Everyone, so now that we've seen density, we're going to talk about a slightly less familiar term which is called specific gravity. Now, if you've never heard that or seen that in your classes, you can probably go ahead and skip this video. But some professors might want you to know it, so it's a good thing to know. It's actually very straightforward, so let's just jump in. Right? So we have the specific gravity which doesn't really have a variable, so we just abbreviate it as SG. It is a term that's sort of related to the density of a material, but it's not exactly density. The specific gravity of any material is defined as the density of that material divided by the density of fresh water. Now, fresh water is always going to be at the bottom and we know that there is a fixed value. So really quickly, what here what happens here is that if you're dividing 2 densities, you're going to divide the units. Right? Kilogram over meters cubed, kilogram over meters cubed. And whenever you divide 2 variables that are the same, they cancel out. So this is just a number that has no units. It's just like 2 or 5 or 0.5 or something like that. Alright? So just to put it into an equation, you know, just to make it a little bit simpler, the specific gravity of any material, which I'm just going to call x, is equal to the density of that x, that material divided by the density of fresh water. Now, again, like we said, the density of fresh water is always a fixed value, it's 1,000. So one really simple way we can sort of phrase this or write this equation is the density over a1000. Alright. So pretty straightforward. So let me just go ahead and run you through some quick examples. If you know that the density of some material is equal to 2,000 kilograms per meter cubed, then that means that the specific gravity of that material is 2,000 over 1,000. So, in other words, it's just 2. It's twice as dense as water. It also works the other way around. If you know what the specific gravity for a material is, I'm going to say that the specific gravity of some material y is equal to 0.7. Right? It'd be less than 1 or more than 1, then that means that the density of that material is equal to 0.7 times a 1000, which is equal to 700. Right? So it works both ways. If you know the density, you could figure out the specific gravity and vice versa. Alright. So really, what we can see here is that the specific gravity doesn't really have anything to do with gravity. It's kind of one of those important terms that later on we figured out that it doesn't really have to do with gravity. But it's sort of what we're stuck with. Right? But it's just a relative number. Right? So 2 or 0.7 or something like that for how many times denser a material is relative to freshwater. Alright. So that's all it is. It's just a number that says how much times denser it is than water. So that's all there is to it. Let me let us go ahead and just work out this example. Alright? So we're going to calculate the volume of some wooden cube. So, in other words, we're going to calculate v, that's the volume, for a specific gravity of 0.8. So we have SG that equals 0.8, and it weighs 16,000 newtons. Now be careful here because we have newtons. So you might think it's mass, but it's actually weight. So, in other words, the w, which is equal to mg, is equal to 16,000 Newtons. Alright? So then how do we calculate the volume? We're going to have to relate this back to the density equation. We have rho that equals m over v. If you rearrange this, you see that v is equal to m over rho. Right? These two things swap places. So in order to calculate the volume, I'm going to need the mass, which I don't know. That's the mass, and I'm going to need the density. So the way I'm going to do this is that this SG here is going to give me density. Right? Because remember that's the relationship between specific gravity and density. And then this equation here for weight is going to give me the mass. Alright? So let me go ahead and just do this one first. So I'm just going to bring this down here. So we have that m equals w over g, which is 16,000. And we're just going to use 10 for g just to make it really simple here. So you can knock off one of the zeros and this just becomes, this is actually 1,600 and this is kilograms. Alright. So that's your mass. And then on the other side, we have the specific gravity. So we have SG is equal to 0.8. So what that means here is that the density is going to be 0.8 times 1,000. Right? It's 0.8 times the density of fresh water. So, in other words, you're going to get 800 kilograms per meter cubed. This makes sense. We got a number that was less than 1, which means that it's less dense than water and we got 800, so that makes sense. Alright? Oh, Actually, I did this wrong, so this is yellow and this should be blue. Alright, so now I'm just going to plug both of these into our equation here and we're going to get that v is equal to 1,600 divided by the density, which is 800, and you just get 2, and the units for that are going to be in meters cubed. Right? So we have kilograms and then kilograms per meter cubed. So you have SI units. So what you should end up with are also SI units. So that is your final answer. So pretty straightforward. Let me know if you guys have any questions.
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