 Now we're going to look at variables and constants in physics. You think about those two words, they mean what you expect them to mean. Constants are values which don't change. Variables are values which can change. They can vary. First type we're going to talk about are physical constants. And these are values that never change. As an example, we could look at the mass of an electron. It doesn't matter which electron you look at in any atom, in any reaction, in any substance, anywhere in the universe, it always has the same value. And that value is about 9.11 times 10 to the minus 31 kilograms. We actually know it more precisely than that. And you can have a more exact value, but it rounds to about 9.11. But it's always this exact value. These are generally the same textbooks in the front or the back cover or maybe an appendix somewhere in between. Then you get into material constants. Material constants are specific properties of specific materials. It's another example we could look at density. Each material has a different density, but water always has a typical density of about 1,000 kilograms per meter cubed. And iron and silver each have their own specific ones. These are generally calculated from experiments using samples of water or samples of iron or whatever material you're talking about. And they're found in tables in the textbook. It can vary which values you're given in each textbook. They're going to list the more common ones. And the slight values can change. And it's going to depend whether it's perfectly pure iron or if it's got a little impurities in it. So you can have slightly different values listed. Then we get to problem-specific constants. These are values which don't change within a particular situation. As an example of this, we could talk about the mass of a bowling ball. Now, if you think about it, once you pick up a bowling ball, swing it back, roll it down the alley, the mass of that bowling ball isn't going to change as it rolls down the alley. Now, the bowling ball doesn't have a constant mass that we can look up in a table because the mass of your bowling ball may not be exactly the same as the mass of the bowling ball with the guy in the alley next to yours. But within a particular situation, that mass isn't going to change for your problem. Then we get into time-dependent variables. These are things where you have different values at different times. Again, let's look at some examples. So your position as you move around. At different times of the day, you're in different places. So your position is not a constant. It's gonna change depending on what time you look at it. Same for the speed of your car. As you start out in the morning, going from home to school, your speed's gonna change. In general, if you don't know that something is constant, you should assume it's a variable. Use a little bit of common sense, but if it's unclear, assume that it might change at different times. Then we look at time itself. Is it a variable or is it a constant? Well, time is always flowing, always changing, so we treat it as a variable. Now, when we solve specific problems, we may put in specific times. But for the entire situation, the event that we're describing, time is gonna be treated as a variable. This wraps up our general discussion of variables in constants and physics. And again, this is mostly just to clarify our descriptions as we go through everything else. So, that's it for today. Thank you so much for watching. I'll see you in the next one. Bye.