 So here's a little bit of an overview of forces in equilibrium and non-equilibrium. Now as a reminder, when we're dealing with forces, we really care about the net force. And that means you must consider all the forces that are acting on an object. Direction matters. And we break it down into the x-direction and the y-direction. So these are our Newton's second law equations where we're taking a look at the net force in the x-direction and the net force in the y-direction. And again, this is the symbol for a summation, so it's the sum of all the forces. And when I take into account all the forces in the x-direction, it's related to the mass times the acceleration in the x-direction. And when I add up all the forces in the y-direction, it's related to the mass and the acceleration in the y-direction. So then we have equilibrium. Well, equilibrium simply means that all the forces are balanced. And that means my net force is zero. And that's for x and y. And technically z, but we're not doing very many three dimensional force problems yet this semester. So the sum of the forces in x are zero, and that gives me an acceleration in the x-direction, which is zero. And a sum of the forces in the y, which is zero, which means my acceleration in y is zero. So not only are my forces zero, but my acceleration is also zero. Now when it comes to this equilibrium then, there are two types. The first is called static equilibrium. And this is when the object's not moving. And if it's not moving, the velocity is zero. And if I have a zero velocity that's not changing, my acceleration is zero. So this object is in balance and not moving. The other type is dynamic equilibrium. And in this case, I am moving, but my velocity is a constant. Because remember, if you're moving with a constant velocity, your acceleration, which relates to the change in the velocity, is also zero. Now thinking about this, this is kind of like Newton's first law, which I'm not going to write it all out here, but remember, an object at rest stays at rest or an object in motion continues in that motion unless acted upon by an outside force. So in this case, I have no outside force. I have no acceleration. The objects continue the way they were. Now the other side of this is the non-equilibrium side. And here, the forces don't balance. And that means my net force is not equal to zero. And so that means I have to have an acceleration. This acceleration could be an x, it could be a y, or both. I could have acceleration components in x and y. Regardless, that means my velocity must change because there is an acceleration. So again, this is just an overview of what we mean by equilibrium and non-equilibrium when it comes to forces in general physics.