It's summer. It's in Louisiana. Now I'm wearing a sweatshirt. We think about that. It just shows how cold it gets in here. Okay, so let me show you a couple examples. I'll part break them into different videos, but let me just start with pretty body diagrams and forces. So this is my favorite example. Suppose I have a one kilogram book and it's just sitting on a table and and let me do the first thing and just sit in there. Okay, so if something's staying at rest or moving at a constant speed, but not changing its velocity, then the total force has to be the zero vector. So if I want to draw a free body diagram for this, what what forces would I have on there? This was not too hard, but generally when you draw a free body diagram, think of it this way. What things are touching the book and then what long-range forces are on the book. Right now you're only going to see one long-range force and that's gravity. The gravitational force can interact with objects without actually touching them. It's true for those things too, but let's just make it simpler. Okay, so I have only one thing touching the book and that's the table and then I have gravity. So if I want to draw a free body diagram for this, I could start with a dot. You could draw it on top of the picture too, but you could draw it as a dot too. I'm going to have gravity going down. I'll call that mg where the vector g is pointed downwards and mg is the weight of an object. And then I have the table pushing up. I'll call that for right now f in. N is the normal force and normal means perpendicular in this case. So tables are going to push perpendicular to the table. That's the way they work. A lot of times I'll just put that as just an n for the normal force. That's it. That's my free body diagram. Now if this is at rest, then I would want to draw the length of this about the same as the length of that, but it's just a drawing. It's not perfect. It's been half-s me perfect. Another thing that may help is to have one here an x and a y axis like that. To so I know which way is the x direction and which way is the y direction. Okay, so let me just go ahead and use this to say, okay, if I know the mass that block, how hard does the table push up on it or what even vector is that? Let's find that vector at n. Well, if it's at an equilibrium, then the forces have to add to zero. So I have f in plus mg equals zero. Now here it's important to say the zero vector. The zero vector equals zero x hat plus zero y hat. And it is a vector. I can't have vector plus vector equal scalar. Okay, so it is important. Okay, so how do you add vectors? Well, you break them into components. So I can do this in the x and the y direction. Excuse me, in the x direction, there's nothing interesting happening. In fact, nothing is happening at all. So in the y direction, I can say, what's the y component of that? I've called f in. What's the y component of that? I'll call it negative mg. And here you see the difference. See here, this is plus mg, the vector, which is pointing down in the y direction. I'll write, I need a new marker. I'll write this as f in the scalar value, which is a component minus mg equals zero. Those are all scalars now, because I'm just dealing in the y direction. So I can solve for f in, which would be one kilobytes times 9.8. And here's, I like to call g 9.8 Newton's per kilogram. And you can see the units work out to Newton's. So if I want to write the, the force f in as a vector, I can write it as zero x hat plus 9.8 Newton's y hat. I note that seemed all crazy complicated for such a simple problem. But it's important to start with the simple things and doom the correct way before you move on to complicated things. Okay. So let's go ahead and make it more complicated. It'd be fun, huh? Okay, how can I make this more complicated? What if I take my hand and I push down with five newtons? So it's a book. I'm pushing down with five newtons. What changes? Let's erase all this. Well, my free body diagram is going to change. So what changes? So I'm going to have to add an extra force right here. This doesn't change because this depends on the mass, the object, and g. G is a, technically it should be called the local gravitational field, which didn't change. So how can I change this diagram? Well, I can add that five newtons force on there. Now there's two ways you could add that on there. I'll do it in red. I could add it right here. I call it F H force from the hand. Or if you wanted to, you could add it right there next to it. It doesn't matter. It really doesn't. I kind of like adding it right here because then you can see the total vector force downward. I can't stop there because if the book stays at rest, then these vectors have to add up to zero. And clearly they don't. So what have to change? Well, this can't change. I set that. This is on in the can change. So this has to get bigger. How big? What should be as big as this? About like that. F N. Now we can do the same thing. Let's see if we can find the magnitude of that normal force. So in the y direction, this marker is dy-in. I have F N minus F H because it's in the, if it's in the right, this is a vector, it would be this. Let me write it like that. F N plus F H plus M G equals zero. In the y direction, now I'm talking about the components of these vectors, not the vector. So it's plus vector which happens to be in the negative y direction. So when I go over here, it's negative now and it's not a vector. So F N equals F H plus M G equals five newtons plus nine point eight newtons. So that's 14.8 newtons. And then if I want to write it as a vector, F N equals 14.8 newtons y hat. Okay? So that's a pretty simple free-bodied diagram. Pretty simple to solve for that. No angles and stuff to deal with. So we'll do that next. More complicated one. But here's the cool thing. What if the table pushed with the force of 15 newtons? It's a little bit more than these forces wouldn't add to zero. And this block would start to change its motion that way. And before if it didn't push exactly the nine point eight, it wouldn't stay still either. So it's the same table and it pushes different amounts. So why is that? I'm not going to tell you. I think I did say it in my book. But, oh well. Okay, I'm going to stop there.