 So I want to do a little bit more on these constant acceleration equations and how you pick out the equation you need to use for a particular situation. As a reminder in my class the symbols that we use are displacement, initial velocity, final velocity, acceleration, and time. Other professors might use slightly different symbols but these are the ones used by our textbook and that make sense to me. Using those five variables we can actually write five different equations for motion with constant acceleration and these are the ones that I typically use in my class. Now what I want you to notice is that each one of these equations has four variables in them. And just kind of going through and writing them in the order that they're shown. You've got delta x, vi, t, and a, vf, vi, a, and t, delta x, vi, vf, and t, vf, vi, a, and delta x, delta x, vf, t, and a. And yes in some of these equations the time is in there twice but it's still one variable. So the next thing I want you to notice is that each one of these equations is missing one of the possible five variables. The top one is missing the final velocity, the next one is missing the displacement, this one is missing the acceleration, the next one down is missing the time, and the last one is missing the initial velocity. This becomes important because it becomes a clue that you can use to figure out which equation. So in my class what I pointed out to my students was what I call the rule of three. And that just means that to solve these constant acceleration problems you have to have three knowns. Once you have three knowns then you can start solving for the unknowns. And you solve for the unknowns one at a time. So your first unknown, whichever one it is that you're going to go with, plus the three things that you know gives you four variables. So the key here then is look for what's missing. You need the equation that has your four variables but one of them will be missing. When you know which one is missing then you can find the appropriate equation that has the four things you know. Now to help make more sense of this let's go ahead and do an example. And there are so many different examples that I could do. Let's say you're looking for the initial velocity and you know the displacement, the acceleration, and the final velocity. Well in that particular case the thing that's missing is time. You don't know the time and it's not the thing that you're solving for. So you go back up to your equations and you find the one which is missing time. Meaning it has the other variables and so we've got the equation here. And so that would be the equation that you would need to use. You would have to use the vf squared minus vi squared equals 2a delta x. In a second quick check we'll show you okay I know vf, I know the acceleration and the displacement which leaves me the ability to solve for the initial velocity. You're going to have to do some algebra in here and learning how to algebraically rearrange the equations is a different skill. It's going to take some practice to get used to picking out which equation to use to make the fastest solution possible of these equations. If you have any questions contact me.