Physics: Physical Quantities & Measurement Units

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Physics is about describing and predicting the behavior and the property of "stuff", things. So, you really need, ultimately, to be able to make quantitative statements about the properties of things. Imagine, if NASA is about to launch a shuttle and they call up their fuel contractor and they say, "We need a bunch of fuel." You really need to be able to make measurements and in order to make measurements of physical quantities; you need some standards. Think about a property of objects, which is their length. You want to be able to describe the length quantitatively of objects. You need a standard of length. So, once upon a time, there was some King somewhere and the King says, "This is the standard of length. It's my foot. And from now on, all lengths will be measured in terms of this unit, one foot." So, if you want to measure this table, it's one, two, it's two and one-half feet long. That's fine as long as everybody agrees, then you've got a standard and you can measure any length in terms of that unit, that standard distance.
Now, what happens when that new King is a little baby with a little teeny foot and all of a sudden, the new standard of length, the new foot, is different. So, now all of a sudden, this table is 90 feet long. You really have to come up with a system, a convention for the basic units that everybody can agree upon. This has been done and it's the metric system. System International. And the units that are defined in the metric system are the basis basically for all of scientific measurements, all of commerce, everywhere in the world, except, I'm afraid, the United States. In the United States, businesses still use the British system, the old fashioned system and it's a shame because it means that we think of things in terms of feet and miles and pounds, which are not metric units. I will define for you the metric units and we will be using them throughout Physics as essentially all scientists use metric units. Because people don't always have good intuitions about metric units, I'll try from time to time to point out the connection between familiar everyday American units and the standard metric units.
For instance, I've been talking about distance. The convention now is the unit of distance is a meter. And here's a stick, which is very close to one meter long. So I can measure things in terms of this reference, the meter. That's fine. A meter is approximately equal to three feet. I've got a yardstick here which I can hold next to it and you can see a yardstick is jut a little bit shorter than a meter stick. A yard is three feet. So, when I think about meters, when I'm thinking about physical situations and I'm working in the metric system, I'm kind of, I'm afraid converting subconsciously into one meter, about three feet. There is an exact conversion and if you ever need that you can just use that.
So, that's distance. How about time? That's another quantity physically measurable quantity and we need a unit. The standard unit in the metric system for time is the second. So that's very familiar. You could use a stopwatch to measure. Press it, go. If you need a precise measurement, a stopwatch may or may not be good enough for you. For many practical purposes, it's fine and so, the idea here is that there should be a well-defined standard of time and then I'm using this measuring device to compare to that standard of time. The standard of time, officially, is not when this watch says one second, and it's not even when some particular atomic clock says one second. It's really defined to be the amount of time, which a cesium atom needs to vibrate a certain defined specified number of times. So, all cesium atoms all around the world, vibrate the same and so, anybody with a laboratory with cesium atoms in it, can compare or calibrate their measuring devices to the standard of time.
The meter is not defined to be the length of this stick. Once upon a time, it was defined to be the length of a very beautiful and specially made stick, metal, with two marks on it, and those two marks were defined to be one meter apart. And no longer. Nowadays, the meter, the unit of length, is defined to be the distance which light travels in a certain defined specified amount of time. If you need that specified amount of time for a problem, you can go look it up. You almost never need it because you'll have some measuring devices in your lab, which have already been calibrated and as long as the devices have been well calibrated, you can just use them. You can always use an interferometer, which is a device, which basically makes use of the properties and behavior and speed of light in order to really carefully measure distances and compare with that defined standard meter.
So, we've got distance and time. What other units are there? Mass is the third central unit, the most important unit when you're talking about how much stuff you've got. Here is my standard of mass. It's a kilogram. And a kilogram, technically speaking, is defined as the mass of an object which looks something like this and it is sitting in a vault over in Europe and you compare other masses to that one and if they have the same mass, then we call them a kilogram. If you chop this in half, it's a half a kilogram, and so on.
When you're making measurements, you want to compare with a standard. It's unfortunate that the standard kilogram sitting over in Europe and we can't just go over and compare with it. So, what we want to do is have measuring devices, which have already been calibrated. If you only need a crude measurement, you can just use, for instance, a little scale and, technically speaking, this scale is not really measuring mass; it's measuring weight. Those two words mean something different and we'll talk about that difference, but here in the laboratory, it's okay to calibrate this scale and it reads "one kilogram" and now I can compare other things like this sack of sugar and it weighs the same and we argue that because of the way that this scale has been calibrated, it's mass is the same. And, kilogram is certainly not the most familiar unit to many people and you can't feel what I'm feeling here, but a kilogram of sugar - if you go to the grocery store, a kilogram of sugar they will say is 2.2 lb. What they really mean is that a kilogram of sugar weighs 2.2 lb. But, anyway, that's a good intuition. A kilogram is about 2.2 pounds as far as you're concerned in the grocery store and we'll talk about the distinction between mass and weight later.
Kilogram. That's one of the standards. So, we've got meters, kilograms and seconds. Just three units and that's all we need to describe essentially any physical quantities that we want. In fact, this metric system that we use is called MKS, for meter, kilograms and seconds.
That may not be obvious, what I just said. Think about it for a second. Supposing you're measuring something that's not a distance and not a mass and not a time, like speed. You're going down the highway and you've got a certain speed and you'd like to measure that. You'd like to quantify it. So what unit are you going to use? What's the standard unit of speed? Do we have to go out and make a new definition and a new standard for every different physical quantity we want to measure? And the answer is, No, because you know what speed is. We'll define it carefully, coming up. But speed is miles per hour, or meters per second. So if you know what a meter is and what a second is, then you don't need a new system for speeds, you already have the base units. So, speed is meters per second. It's a derived unit, in terms of these three basic underlying ones and it turns out that as we go in Physics, and we talk about more and more complex properties of systems. We talk about things like power and momentum and energy. These all have complicated and very well defined units and all of those units are derived in terms of M, K and S.
In the metric system, you can add prefixes. So, for instance, you can talk about a centimeter, that's one one-hundredth of a meter, or a millimeter, that's one one-thousandth of a meter. You can do that, it's still metric and you can work in some convenient scale. We like to use kilograms in Physics because this is sort of a normal, ordinary, everyday mass. If you're a chemist, you're using small amounts of chemicals. This is ten grams. Even that may be too much for you, so you might prefer to talk about things in terms of grams, in terms of kilograms, and so chemists will use the C, G, S system - centimeters, grams and seconds. It's still metric, but just slightly different conventions about what's your base unit.
In general, in Physics, the central idea is that complex systems can be understood in terms of a few basic underlying principals. And it's the same way with units. Very complicated physical properties can be described in terms of just these three basic units and now I should be careful, there's really a few more in the metric system. When we get to talking, for instance, about electricity and magnetism. We'll have to define a standard of current. It's something new, it's a new unit, but basically complicated units can be always described in terms of these underlying basic units. Let me give you an example of a non-metric unit - a millihelland. It's the amount of face required to launch one ship.
Preliminaries
Measuring the World Around Us
Physical Quantities and Units of Measurement Page [2 of 2]