 In this video I want to talk briefly about free body diagrams, a little review of free body diagrams. So free body diagrams, which I already kind of used in a previous video, are of the basic form that we have something, an object, right? And this object can be whatever it is we're talking about. We take this object and we apply any external loads that we might have to it. So it might be resting on a surface, which means there's going to be a normal load applied to it. It might have some mass, which means there's going to be a weight applied to it. We might apply an external force to that. And great, we've got this free body diagram. Also, if we're applying an external force, there's got to be something balancing it, which most likely is going to be a force of friction, as it is being resisted on that table that it's sitting on. Now, this all has to balance, right? So we have our standard equilibrium equations. Sum of forces equals zero. Sum of moments is equal to zero. But it's important to remember that these are vector quantities, right? So in a planar sense, as I've drawn it here in just two dimensions, I have an x and a y component of those forces. And we can see that by the forces we have applied here. Of course, if I was talking about a three-dimensional, I have x, y, and z. And similarly for rotation, which is our moment balance, we have a rotation of one degree when we're talking about a planar sense. So it can rotate around a single access, which is pointing in and out of the page. If it's 3D, we have three rotational axes that we might consider. Now, this situation is true if we have static. Static is also the same as constant velocity. So it's not accelerating. It's not changing its acceleration. Of course, I can also have the same sort of object with the same forces, just drawing those on here quickly. And if it's not static, then I have an equivalent diagram to this, which we call our kinetic diagram. And that indicates that it's moving, or if it's moving, then we have this present, right? So if we have a kinetic diagram, we might say, oh, because of this force that I've applied to it over here, f, it's moving to the right with an acceleration a, right? And we have a similar set of equilibrium equations for this, sum of forces, sum of moments. But of course, now they're not necessarily equal to zero, right? f equals ma, m equals i alpha, so rotational inertia and angular acceleration alpha. So in this case, we're talking about things that are in accelerating motion. Now, of course, you're probably familiar with free body diagrams. I've drawn a simple example here. You know, if I wanted to draw a more complex example with my super complex drawing of a car, it has a mass, of course, so it's got a weight. It has force of the engine pushing it forward. It's got wind resistance resisting that motion. And because it makes two contact points with the road, it's got two separate forces here. And again, I can solve for those with my force balance. I can balance my equations. So free body diagrams, of course, are a foundational thing that we need to know about.