 Now we get to the concept of free fall. Objects fall because of gravity. We all know that. But sometimes other factors also influence the motion, and these could be attached forces like maybe a bungee cord, or thrust if you've got a motor or an engine or a rocket, or resistance. And this could be air resistance, fluid resistance. Falling through water is different than falling through maple syrup. Now free fall is a very special case where gravity is the only factor. And that means all of those other possible forces are absent, or at least so small that we can neglect them. And you'll see that often written in physics problems where it says neglecting air resistance. If air resistance is small enough, it doesn't affect the motion enough to change our calculations. So when does free fall apply? First of all, no strings attached, or hands, or rockets, or anything else that's going to directly apply a force to that. So it's got to be an object which is free from anything connected to it. Now if you're in a vacuum, you have no resistance. So that's good. Air has very low resistance, so we can assume free fall for air in some cases. Which cases? In particular when the object is fairly compact, meaning it doesn't have a large surface area. The larger the surface area, the more air resistance you're going to have, and eventually you're not going to be able to neglect it. So let's talk about the acceleration. Near the earth's surface, gravity is fairly constant. So we have a constant acceleration if gravity is the only thing affecting it. Now this isn't the case if you're going way above the earth's surface, like rockets going into outer space. That acceleration of gravity is about 9.8 meters per second squared, and it always faces downwards towards the ground. Now we'll often see G shown as the magnitude of the acceleration of gravity. So we've got the 9.8 meters per second squared. And it doesn't include the direction because this is just the magnitude of the acceleration. Now since this is a constant acceleration, we can use our standard constant acceleration equations for y. And for our displacement, that means we've gotten delta y. And for our velocity, we've got our regular velocity equation. But these are the velocities in the y direction and the accelerations in the y direction. Using our value for gravity as our acceleration, that means my displacement equation, the one-half A, becomes minus 4.9. And over here in my velocity equation, I've got minus 9.8. And so I still have my initial velocity and my time in there to help me find my final velocity or my vertical displacement. Let's talk about something that's dropped from rest. And this is a really common case and it gives us an overview for what we're seeing here. From experience, if you're holding an object up and you let it go, the moment you let it go, it's now free to fall. And assuming we can neglect air resistance, it's going to fall downwards. Now if we plug things into our equations, in this case, I'm starting at a height of 4 meters above the ground and it falls downwards. And this is my parabolic curve caused by that t-squared factor. If I was going to transfer this data into a dot plot for the motion, I see here my positions as it falls. And because this is a dot plot, we want to indicate that in general it's moving downwards. Now I can take these same things and take a look at the velocity. So I've got my same dot plot here. But I start to notice up here at the top, my dots are so close together that they're almost on top of each other. As I move further down, the distance between my dots gets a little bit larger. And finally it gets much larger as it approaches the ground. So what this means is I start with a velocity of zero. It was starting from rest after all. And over time, my velocity is negative and it grows to larger and larger negative values. The slope of this line is my minus 9.8 meters per second squared. That's my acceleration. And because it's a nice constant acceleration, we get a nice flat line for our velocity where I've got the same slope everywhere, meaning I've got the same acceleration value everywhere. So this introduces the basic concept of free fall. We're going to look at some more specific examples.