Hey, everyone. So in the last couple of videos, we were introduced to forces and Newton's laws. Now, many problems in this chapter are going to involve multiple kinds or types of forces that are going to be pushing or pulling on your objects. So what I want to do in this video is I want to give you a brief overview of the 5 most common types of forces that you'll see throughout this chapter. Any for every single one of your problems is going to involve some combination of these 5. So we're going to introduce them really quickly, and then we'll go into more detail later on. So let's get started here. The first one is called the applied force, and we've actually seen this already before. This just happens anytime someone or something directly pushes or pulls on your objects. The letter that we'll use for this is going to be f_a. And the simplest example is if you put your hand on a box and you push it. Right? So how do we draw this force? You might be tempted to draw it like this because that's where your hand is coming from, but what I want you to remember is that we're always going to draw forces as a pull arrow, not a push that is acting away from the object's center. So we're not going to draw our forces like this. We're going to go to the object center and they're always going to be sort of as if they were pulling forces away from the center. Alright? So this just gets a symbol f_a, and what you need to know is that it's always in the direction of the push or pull. You're pushing the box to the right, your force is to the right. Let's move on to the second one here, which is called tension. Tension happens whenever you have some kind of a rope or a string or a cable or something like that that's being pulled. The letter that we'll use for this is capital T. So here's what's going on. You take your hand and you put it on a rope and instead of pulling the box directly, you're going to pull in the rope which is attached to the box. So imagine you start pulling on this rope with an applied force of 5 newtons. What's happening here is your 5 newtons sort of gets transferred throughout the rope like this, and it's kind of acting as if you were pulling on the box directly. It's just a way to sort of transfer your 5 newtons and then it just becomes a different force. But remember, we're always going to draw them from the object center. So basically, this is how you draw your arrow. And this is T and it would also equal 5 Newtons. Alright. So you pull with 5 and the tension becomes 5. It's always in the direction of the pull. So if I'm pulling on the rope to the right, then the arrow goes to the right from the object's center. Let's move on to the 3rd force which is called the weight force. And this is really just the force of gravity, presumably by the Earth or whatever planet you happen to be standing on or near. The symbol or the letter that we use is going to be a W, and what you need to know is that we're always going to assume that there is a weight force. Unless the problem explicitly tells you that there is none or that you're out in space or you're way far away from any planets or something like that. So here's how this works. This weight force is always going to act towards the Earth's center. So in your problems, you can usually assume that it's going to be straight downwards of the object's earth centers like this. So in this case here, when you have this object that is sort of near the Earth's surface, you're going to draw an arrow that goes straight down towards the earth's center. Alright, so what about this object over here that's on the surface? It actually doesn't matter whether it's on the surface or whether it's inside of the Earth or floating out near space. Basically, it's always going to have a weight force, but in this situation, it's not going to be acting straight down. It's actually going to be going in this direction. That is your weight force. And finally, for this object here, it's inside of the Earth, but it also still has a weight force that acts in this direction over here. Alright. So that's the weight force. Let's move on to the 4th and 5th one, which are kind of similar related to each other, which are the normal and friction force. The normal force is just a reaction to whenever you have one surface that's pushing on another one. So basically, whenever you have 2 surfaces that are in contact. The letter that we'll use for the normal force is big N. And so here's what's going on. Right? So if you have a box that's resting on the ground, you have 2 surfaces that are in contact. They're touching each other. What's going on here is that this box has a weight force that's acting presumably straight down. Right. We can assume that the Earth's center is straight down, and the reason it doesn't go crashing through the ground is because there is a reaction surface to that surface push that's sort of helping support that box. So that's what the normal force is. It's that reaction force. What you need to know is that this reaction force is always perpendicular, meaning it's always 90 degrees to the surface to the surfaces that are touching. So for example, the surface here is really just the ground. So notice how the ground here and this normal force make a 90-degree angle. Alright. Now you can have other directions. The normal force isn't always going to be acting straight up. It could be going at an angle like this or even sideways. So for example, imagine that you're pushing a box against a vertical wall. In this case, we have a weight force that's acting straight down, right, because it's a vertical wall, but you also have an applied force from your hand that's pushing the box into the wall. The reason it doesn't go crashing through the wall or break through the wall is because there is a reaction to that surface push and it points outwards like this against your hand. That's the normal force. In this case, notice that the surface is the vertical wall like this and this also makes a 90-degree angle with the normal force. Alright. So it's always going to be 90 degrees. Now last but not least, we're going to talk about the friction force, which is really whenever you have a rubbing of 2 rough surfaces that are in contact with each other. So whenever you have something like this where the two surfaces are sliding against each other, the symbol that we use for this is going to be a little capital curly F like that. That's the symbol I'll use. What you need to know is that usually it's going to be opposite to the direction of motion. So imagine a box that's sliding across the ground like this with some velocity. There's actually a couple of forces here at play. There's a weight force that's acting straight down. The reason it doesn't go crashing through the ground is because there's the normal force that's acting straight up, but now there's an extra force because these two surfaces are rough. That's the keyword that you need to know. That rough is usually going to be told, you're going to be given that information in your problem whether the surfaces are smooth or rough or something like that. And now what happens is that there is a friction force and it's going to want to oppose the direction of motion. We'll talk about that more a little bit later on. So basically, you know, if you've ever slid a box across the table or something like that, it eventually comes to a stop and that's because of friction. Anyways, folks, that's it for this one.