 Hello. Today we're going to begin with a particular question. What is air? Now, that might seem like a little bit of a strange question. Why are you asking what air is? Well, it is a question that humans had to answer over a period of time. In order for us to sort of determine what something is, we need to observe it with our different senses. So let me ask you a question about air. How do you observe air? How would you prove to somebody, maybe a young child, that what is this thing called air or somebody who speaks a different language and they're trying to determine what air is, this word that you're using that they don't know in their language that they have another word for but they don't know. How would you tell somebody what air is? Well, let's talk about it from the context of your senses. How do you sense air? So let's work through some of the senses. Can you see air? Now, this is the hardest one. We're made of the hardest one. But this is an interesting one because air, unlike many things, is not easily visible in most cases. It's hard to see the air around us. It's usually transparent. We can see through it. Now, occasionally we can see some air. Perhaps we see interactions of air where there's a mirage and there's some differences in the air density that we talk about. Perhaps we can see stuff in the air like smoke or if we color the air a certain way with some sort of smoke or dye or something like that, then we can see the air. But usually air is a little bit difficult to see and that's why the question about what is air is a little harder to answer because you can't describe it easily in terms of your vision. Can you hear air? What does air sound like? Well, if I ask you to make a sound like air, go ahead and make one now. I'm betting it'll sound something like a whoosh or some breathing sound or some kind of sound like the wind. Usually this means this is the interaction of air with something that's moving. As the air is moving, it interacts with something, it crashes into something or we associate the sound of air with the sound of the motion of air as it interacts with things. Maybe you can even say the tinkle of some little wind chime or something like that that lends sound to air. Or if you really wanted to get theoretical, you could argue that air has the sound that all sound we hear is the sound of air. It's the interaction of air with your eardrums if you wanted to get really sort of scientific about it. Well, how about can we smell air? Well, air itself is really the means of which we convey smell just like it's the means that we convey sound. If we're trying to smell something, it's little particles that are in the air. We need air in order to be able to smell. When you breathe something in, you're smelling the air and what's in the air, whether it's the smell of your grandmother's cooking or the smell of something that doesn't smell very good, the smell of some garbage somewhere, that's stuff that's in the air that the air is composed of. Can you taste air? Well, taste and smell are very similar. They're related to each other in many ways and in some ways people could say that you can maybe taste salt in the air if you're near the ocean. All right, maybe you feel in your mouth a little bit. It's so akin to smell that it's hard to really argue about what the difference is between taste and smell. Maybe you can't taste air specifically or maybe you can, but we're usually not using taste to describe air that much. So here's the last question. Can you feel air? And this is, again, a matter of how is air interacting with you? Can you feel air? I'm sure you can. Take a deep breath. You can actually feel the air coming into your lungs or blow out. You can feel the air and if you put something in that air stream, you can feel it pushing on your hand. You can feel the interaction of the air as it's moving with your hand. This gives us all some evidence that air exists. We can observe this thing even though we can't observe it as we can many things. It's hard to see and we have to sort of observe the interactions of the air with other objects. There is some evidence that this thing called air exists. So then the next question is, is air made of stuff? The scientific term we use is matter. Does air have matter? It's out of stuff. Can we demonstrate that? Well, that's not too difficult to demonstrate. Here's one of the things. One of the things we know about stuff is that stuff matter takes up space. And I can take a balloon, fill a balloon and I can blow it up and when you ask me what's inside the balloon, we all say air. That's the stuff that we breathe in and we breathe out. Often descriptions of air talk about breathing. And so now I have this balloon and this balloon has there's something inside it that's taking space. And another way I can know there's something inside it is now I can feel it even though it's not moving when I push on it, it resists, it pushes back. And it pushes back in a different way than the balloon pushed back when it was empty. So we can sort of tell that there's something in there that is resisting that's pushing back. And that push, we know that air pushes or at least responds to our pushing. Another piece of evidence like that is I know I can easily blow up a balloon when I blow it here. But if I take that same balloon and I put it inside of a 2 liter bottle, let me go ahead and insert it into this 2 liter bottle here and I'm going to put the mouth of the balloon around the cap of the bottle. Hold on one second, let me complete that process. Apparently this is going to be a little harder than I thought. Okay, now you see there's a hole so I can blow into the balloon inside the bottle. Now I'm going to take the bottle and I'm going to attempt to inflate the balloon by blowing into the balloon again. Well now I'm able to do that. I can fill the balloon, but one of the things I'm noting here is it's very hard to fill the balloon. It's much harder than it was when I was blowing it before. Why? Well now you'd have to think about it. Well if the air is taking up some space this bottle is already full of air. And if I take the balloon and I try to put more air in the balloon but also in the bottle, there's no more room in there and the air that's in the bottle is resisting my ability to put it in more. So the air is pushing back. There is an interaction that we can observe or at least we can describe as being the action of this air in response to the action that I'm trying to do there. Another way to show that is if I take this and I squeeze this bottle, well it's kind of leaking now but you'll see that it's pushing the balloon out. Turns out I tore the balloon a little bit there. It's pushing the balloon out of, presses the balloon and tries to push it out of the bottle. So there's some evidence that air exists. There's some evidence that air exists from the perspective of there's some evidence that air exists from the perspective of it pushes. It reacts to our pushing back and forth on it. So I'm going to go over here and I'm going to go take a look here at this a little experiment here where what I'm going to do is we're going to take a look at a surface of water here. There's a surface of the water and we know this water is contained in this container. There's the sides of the container that are keeping the water where it is. Okay? So then the question is what about this surface here where the water exists? Why is there a surface? What's happening on that surface? Well the water, there's always so much water the water fills up and then it levels out and creates a surface here. You might not have spent too much time thinking about what it means to have a surface of water but can I change that surface? Can I manipulate that surface? I'm going to try something here. I'm going to take a little tube. Here's a little tube, okay? And this tube it's just clear if it's all the way through it's not a test tube with an end it's just a clear plastic tube with open on both ends. But I'm going to plug one end well I'm going to put that tube into the water surface and when you do you'll see that there is a surface inside the tube and then there's another surface inside the tube. So there's two different surfaces and if I wiggle it a little bit I can take advantage of the fact that water's a little sticky and I can create sort of a little difference in surface as the water kind of holds on there. Okay? But you'll see there's two different surfaces. If I move the tube up or down you'll notice there's a little difference in the surfaces but for the most part those surfaces are about the same place. Now I'm going to change things a little bit zoom out just a little bit here I'm going to change things I'm going to take my finger and I'm going to put my finger can we zoom out just a little bit I'm going to take my finger here and I'm going to put it on top of the tube so I'm going to plug the top of the tube so that we no longer can let air in or out of the tube. When I do that I am now able to do something slightly different. Now let's go down and look at that surface again if I look at that surface when I push down I can push and you can see that there is now different surfaces the surface that I had before is now down here lower there's a spot down there where I have the surface inside is different than the surface outside. The air that has been captured inside the tube is pushing down and moving the water surface so it's interacting with the surface of the water which means there's probably air also interacting with the surface of the water up here that's what's touching the surface of the water the water is actually being contained not just by the sides but also by the air at the surface Now notice I'm going to do the same thing but I'm going to leave that there I'm going to leave it plugged I'm going to lift the tube out wait a minute that water surface is also being lifted I can lift up that water surface but how am I doing that? Is air actually pulling? because there's air inside the tube but when I lift this up I'm keeping that amount of air inside the tube and I'm able to lift the surface of the water in another way I'm effectively pulling the surface of the water another way to think about this however is to think about at the moment if air was pushing here and it's pushing there at what point does it start pulling? well, if we think about the air inside is pushing and the air outside is pushing we can think about this a little bit like it's a seesaw or a teeter tot at the moment when they're even here at the same place the air inside and outside have the same amount of push but when I lift this up the air down here you'll notice this surface actually goes down a little bit I'm pushing more on the outside and when I go down I'm pushing more on the inside so we can think about this as pushes and pulls but the other way to think about this as pushes I'm pushing down more on the inside so it goes down and the water outside goes up and then when I reverse that I'm pushing more on the outside and less on the inside and so now I get a difference in heights where the inside is further up so this evidence of the air and water pushing that evidence of the air and water pushing is important to see the question, well we know this idea of pushing something called a force but we also know that anything that we've observed anything solidly observed has a special force where it's pushing down in this case everything's pushing down and we call forces that push down as a result of gravity we call those forces weight so the question becomes does air have weight and if it pushes down so does air have weight and if it does have weight how do we go about measuring that weight now before I answer that question I would like to ask another sort of question here if I am able to take the water inside there and plug and lift I can lift that water up well in this case I can only lift it so high because once I pull my tube out of the water it's all going to drain out but is there a limit to how high you could lift water up just using a tube by plugging the tube and lifting it up because you can see how people a few hundred years ago might say that would be pretty useful think about a well the water's down there I want the water up here and I lift the water up and if so how do I do that well that leads us to a story about somebody named Evangelista Torricelli and I'm going to move over here I'm going to talk a little bit about Torricelli's story here Torricelli was among the other jobs he was a scientist and one of his jobs though was he was also responsible for working and engineering in a mine and in the mines where he was was working one of his jobs was to get water out of the mine if you're a miner you don't want water in the mine you don't want the mine to be flooded so you have to pump water out of the mine you have to be able to take water out of the mine and at this point in our history our human history we had the ability we had learned about suction to decide something to create a vacuum and we would be able to create that vacuum and use that vacuum to lift air up but what Torricelli noticed with this creation of the pump is that no matter what they did if you had a mine let me go ahead and sketch something that looks a little bit like a mine here okay here we have some deep mine hole and there's water pooling down at the bottom of the mine and Torricelli and the people that worked with him would put a long tube down into that mine and they would remove the air from that tube and when they remove the air from that tube the water would come up the tube and they would be able to pour the water out of the mine but one thing that Torricelli discovered is there was a limit to how high they could pump they could not pump water any higher than 33 .8 feet I think Torricelli actually originally said around 34 feet but we've got a little bit more precise number now it was impossible for him to pump the water any higher than 33.8 feet now they could pump it into one pool then pump it into another pool then pump it into another pool 33.8 feet at a time but they could never get a single pump to pump more than 33.8 feet there was a limit to how much water you could pull so if you were around that time like Torricelli you might ask yourself why why is there a limit of 33.8 feet and Torricelli said well I'd like to explore this idea but I have an idea about how I could think about this and see he said I know of a liquid a special liquid the most dense liquid the heaviest liquid that we actually have and that liquid is mercury so what he said is I'm going to try something I'm going to go ahead and I'm going to take a long thin tube pretending like it's my mine I'm going to take a long thin tube and I'm going to fill that long thin tube with mercury I'm going to fill it all the way up to the top so it's filled with liquid mercury well mercury is liquid at normal room temperatures okay and then he said I'm going to take that now and I'm going to put that and cap it off I'm going to put it under a whole pool of mercury now at this time I don't think they were aware of how poisonous mercury is and so Torricelli was probably poisoning himself somewhat at this time but I'm going to make sure that the bottom I'm going to cap it I'm going to put the bottom of that tube into the pool of mercury and then let me make this bowl a little bit bigger I'm going to flip the whole thing upside down okay and he flipped it all the way upside down took the tube and so that tube was full of mercury and there was no air in the tube what Torricelli discovered when he did that is that the surface of the mercury didn't stay the whole tube did not stay full that there was a certain point at which the mercury lowered itself it could not go any higher than a certain level okay and he said hmm how high is that and he measured that level today it's about 76 centimeters or 29.9 inches now I don't know exactly what Torricelli measured alright but he found that there was a particular level that that um a particular level that the mercury would go to this was the first barometer as it turns out there's also one of the first recognitions here of what was that little space up there in the top that space up there there's no air in there what's in there okay it's essentially a vacuum now it isn't entirely a vacuum we could argue a little bit about the details of what's in there it probably has a lot of vapor of mercury inside but effectively there's nothing in there there's a vacuum in there and what we have is just a stack of mercury but the question is if there's nothing in there there's nothing to pull there's no air pulling it what is keeping the stack of mercury up and the argument here is well what's really happening there is a little bit again like our seesaw that but we have this big stack of mercury that's pushing down and it's pushing down on this water here but what's pushing down outside well what's pushing down outside is the air so the idea is there's a balance here between air and mercury that the amount of air pushing down or the air on the outside is pushing down just as much as the stack of mercury is pushing down on the inside that's the idea and that's the concept and that's the thought about how heavy air is that the air how tall is the air stacked up well if we think about air we can think about the air as being a stack of air now we don't really know it's everywhere but if we just look at where it's pushing on the surface of that bowl we can think about the air going up until it's stacked up as high as it goes to the top of the atmosphere whatever the top of the atmosphere means and this created idea of ok so the amount of air we have now measured the weight of the air of the entire atmosphere as compared to the weight of the mercury now how does this go back to our 33.8 feet of water well it's interesting mercury as it turns out is mercury is 13.56 times denser than water so if you take that 29.9 inches and you multiply it by the 13.56 you take that 29.9 inches and you multiply it by that 13.56 and then you also want to convert that from inches to feet let's see here we have 12 inches is one foot so in other words that gives us I believe about 2.49 feet multiplied by 13.56 and we end up with a number that I believe and you can check those numbers yourself is something around the 33.8 so the weight of the mercury in a column is the same as the weight of the water in a column which is the same as the weight in the column so it might seem a little strange that we're talking about weights in terms of heights that we talk about the weight of the mercury as being how high it can stack up before it can't be pushed any higher or the weight of the water at 33.8 feet as how high it can stack up before it can be pushed any higher or ultimately the weight of the air that we're measuring here how much air is stacked up above us up into the top of the atmosphere well those weights can change depending on how big of tubes you actually put together for example if Torricelli had instead of taking a long thin tube it had more mercury and he created a great big thick column of mercury well that would have been more mercury and more weight if Torricelli did that or if they did it with water you'll find it does not change the height to which that raises why? why is that? what is true about that? well in this particular case the main thing we're looking at is we're realizing that what happens is when we take that entire weight that weight is actually distributed over some space and if you take that area bigger and fill everything up above it then you will also add more weight and the relationship between those two things however will stay the same that when you make more area you will add more weight and there's a relationship between those two well we have a name for that relationship and that is called pressure pressure is a derived unit it's a relationship between weight because we will see later forces as applied to an area in this case it's the cross-sectional area or the area that's facing up the surface of the water or the interior area of the little tube and we will talk more about how we measure and how we use pressure in a future video