 Okay, let me get this door. Okay, so I want to do another, I want to look at another Kahn Academy video, but I'm not going to go through the whole thing. I want to start with this video. The title of this is called Newton's Second Law of Motion, and this is near the end, and let me just go over essentially what he talks about. Kahn comes up with this idea, essentially he says, here are four possible ways that force can do things to an object, and then he goes through and says logically which ones could work, and then he gets down to this force equals mass times acceleration, and he doesn't divide by a vector. Good job. Okay, but so I think the idea is how do we get across what forces do, and this way he kind of shows a logical argument about what it couldn't be, and therefore what it should be, but it's kind of from an authoritative standpoint. This is even the description, Newton's Second Law says that Newton is the authority. Well, maybe that's not always the best way. There's another way to look at what forces do. So let me go ahead and show you some experiments so that you can have an idea of the model of force. I wouldn't call it Newton's Second Law, and to build that model, how you could possibly build that model yourself. Let me turn this light. Okay, so let me close this. Okay, so what I have here is a motion sensor, motion detector. Basically what it does, it sends out a pulse of sound that bounces off something, and it can give you a position versus time graph, and this also gives a velocity versus time graph, so it gives both of them. And with that I have this track and a cart, and this is a fairly low friction cart. So it's got nice wheels and stuff like that. Let's just do a couple of experiments with this to see if we can come up with an idea of what does force do. The first experiment, and I'm not going to tell you the answer, I'm just going to show you some data. The first experiment I'm just going to take the cart and I'm going to give it a push, and let's record data and see what happens. Okay, so here's the cart, start the data. Okay, so this, just so you can help you interpret what this data means, this is position versus time. Every small time interval, the position increases, and it doesn't matter where you're at, it seems like the position increases the same amount during each of those time intervals. If you were to plot this as a velocity graph, if you could watch for it closely, this is about the point where I stopped pushing it, this just shows that the velocity is essentially constant. These little bumps in here, it's taking position data, so it has to do a numerical derivative, so there are some small errors in this would lead to bigger errors in that, so that's why it's bumpy, but that's just pushing a cart. I could push it faster or slower, but I'm not going to do that. The question is, so in this case, what happened to the velocity of the cart after I pushed it? You may say, well, it's hard to say, maybe this does mean force, constant force means constant velocity, maybe the force in my hand went along with it after I pushed it, and there's no really great way to see that at this point. But I could do another force. Here I have, this is a little fan, and I can clip it on the cart. This one has four batteries in it, and it will push on the cart, but is that a constant force? Well, there's one simple way to show this. I have here a spring scale, and I'm going to just hook it on here, and you can't really see it, so I'll add another little video right here. There's my scale. I'm going to turn it on, and then I'll look at what that scale reads. I'll record it with this. So I'll turn this on, and so this reads about 0.14 newtons. Let's see what the motion of this would be like if I turn it on and record. I'm going to store this last run just so we can compare, and then I will turn this on. So it seems like a different motion. Here, again, if I look at how far it went, this is the distances up and down. In a certain amount of time, I could look at the slope of that line right there. Over here, in that same time interval, it went further. So what does that say about the velocity? Well, the velocity graphs right down here, too. Compare this to the case where it has pushed it, and this is the case with the force. You're going to have to take on faith that it actually is a constant force over some time. I held there and showed you the force is constant, but as it moves, you'd have to do some other experiments. Really, this shows a problem with collecting evidence. How do we really know anything? It takes a lot of experiments. We can set something up like this in a class and say, well, just believe me that the force is constant. But it's not too hard to determine that for yourself. What if I do something different? What if I change the force? What if I have a weaker force? So this fan has four batteries in it. This one only has two. It has two dummy batteries in there. So this is going to produce a lower force. I won't measure it just for time. Let me store this last run, and let's just repeat the experiment and see what happens. There you see what's the difference between the blue line with four batteries and the blue line with two batteries. Maybe you'd prefer to look at it down here in the speed graph. What happens if I... This is about 200 grams. I think this is 500 grams. So what if I put this bar on top of it and I put the stronger fan back, the one that made the blue curve. I'm going to put this on there. So I'm going to change the mass of the whole thing and I'm going to let it run. Let's see what happens then. Let me not store that latest run just so we can just compare it to the previous one. There you go. Let's see what other experiments could I do. You could continue to change the mass and things like that, but right there, that should be enough evidence for you to build a simple model of what does force do to an object, what does a constant force do to an object. Now, if you wanted to do this in a clash, students could do this on their own. You can't just let them just go explore this completely on their own because there's too many questions to ask. So a lot of times what we'll do is we'll give them very specific questions to ask, very specific experiments to do like this. But you can compare the difference between this and a case of Khan Academy, which does have some benefits too.