 Okay air resistance Take a look at this. Let me just do a little impromptu experiment here Okay, so here are two sheets of paper Which has a greater mass? The answer of course is that they're the same So if I drop them do they do they fall the same way now crumple that one up and I drop them No, they don't so there's something else going on here other than just a gravitational force There's when an object moves through a fluid or a gas there is an air resistance force You can feel this when you drive through the car stick your hand out the window as a passenger and The faster you go you can you can feel that air pushing harder against your hand, so it depends on the speed It also depends on How big your hand is so if you make it smaller it's gonna be less Okay, it also depends on Some other factors you could put like a cone on there so We have it's a complicated thing though I mean you're moving through here in these air molecules or colliding with your hand It's not simple. We can model it in a way that works pretty well We can model the magnitude of the air resistance with this model This says that the magnitude of the air resistance force is equal to One half row is the density of the fluid the air or whatever you're in a is the cross-sectional area of your object so If you're looking at it head on what kind of air would it have? C is a drag coefficient, so if I have a sphere and a flat disc that have the same area They would have different Coefficients and V is the magnitude of the velocity squared So let's look at this case for a dropping coffee filter Coffee filters are nice in that I Can drop I can stack them and it doesn't change any of these things doesn't change the area or the coefficient But it does change the weight so I can change the weight without changing the object kind of So let's say I have a coffee filter like that Right when I let go What happens? Well when I let go there's there's a gravitational force on it So it's going to start accelerating. It's going to start speeding up going down and after a little bit You'll have some velocity V and so I'll still have the same gravitational force but now there's a Velocity, so there is an air resistance force and this is always in the opposite direction of motion So it's going to be up a little bit like that. So say here a equals negative 9.8 meters per second squared here is a y a Y now we have f air Not quite as big so this is going to be say It's going to be less than well Let's say it's going to be equal to negative 6.2. I just made up something So it's not as accelerating as much because now the net force and y direction is less in magnitude because this is working against it But it'll still speed up But eventually it will get to a point where it's almost Here but these two forces are the same the air resistance is equal to the gravitational force and in which case the acceleration is Zero and we call this velocity terminal velocity Vt So at terminal velocity then one half row a C v squared equals mg Now in this particular Lab we're not changing a we're not changing C. We're not changing row if you want We could just say this is equal to some constant times v squared Equals mg and that's the terminal velocity, but we want to find that constant. That's what we want to do So how do we do that? Well, if I take a coffee filter and I drop it and and it and I let it accelerate and it reaches terminal velocity then I Can measure the terminal velocity with the motion detector And then I can change the math and measure the new terminal velocity and change the math I can change that and that and By looking at how those change I can make a graph of something versus something. I'm not going to tell you what I'll let you think about that But from that graph make it such that it's a straight line and From that slope Make something meaningful from that So that's what we're going to do in the first part and the second part You're going to say once you know that constant K, then you can model this If you look back at your first time you did a numerical model where you threw the ball up You had the acceleration is negative 9.8 But now every line has going to have a different acceleration because it's going to have a different air resistance force But you can do it. Okay. I think that's enough