 Now we're going to look at wave speed. Now to understand wave speed, we have to remember that a wave is a disturbance that travels through space. So the wave speed is the speed of the disturbance, and this is not the same as the speed of the individual particles. Now these statements are going to make more sense if we take a look at a simulation here. And the one that we're using is the P-H-E-T, which is out of the University of Colorado, waves on a string. And I've got that ready and loaded up here. So for this particular situation, if I start a disturbance moving, it's the speed of that disturbance is my wave speed. Now I could very easily make this an oscillating wave, so I don't have to keep moving that up and down. And we again see that our wave speed is how fast the speed is traveling along the string. Now I'm going to put this into slow motion here for just a minute. And again we see our wave speed. But now I want to get to that second comment. You see each individual particle is moving up and down. And in this case it's actually moving up and down with a velocity that can be described by simple harmonic motion. So it's not the velocity of the individual particles as they're moving. It's the velocity of the wave, the disturbance as it travels along. So that's our wave speed. Now in terms of an equation, any speed can be written out as a distance traveled over a certain amount of time. And we could note that here with V being our speed. Sometimes referred to as a velocity, but in this case we don't care about the direction, a distance and a time. Now we can use this basic equation for just any sort of disturbance, a wave pulse, something that goes through. If we've got periodic waves, meaning they're repeating like our oscillating waves, then I could also express the equation using this one, where again V represents my wave speed. But now I've got this lambda for wavelength and capital T for period. And this equation makes sense if you think about the fact that the wavelength is the distance for one cycle and the period is the time for one cycle. So once again we've got a distance per time and that's our wave speed. So each particle, the disturbance will move a distance lambda in a time of T. Now when we think about units, regardless of whether we use the general equation or the periodic equation, you've got some sort of distance over time and so you're going to have standard units of meters per second. Wavelengths are measured in meters, periods are measured in seconds. Now you could have feet per hour or any sort of distance per time unit, but our standard metric is going to be meters per second. So this introduces the concept of wave speed.