Okay, so we've spoken quite a bit about mechanical properties. I'd like to talk about another property that is quite important, and that is the density of the material. So the density, as you probably know, is a measure of the mass per unit volume. Okay, typically it has units of, this is my notation, just my notation, I'll make for referring to a dimensional analysis, okay, dimensions only. So the square bracket stays, that's what that means, in my notation. The units were typically the grams per cubic centimeter, or in proper SI units, kilogram per cubic meter, those are very common mass units per volume. Why is density important? Well, a lot of, there's certainly a lot of need for low density materials to improve global sustainability, near and for decreased fuel costs, transporting things around the world. It's often an objective in ours to have a lightweight material, and often also high strength or high modulus. So actually what I wanted to do is explore a little bit, this property here, the young's modulus, but plotted against the density, and look at some of the different materials in the world, and how they can rank up. That's a horrible looking row. Let me just see if I can sort that out. That's a little bit better. So we're going to look at modulus versus density, but the thing is, you know, margins, if we want to look at all the materials in the world, we're going to go from some foam, rubbers and stuff that are really, really soft, like you might make a bed out over something. Sorry, I'm all like you'll just fell off my shelf. Or in all the way up to like a really, really high strength ceramic, high modulus ceramic material. So we're going to cover many orders of magnitude. Now when we as scientists, as engineers want to condense data down onto an axis, or many orders of magnitude, what we do is we take the log, or we can plot it on a logarithmic axis. So we're going to do that for modulus. And in fact, we're also going to do that for the density. Now let me just put a few values on here. So we've got one unit. Okay, these are going to be units of grams per centimeter cube. So you may know that water, for example, has a density of one gram per cubic centimeter. The modulus that's put this here, we've got one there. Let's put 10 here. Maybe 100 there. So we're going up in orders of magnitude, right? And a thousand. And these are units, the typical units for young smogs, just sort of gigapascal. Okay, so now let's see if we can, we can't just, you know, maybe we can, we can position some, some materials somewhere on here. And let's, let's start with something close to one gram per cubic centimeter. So something that is close to the density of water. Well, what floats on water? We often think of something that floats. What? So it's going to be, would would be a little bit less. So would should be somewhere along this line here. And what would be the modulus of wood? Well, would have modulus in the sort of the tens, you know, tens of gigapascal rains. So, you know, it depends on whether you're going parallel to the grain or per vernacular to the grain. But we could position wood in there somewhere. And you know, it depends on the species of wood and all that kind of stuff. What about something else like say, say metals, you know. So we'll put higher density up there. Well, metals have a density that sort of resides in between there. Five to eight mine grams per cubic meter. cubic centimeters here I mean. And a young's mod was in the hundreds of gigapascals. So I mean, don't quote me on exactly where I'm drawing these, these regions, but typically you'd expect metals to be in here somewhere. Ceramics would often have a lower density than a lower density than metals. And even a higher, a higher young's modulus. So you might have some ceramics say in this area. All right. And again, you know, don't please don't, don't quote me on exactly where this is. I was trying to give a bit of an intuitive sense. A lot of times we can decrease. What do you, you know, if you have something that you want to have a high strength or high young's modulus with a decreased density that is quite lightweight. Well, as I think I showed you in the previous video, how about some carbon fiber? That's very, it can be quite high stiffness, high modules for a very lightweight. So we can have some exactly that. We can have sort of a region around in here. We might have some composite materials. Fiberglass, carbon fiber, different, what's different, composite materials. What else might you have? Well, close to one, grand per cubic centimeter. Again, things that roughly float on water, but lower strength, you might have some of the polymers. Well, you wouldn't, in fact. One else, you know, really low modulus, low density, some polymer foams. Again, likely, you know, you might have, if you had a foam mattress, polymer foam, very lightweight, definitely float on water easily, if it's a closed foam and a closed cell foam. But again, but very, very, you know, it's easy to deform them, right? A piece of foam, so low, low modulus. And then finally, we can kind of pop that. Elastomers in here somewhere roughly. All right, so this is not beautiful, but it's a kind of a nice way of characterizing the world of materials. But what I want to draw your attention to is that this region here remains largely unpopulated, well, completely unpopulated, in fact. And it's that region right there that corresponds to high young's modulus and low density. It's an area of current research. You know, if we can populate that area, we could do some very exciting things. Decreases in fuel efficiency, or increases in efficiency, decreases in consumption. Lots of great things there. You know, I mean, even things like sporting goods, we're trying to move more and more towards that high stiffness, low modulus area. High modulus, low density, sorry. Right, so that's a quick look. Next video we're going to look at an example of selecting from these different material classes based on the modulus and the density. Okay, thank you.