 So if I'm given a physical quantity with a number and a unit and I don't like that unit I know how to change it into the different number and the other unit that I want just by multiplying by one a few times But why would I prefer one unit over another? There are many reasons to do so for example as a unit of length a rough isn't very good because there's no standard height for a Scottish Terrier so it's very hard to check exactly how many roughs tall you are and same with meows It's very hard to stack cats to very high sort of piles particularly above two or more And so why would I change to different units? Well, sometimes I'd like the numbers to be reasonable I mean I could take a very very tiny unit length like a micron and measure myself in microns but I'll come out to be millions of them and Humans don't really like counting around really large numbers You'll notice that when I started talking about three and a half million joules in a previous example I convert it to mega joules because three point six is a kind of human-sized number I can think of three things or four things or three and a half But three and a half million is just a little bit hard for me to conceptualize And so we often change our units to try and make a more reasonable choice of number The absolute easiest way to do this is to use the metric multipliers So when we had three point six million joules, we already introduced the mega mega joule So mega anything is a million of them Or one with six zeros We're familiar with kilometer being a thousand meters and indeed a kilo anything is a thousand anything's There's also hecto for a hundred and deca for ten and desi for a tenth Probably the next most common one would be the centi a centi something is a hundredth E.g. a cent is a hundredth of a dollar and We have milli a millimetre is a thousandth of a meter and milli is a thousandth of anything We also have micro and nano below that and giga and terra above that and many more besides Once you know those and you know the symbols for these things so giga is Prefixed by capital G and mega is a capital M and kilo is a little K Micro is the Greek symbol so once you know those symbols you can make up all sorts of units anytime you have a unit You can always have a giga version of that unit or a kilo or a micro So for example, we might have seen Micrometers, so you know what a meter is that's a micrometer. That's a millionth of a meter. You might know kilometers You might be familiar with megahertz a hertz is a unit of frequency It's how many cycles per second so a megahertz is a million cycles per second or in terms of Computers you might have heard of gigahertz. You might have heard of centimeters. You might have heard of millilitres You might have heard of Nanometers, so many of those examples were meters But in fact all those things could work just as well for jewels You can have mega jewels or micro jewels or you could have mega watts You can have giga watt power stations and so forth So if you say that it's 1.04 mega roughs from here to Sydney and you tell someone that Then they're not gonna know what you're talking about because they don't know what a rough is And if you tell them it's the height of a Scottish Terrier and they measure their Scottish Terrier They may well get the wrong idea because Scottish Terriers aren't very standard and there's two important things You have to have in a standard system of units The first is it has to be easy to measure and the second is it has to be the same for everybody So we all have to use the same units and over the last few hundred years people have moaned and negotiated and tried to get towards something and we've done pretty well, so now all Countries in the world except for three except for Burma, Liberia and the United States of America Use what's known as the international system of units or the system in international the SI units the metric system Now there are enormous number of different SI units And that's just a small sample of them So don't be surprised if you don't recognize some of these and there are many others besides you almost certainly recognize the second and the kilogram and the vault and The Hertz and the meter or the Ohm maybe and the Newton But maybe you haven't heard of the Weber and perhaps there's no surprise There's such a large number of units and remember this is just a subset Because there's such a large number of different things we might want to measure the weight of a swimming pool The height of a tree the pressure under a shoe heel the conductance of a nerve Speed of a bicycle the brightness of a star maybe the wind velocity of course two of those the same So the speed of a bicycle and the wind velocity are both velocities in other words You both measure them in terms of distance over time Now one of the things that's done right in the metric system is that when you have a unit of velocity It's not a completely independent thing from the units of distance and time So we already have a unit of distance in the SI system and that's the meter And we already have a unit for time And so the conversion becomes very simple when the SI unit for velocity is indeed the meter per second And in fact there are lots of relationships between all these different units for example A Newton is a unit of force and force we will find is equal to a mass times an acceleration Which is the same as a mass times a distance divided by time squared and so the unit has got to be kilograms meters per second squared and indeed one Newton is exactly one kilogram meter per second squared So if I know what something is if I know that a force is a mass times an acceleration Then I can figure out that the unit for force must be a kilogram meter per second squared And I don't necessarily even have to know that that's called a Newton So you don't necessarily need the SI name although it'll often have a name All you really need to know is how it's built up of these other more fundamental things and that powerful thinking can even go backwards Sometimes if you know that the unit very well you can actually make a fairly good guess as to what that thing is So for example a Pascal, which is a unit of pressure if I tell you that that's one Newton per meter squared Which we could turn again into kilograms and seconds and meters if we wanted to if you knew that then you can kind of tell That it's a force per unit area So Newton is an amount of force and meter squared is an amount of area and indeed pressure is how much force per unit area Something has so how many of the really fundamental quantities are there well It turns out that nearly all of the SI units can be based off only for the kilogram the second The meter and the Coulomb so there's this stuff called mass. There's this stuff called charge Coulomb as a unit of charge There's this stuff called distance and stuff called time and out of those four You can build nearly all the others the only exceptions are the candler Which has something about how the standard human eye responds to light and the Kelvin which has something to do with temperature So these four fundamental quantities and nearly all physical quantities are related to them because the SI units have to be designed to be Easy to measure as well as standard for everybody. They don't actually contain these four fundamental quantities They do contain the meter and the kilogram and the second but the SI base units don't use the Coulomb They use the ampere which is the unit of electric current. That's Coulomb's per second That's the one that's defined because that's easier to measure it turns out There's also in the base units the Kelvin mole and the candler and those base units can be combined to produce all the other SI units