell." And I have to tell the kids that only an idiot would put liquid nitrogen in a Pepsi bottle! That's okay, this is a Barq's root beer bottle, so it should be fine. Actually, of course, the putting it in the bottle isn't the dangerous bit, right? The dangerous bit is if you cap the bottle off. You really don't want to put liquid nitrogen in a sealed container. So I'm going to just take this bottle over here, and we'll see why you don't put it in a sealed container. The reason is what? When the liquid nitrogen turns into a gas, the molecules get to be ten times further apart. That's ten times this way, ten times this way and ten times this way. So that one cup of liquid I just put in there is going to try and turn into 1,000 cups of gas. I'm pretty sure 1,000 cups of gas are not going to fit into that bottle anymore. Let me do a mini version of this, and show you why you don't put liquid nitrogen in a sealed container. Now in here, yeah, I'm hoping is a dipper. We're going to get this cold and we're going to dip some liquid nitrogen. While I do that and get that nice and cold-- That's pretty much cold. I want to show you something else I've been practicing now for quite some time. I intend to nail into that block of solid wood, this nail, using for a hammer, a banana. Don't make me use this! ( laughter ) Now, I need absolute silence to do this, so if you would, turn off your cell phones. ( soft thud ) Apparently not that banana. So I tell you what. Let me get a glove on my hand, and we'll see if this other banana will work better for me. I'm going to tip that up there so that dipper is getting nice and cold. We'll see if we can nail this nail-- Uh-oh. I need to get the frozen side. ( tapping banana on table ) That's the frozen side. ( hammering sound ) There, we've pretty much nailed that nail all the way into the block of wood. ( applause ) Now, let's go back to putting liquid nitrogen in a sealed container. You might think I'm being a little nonchalant here, as to get my hands in this liquid nitrogen. But honest, I'm not. I'm being real careful not to. We'll put this in here. Notice it doesn't immediately turn into a gas. The container is very cold. If I lower this in here really gently and not spill it. I can actually put a cap on this. Now if I do spill it... ( popping sound ) There we go. Well caught! You can keep that as a souvenir! ( applause ) That happens. Now, I said I was being real careful about not getting that liquid nitrogen on my hands. I don't think we've got enough left to freeze this flower. This flower is mostly water. And if I can freeze it, even just a little, I'll show you what would happen to my hands if I got them frozen this much, and then hit them against something. It shatters. I wouldn't want that happening to my hands. Okay, let us move on. I've got a shop vac here, a means of making air move. When air moves, it gets to be... ( loud bang ) ( audience exclaims ) Yeah! That's why you don't put liquid nitrogen in a sealed container! Well, not a good idea at all. ( applause ) Okay, let me see if I can get air to move here and be at low pressure. I'm not sure which of these... ( turns shop vac on ) ( turns shop vac off ) I'm going to put this ball in the stream of air. It won't be able to get out, because the stream of air is low pressure. If it tries to move sideways, the air at the sides that isn't moving, that's at high pressure, will push it back into the stream of air. So let's see. ( turns shop vac on ) There we are. I can even turn it from one side to the other. ( turns shop vac off ) This has some pretty neat consequences, and some practical applications. Let me tell you of a few. It's not a real one, but have you ever wondered why we still put stitching on baseballs and softballs? I mean, we could obviously make them these days, smooth, yet we don't. Why not? We want the stitching there so that if you put a spin on it, the ball drags air around with it. You think what's going to happen to a ball if it's moving through the air as well as spinning. The air on one side is getting a kick in the direction you want it to go anyway. So it gets faster. The air on the other side encounters the air trying to get past the ball. They get in each other's way and slow down. That side's at higher pressure, then, so it pushes the ball sideways. This is why you can throw a curve ball. We didn't realize that we could do this until high-speed photography showed the ball really does curve sideways. I'm not talking about curving it up and down. Obviously, gravity does that. If I throw this ball toward the lady at the back of the room in the red, I'm hoping I can pretty much get it to you with a curve up/down, but it won't curve sideways. There we go. Now, on the other hand, if I throw this one and I put a spin on it, we ought to get it to curve sideways. I can't throw it and put a big enough spin on it just by hand. But I can throw it and put a big enough spin on it with this. Let's see. If I throw this-- I'll again throw it toward you, but you don't have to be ready to catch it. Somebody over there probably has to be ready to catch it. Let's see. That did that. Let me try again. If we've got another ball here, I'm pretty sure. Ooh, we've got another couple. Okay, so let's see if we can get a good spin on this one. Yeah, it goes over to the side. If I throw it and I put a backspin on it, we ought to actually generate lift. So I'm going to try and skim this over your heads and have it lift up toward the back of the room. Now sometimes I miss and I hit people, which is why we stopped using real baseballs for this demo. ( laughter ) There, see it actually went up before it went down. That's called what? That's the Bernoulli Effect. And it has some pretty neat applications besides just softballs. Watch. Glider wings. Curved on the top, flat on the bottom. The air has to go further over the top, so it has to go faster. That generates about 20% of the lift that the glider uses to keep it off the ground. It's not all of the lift. Some people think that. Most of the lift comes from the fact that the wings are angled upwards. Air hits them, air gets pushed down. Newton's third law says what? Therefore the glider must get pushed up. It's the same, I'll bet, as you did as a kid when you put your hand out of a car, and did that and it pushed your hand up. And you do that and it pushes your hand down. So let's see. This ought to just glide out. There we go. See? And this is so much nicer than a bunch of little kids. Because that is usually-- By all means, throw it back. That's usually another science model. I throw a big glider out. And then just for the teachers, I tell them that's a science model of piranha feeding. ( laughter ) Oh, yeah, because every little kid in the audience grabs the glider, tears it apart. See? Just generating an awful lot of lift for you, isn't it? ( laughter ) Anyway, that's not the best way to use the Bernoulli Effect. When I came to this country, and you can tell by my voice, that I talk funny. And it's not just the helium. ( laughter ) It's because I'm British. When I came over here, it was Halloween. And I found something out immediately. You people are crazy. ( laughter ) No, you are! They don't soap windows and toilet paper-- Nice shot! Toilet paper houses in England. No, they're much too refined for that. That's probably why they threw me out. I thought that's pretty cool. I've got to try that. So, I toilet papered the neighbor's house. I took way too long. I got caught. I figured you've got to be able to do this quicker. Science to the rescue. Bernoulli Effect. How can you toilet paper a house? ( machine roars loudly ) I'm going to turn that off before the whole roll comes off. But I got another show tomorrow, so if you would-- ( laughter ) Very carefully. Very carefully roll that back up for me! ( laughs ) Okay, I've got two more demos left. If you would. But these last two demos really do involve a little bit of danger. So one, we're going to move these tables so that they're out of our way and we're not knocking anything down from them. And then, I'm going to show you why I teach physics. People come up to me after the show sometimes and they ask me, "David, why do you teach physics? Is it because they pay you big money to do it?" ( laughter ) Oh, I must have some teachers in the audience laughing, because you know full well, no, they do not pay me big money to do it. After one show, this one lady comes up. And I said, "No, I don't do this for the money." She looks at me and she goes, "Oh, you do it for the children." ( laughter ) That was my reaction, too. No! I don't do it for the children. I do it for the toys! This is one of my favorite toys. Bed of nails. I'm going to lie on this bed of nails for you in a minute, once we put this down here. Let me get it in the right place. And when I do, I'm hoping I'll be okay. I have to take some precautions. One, I'm going to need a pillow when I lie here, because otherwise, I get marks all over my bald head. Small children come up and join the dots together. "Oh, look, it's a giraffe." No, it's not. But I'm going to give you a piece of advice now. This is serious. Should you try something like this yourself, never lie on a bed of nail. ( laughter ) Yeah, think about it for a minute. You do not want to lie on a bed of a nail. You want lots of nails. When I lie on here, about 160 nails are going to support my 160 pounds of weight, which means that any one nail is only going to have to push on me with about one pound. And a nail pushing on me with one pound doesn't hurt. If I push on this nail with one pound, it pushes back on me with one pound. That's Newton's third law, right? It doesn't hurt. Two pounds, doesn't hurt. Three pounds, still not hurting. Four pounds, beginning to get uncomfortable. Five pounds-- Ow, five pounds is too much. I would not want to lie on here weighing five times what I do right now, but still only be the same size. But twice as much would probably not be a problem. So Rachel, come on up here, will you? I'm going to lie on the bed of nails. We're going to put my beginner's bed of nails, nails down on my chest, and Rachel's going to stand up on top of me. You're going to help her keep her balance. Now if you decide to try this at home, do not sit down fast! ( laughter ) I will never do that. Again, it's okay. Lie down. If you would. Oh, got to get the pillow in the right place. Put that bed of nails up on my chest. There we go. Keep it away from the chin. Hop up there, Rachel. He'll help you keep your balance. And there! This is not bad. ( applause ) Thank you. Never try to do that demo with Rosie O'Donnell. ( laughter ) Now you remember when I started the show, I told you about the demo we do with the pendulum? A big, heavy weight moving fast? Has a lot of energy. I've got another big, heavy weight around here that we're going to make move very fast. And I have a lot of energy. I'm going to make this sledge hammer move fast. What we're going to do with it, is we're going to break this brick. Now that doesn't seem like anything unusual. Except where that brick is going to be is-- I'm not lying on the bed of nails, this good gent is. He's going to lie on this bed of nails. We're going to put the other bed of nails on top of him, put the concrete block on top, smash the concrete block. Yeah. Now before we do, just in case anybody was thinking, "Oh, well, look how he's laying on that bed of nails. It's not really sharp." Oh, yeah. Oh, yeah, I'm betting when I pick this up, yep, the entire bed of nails comes up with it. I'm going to have to put my feet on here. ( grunts ) You can tell from that, that oh, yeah, those nails really were sharp. What this demo illustrates is why you want the front of your car to crumple up if you're in an accident. I'll bet there's a lot of you in the audience that remember back when they made cars really strong. You're in an accident, the car hardly bends at all. They thought that was the safe thing to do to protect the people inside. And of course, it didn't protect the people inside that much. Because while you've stopped the car, but how fast are you going to stop the people? What you want is for the impact to take longer. You want all the energy, the impact, to go into bending the front of the car, not going into doing damage to the people. So when we break the block, what I want is for all of the energy of the block, say, all of the energy of the hammer, to go into breaking the block, so that none of it goes into my friend here. Come over and give me a hand laying him down on here. We've got to get some safety gear on him first. The very first time that I did this demo I knew enough to wear eye protection. In fact, I'm going to quickly dig in here and see if I've got some eye protection for myself in here. Yep, I do. I knew little bits of concrete might come off and go in my eyes. But they didn't. A piece of concrete the size of a golf ball came off and hit me smack in the nose. Oh, that hurt! So now, we put a face mask on him, as well. I was so proud of thinking of the face mask the next time I did this demo. I put goggles on. I put a face mask on. I put my arms in front of my face. That's what you're going to do. And I still got hurt. But this man's not going to, because we're going to take even more precautions with him than we took with me. Real gentle. Don't get your feet on there. Come down and lay backwards. There we go. Now we'll put this on top. I'll put it there. Yep, a very necessary piece of equipment. ( laughter ) Now you know how I got injured the second time I did this! ( laughter ) So, I can't leave him lying on there too long, because this is worse than a Motel 6 bed. I'm going to put this on. And let's see if we can do this without hurting him. ( loud crash ) Now quite a significant smash of the concrete block. You can see why he needed the face mask. And are you still living? Yeah? Good. ( applause ) Thank you. Show these good folks your shirt. See? That was my show, folks. I hope you enjoyed it! ( applause ) Thank you very much. You were a great audience. Give this gent and yourselves a big round of applause. Thank you very much. Well done, excellent.