 Now we can look at a type of motion called projectile motion. In projectile motion, this is motion where the acceleration is due to gravity. Just like we saw in freefall motion, that means that I have an acceleration of 9.8 meters per second squared downward. But now I've got both my x and my y components of that vector that I wanted to take into consideration. So the y component is my negative 9.8 meters per second squared, and my x component is zero. Gravity only pulls downward, so I only have an acceleration in the y component, no acceleration in the x component. So it's like freefall, but now we're doing our calculations in two-dimensional motion. When does it apply? Just like freefall, there have to be no strings attached or hands or rockets or anything else that's giving a push or pull directly on the object. If we have a vacuum or air, if we can neglect air resistance, and if the object is fairly compact, that's the in some cases. The main point here is that we don't want air resistance to significantly attribute to the motion, and that would happen if I have a large surface area on my object causing air to have a greater effect. If we look at our formulas, we've got constant acceleration equations for y. Just like we had for freefall, we've got the specific acceleration of minus 9.8 meters per second squared. In our displacement equation, it was one-half the acceleration, so that's why we've got minus 4.9 meters per second squared. And again, these are our two-dimensional equations of motion, so I'm tracking just the y components here. If I look at the x direction, I've got zero acceleration. So if I look at my displacement and velocity equations, they simplify down a lot. My velocity doesn't change. Whatever velocity I start with for the x component is the final velocity for the x component as well. And my displacement in x is related directly to my initial velocity in the x and how much time has gone by. Graphically, if I were to plot out the x and y positions of an object going through projectile motion, it's going to look something like this. It's always going to be a part of a parabola. For my first little chunk of time here, I can use these arrows to show me what my horizontal displacement and what my vertical displacement is. And because these are at set time intervals, this kind of gives us an indication of what our velocity is as well. So I've moved to the right and upwards over that time period. Well, if I chart that out for each and every one of my dots on my motion, I see that I've got a bunch of arrows that always moves towards the right and my x component isn't changing. So my velocity in the x direction staying the same means that in each second interval, I've moved sideways the same amount. But if I look at my y components that finish out those triangles of motion for each of my one second interviews, I see something very different. Let's clear out the x components just to make it a little clearer. I've started by moving upwards, but as I move up, I get slower and slower until finally I reach a highest point and then it starts moving downwards faster and faster, just like we had in free fall. So there's a connection to free fall. The y motion is just like we had it in free fall. Same equations and everything. But my x motion is constant speed. As a matter of fact, free fall is a special case of projectile motion where specifically I have no velocity in the x direction and therefore no displacement in the x direction. But projectile motion doesn't have to have that. That's free fall when I'm only moving vertical. In projectile motion in general, I'll have some sideways motion as well. So that's our brief introduction to projectile motion. We're still going to have to come back and solve some problems so we can understand it a little better.