 Now we can take a look at the forces which cause simple harmonic motion, abbreviated as SHM. Now previously for simple harmonic motion, we've described the general position equation and the general acceleration equation given by these ones here. There was also a velocity equation, but we're not really concerned about that one at the moment. Let's take a closer look at this acceleration equation. And notice that these two terms out here in front, the A and the omega squared, since they're just multiplied, I can very easily switch the order of those two terms. Now if I look at the last part of this equation, it should look familiar because that's really the position equation. That means my acceleration could be rewritten in terms of the position. Now my equation is generally written without the representation for it being a function of time. So I have A equals minus omega squared x. Now in order to have simple harmonic motion, it has to follow this kind of pattern where my acceleration and position are related with a negative sign and some sort of constant in there. Now that brings us to restoring forces because restoring forces have this sort of relationship where the force and the position are related by a negative sign and some sort of constant in there. And if I have this kind of force, then the acceleration it will cause also has that negative sign and a related constant. Now for this to make sense, let's give us an example. And one of the most common restoring forces we work with is the spring force. And we saw earlier this semester that that was equal to f equals minus kx where the force is equal to the negative of the spring constant k and x, the position. Since acceleration is the force over the mass, then my acceleration would be equal to minus k over mx. And that is of the same form here that we were talking about with simple harmonic motion where my omega squared and my k over m are in the same positions. So that means for a spring, omega squared is equal to k over m. So that's the introduction to simple harmonic motion forces. They have to have that kind of restoring force format.