 So, we talked the last section about what a unit was, basically, hopefully, that made some sense. At this point we're going to talk about different units of measurement, one of the units that we talked about, and the previous section was the inch, but there are many other units. So, basically two things that we're going to cover now. What are units, some examples of them, and why do we need them? So, we kind of covered why we needed them in the previous section, but maybe a little bit more information forthcoming. So, this is a picture of, I don't know, some college campus that I think I took from an old version of your book, and basically they show these two people walking to probably chemistry class. They look very excited. This guy's got a skateboard, so apparently he's going to try to get away. Let's see. But there are a bunch of numerical measurements that are shown to you that relate to the picture. It's 8.30, so that's a number, but with just the number there's not enough context, so you need a unit, in this case, AM or PM, so again, number, unit associated with it. And apparently it is pointing to this tree, or a pole, so the pole is very good at telling time, I guess. It is 22 degrees Celsius out, so that's measuring the temperature, so 22 is the number, 22 degrees Celsius is the unit. You can also say the same thing, 22 degrees Celsius is another way of saying that, is 72 degrees Fahrenheit, so a different unit for measuring temperature, and because it's a different unit you're going to get a different number. This student here on the left is carrying around 26 pounds worth of textbooks, which is probably about $3 million worth, the way that you're charged for them these days. I'm more concerned that he's carrying around 26 pounds worth of books, he should probably think about investing in electronic copies. But anyway, the punchline here is that any time you make a measurement and get a number out you're going to need some type of unit, centimeter, kilogram, pound, degree Fahrenheit, you're going to need some kind of unit to allow other people to understand what your numerical measurement actually means. So we're talking about units, and I showed you a bunch of different measurements on the previous slide with different units, but there are times when many units are sort of used as a collection, like think of units as falling into different categories, so there are certain, there's a certain collection of units that are all grouped together and are used a lot by scientists. That collection of units is called the metric system and we're going to talk in detail about that over the next few slides. It is probably the most common collection of units, or if you want to call it the stuffier term, system of units that scientists and a lot of people in the world use for measuring things. We're not going to cover all of the units in the metric system, but we're going to cover some of them. There's another system or collection of units that's related to the metric system, and I am not even going to try to pronounce this. It's French, and I don't speak French, but I can figure out that it stands for the international system of units. And thankfully, you don't have to memorize that. You probably don't even have to memorize this, but it is abbreviated the SI system. So it's related, some of the units that are in this collection overlap with the collection of units in the metric system. There's a little bit of a difference, but they are many of the units in the metric system and in the SI system are used by scientists. So we're going to talk a little bit about those units. OK, here is a collection of some of the units used in the metric system, all of these here. And then this is a collection or a list of some of the units used in the SI system. And I'm going to go through these a little bit and sort of explain what I want you to know, what I don't want you to know. First of all, you can measure the length of things, right? The length of things is basically the distance between two points, let's say. You can measure how tall you are. That's basically the distance from the top of your head to the bottom of your feet. The metric system has a special unit for measuring length. It's called the meter. You're going to have to know that. The abbreviation for the meter is a lowercase letter M. You're also going to have to know that. Nicely enough, the SI system also uses the meter. Mostly, I'm not going to care that you know the units in the SI system, except for one. And we'll get to that in a minute. Volume is another thing that can be measured by people. Volume, my informal definition of volume is how much space something takes up in three dimensions. We'll talk about that in a minute as well. But again, how much space something takes up in 3D. The metric system uses a unit called the leader to measure how much space things take up in 3D. The abbreviation for leader is sometimes it's a capital L. Sometimes it's actually also a lowercase L. So that is a horrible looking lowercase L, but there you have it. And you should know that the metric system typically uses the leader for measuring volume and the abbreviation for leader is an L. Mass is very difficult to define. We are going to cut a huge corner and basically say that mass is the same thing as weight. Now, if you talk to someone who is very stuffy and is very into science, they are going to tell you that mass is not the same thing as weight. And they are correct. However, unless we start doing measurements on the moon or in weirder places, it's not really going to make that much of a difference to us. So as far as I am concerned, in this course, mass is the same thing as weight. I just want you to realize that we are cutting a corner there. Mass, a more informal definition of mass or another informal definition of mass, is the amount of stuff that you have. But if you go to Wikipedia or something like that and look up mass and the definition of mass, your brain is probably going to explode because there are very technical definitions there on Wikipedia and you don't really need to know them for this course. However, the metric system, what you do need to know is what the unit is for measuring mass in the metric system. And that is something called a gram. And the abbreviation for gram is lowercase letter g. So hopefully it's easy to memorize the abbreviations at least. Let's see. So you don't need to know the SI system. Use this cubic meter or kilogram. You can sort of flush those from your mind. Temperature, we're all pretty familiar with temperature. How hot or cold something is. The metric system has its own fancy unit called the degree Celsius. And that is usually abbreviated with a little degree symbol. That's that little circle that's sort of lifted up high. And then a capital C. That is the metric unit for measuring temperature. Degree Celsius, you should know that as well. Here's the only SI unit that I really want you to know. That's a little bit different. The SI system uses something called Kelvin. That's the SI system's unit for measuring temperature. And it is abbreviated with a capital K. And I don't really care that you know this or not, but just to point it out, just to be a little bit fancy, the degree Celsius actually has a little degree symbol there. Kelvin, the abbreviation does not have a degree symbol. So for what it's worth, it doesn't. And then time, the metric system, and pretty much every system that I can think of. I'm sure there are others that don't use this unit. But the standard unit for measuring time is the second and the abbreviation for the second is the lowercase letter S. Same thing for the SI system, but you don't really need to know the SI system except for temperature where Kelvin is used.