Hey guys, so in this video we're gonna start talking about pressure and atmospheric pressure, both of which are huge topics in this chapter. Let's check it out. Alright, so pressure is defined as force divided by area. Force divided by area. So it's a measurement of how much a particular force is spread out over a surface area, and it has units of Newton per square meter, and that's because force is measured in Newtons, and area is measured in square meters. Now they got tired of writing Newtons per square meter over and over again, so they decided to call this something, and this is called a Pascal. Pascal, named after Mr. Pascal, abbreviated PA, and it just means that if you have 1 Pascal, you have 1 Newton per square meter. Let's look at a quick example here. So, two identical wood blocks, these two guys here, 12, and it specifies here that they have 800 kilograms per cubic meter. Hopefully, right away, you identify that this is density because of the units. It doesn't say that, but you need to know that. So the density, which is ρ, the Greek letter rho, is 800 kilograms per cubic meter. This is why we covered density earlier. And these are the dimensions of the blocks. So 0.2 by 0.2 by 1. So if you notice, this is a long side here. This must be 1 meter, and these guys here will be the point twos meters. And here, it's just oriented in a different direction. This is the long side so this is going to be 1 meter, and this must be 0.2. Heights. And the depth here must be 0.2 as well. Okay. So, they're placed outdoors, meaning that there's a bunch of air around them and horizontal surface, on horizontal surfaces. So the idea is that it's placed on a sort of surface here, the floor, something like that. We want to know the pressure of each block on the surfaces that they sit on. So the idea is that if you have a block and it sits on a surface, it is pushing against the surface, and it's applying a pressure. Why? Because there's a force over an area, and then whenever you have a force over an area, you have a pressure. So I want to know how much pressure is this block over here applying on the surface right underneath it. So you might imagine that it looks kind of like that, right? If you draw sort of the 3D version here. And you might imagine that there's the bottom here of this guy is also pushing against the surface, against the floor. And I want to know the pressure. So we're calculating pressure. Pressure against the floor, let's call that How do we find pressure? Well, the equation for pressure is force over area. So let's write that. It's the amount of force that the block applies on the floor, divided by the area, the area that they're touching. How much area is that? It's just this area down here, the area of interaction. Okay, so what is the force? This block pushes against the floor because it has weight because of gravity, right? So gravity pulls on the block down, the Earth pulls down the block, the block pulls on the table, or on the surface, on the floor. So the force that's causing the block to push against the surface is mg. So I'm just gonna rename this to mg, and this happens a lot, by the way, that the force on a pressure problem is the weight force divided by the area. And I can just sort of start plugging in that the area here is going to be 0.2 times 0.2. Okay? Obviously, we know gravity is 9.8. For the sake of this problem, to keep it simple, we're going to use that gravity is approximately 10 meters per second squared to make our lives easier. But I still have to find the mass. And once I find the mass, I plug it in, and we're done. How do we find mass? You may remember that if you have density, which you do, and if you have volume, which you do, you can find mass. Because density is mass over volume, therefore, mass is density times volume. Now, please don't get the little p, the little curvy Greek p, which is rho, confused, that's density. Don't get that confused with big P, pressure. Okay. Those are different things. It's unfortunate that they look so similar. So, do I have pressure and volume? Yes. So that I can find I'm sorry. Do I have density? See, I just did it. Do I have density and volume? We do. So we're gonna be able to just plug all this stuff in and figure out the mass. So let's do that real quick. Mass is gonna be density, which is 800 kilograms per cubic meter. Remember to always put units like this, it's easier to cancel, times the volume. The volume is just the 3 sides multiplied. So 0.2 times 0.2 times 1.0. And because this is meter meter meter, this is cubic meter. And this is nice because cubic meter cancels here and we end up with the mass. The mass will be, I have it here, 32 kilograms. Now that I have the mass, I can plug it in here, 32. Gravity is 10. This is 0.04. And if you do this entire thing, you get that the pressure is 8000. Now the question is, what are the units here? Well, because I'm using the standard units, this is just going to be Pascal. Now if you don't see that, just keep in mind that mg, because this is in kilograms and this is in meters per second squared, this mg here is in Newtons. And this was meter and meter, so this is meter squared. So Newtons per meter squared gives you a Pascal. So that is the answer to this one. Okay. Now I'm gonna do the second one in a different way so you can see another way that you could have done this. That is gonna be a little bit easier, and it's gonna be helpful later on. But this is sort of the most straightforward way. You could have done it without anything fancy. Okay. So let's do this a little bit differently. And the first thing you might be wondering is, isn't it just the same thing because it's the same block? Well, pressure is force over area. And while the force is the same because the mass is the same, because it's the same block, right? The area is different. The floor is touching, is interacting with the surface underneath it, via a much larger area. So the area that they are touching against each other is much bigger. And if the area is bigger, you might imagine that the pressure will be smaller. Okay. The pressure will be smaller. But we're gonna calculate this differently. So the pressure with the floor is still gonna be the force against the floor, divided by the area. And the force against the floor, by the way, it's still mg, divided by the area. But I'm going to show you something a little bit different now. So what is the area? The area is these two dimensions here, right? These two dimensions here, not all 3 of them but just 2, which is the width times the depth. Okay? So let's leave it there. And in mass, remember we just did this here, mass is right here. Mass is density times volume. But what is volume? Volume is width times depth times height. Okay? Width, depth, and height. So I can plug in this stuff in here, and I can say the m is going to become row wdh, don't forget the g over here, divided by w times d. Okay? And this is the only time I'm going to do this, just to show you this is actually very helpful for you. W cancel, d cancels, and you're left with that the pressure against the floor is gonna be ρhg (row h g) . So this is interesting because the pressure actually does not depend on the area. It only depends on how high this thing is. Why doesn't pressure