 Now we're going to talk about the equation for kinetic friction. So in kinetic friction, we still have two solid surfaces that are in contact, but in this case, we have motion between the surfaces. Because of this, kinetic friction is also sometimes called the sliding friction. So you have one object sliding across another object. The kinetic friction will impede the motion, but they do actually move past each other. Now the kinetic equation is what's shown here. Now this is a little bit similar to but simpler than the static friction equation. So if you haven't watched that video, you should go look at that one as well. Over here, again, we have the frictional force. But you'll notice that we've got the F sub k, where the k stands for the kinetic. Again, our textbook uses the standard of a lower case F for frictional force as opposed to the typical upper case F. Over on the other side, we still have the normal force. And now we have the coefficient of kinetic friction. So you notice we've got our mu, but the mu has a little sub k on it. So this lets you know that you should be using the kinetic friction coefficient as opposed to the static coefficient. The big difference between the static equation and the kinetic equation is the symbol here. For kinetic, it's always exactly equal to. You may remember on the static case, it was less than or equal to, and you had to just sort of figure out how much force was needed. But in this case, it's exactly equal. So if they're moving, you know exactly how much frictional force is there, provided you know the coefficient in the normal force. So that's our introduction to kinetic friction, the equation. Again, you'll need to actually work some problems out before you fully understand these equations.