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Anderson Video - What Do Forces Do

Professor Anderson
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>> Hello class. Professor Anderson here. Let's talk about forces and see if we can make some assumptions about what they actually do. All right? We talked a little bit about forces, gravity, electromagnetic force, tension, friction, so forth. What do forces do? Let me ask you to answer this question. What do forces do? Yeah, Sean? >> (student speaking) Forces cause motion. >> Forces cause motion. All right. We said that everything is in motion in the universe, and there must be something governing that motion. So maybe it's these forces that are causing the motion. Any other ideas? Yeah, Chris? >> (student speaking) Does force cause pressure too? >> Force can cause pressure, absolutely, right? We are under pressure right now. Why are we under pressure right now? Probably because you have midterm coming up in another week or so, right? But more specifically, we are under pressure from the atmosphere, right? There is something called atmospheric pressure which is all the air above us pushing down on us all the time. You don't really notice it unless you do a couple things, like go up in an airplane or go up to the top of the mountain. You'd feel that pressure change. Or if you go underwater, you can also feel that pressure change. So pressure is certainly related to force. What else can we say about what forces do? Somebody over here had a comment? Yeah, Martin, what do you think? >> (student speaking) I think in the most simplest terms, it keeps things together. It holds stuff in place. >> Keeps things together. All right, I think that's a great notion, right? Atoms, we know, are protons in the center, electrons in the outer ring. And those things are held together by forces, specifically electromagnetic forces. If those forces don't exist, then the electron just flies away. We don't have atoms anymore. We don't have you. We don't have me. We don't have our universe as we know it. So forces are responsible for keeping stuff together. Absolutely. All right, I liked all those answers. But let's focus on one, which was force causes motion. Somebody said force causes motion. And the reason you might say that is because when you run out of gas in your car and you've got to push it to the gas station, you get behind the car and you apply force to it, and you get that car moving. Your force causes motion. But that's not quite specific enough for our purposes. What we want to say is the following: Force causes acceleration. And this is the really key concept. Force doesn't just cause motion. It causes something called acceleration. We know little bit about acceleration. And specifically, when we talk about force, we want to talk about the net force. The net force is written like that. This is the summation sign from mathematics. And it says you have to add up all the forces that are acting on that object and see what the net force is on the object. And if the net force is equal to zero, then the acceleration A is equal to zero. Okay, so this is kind of a weird concept, but if you're driving your car -- At constant velocity -- Now, you're driving down the freeway in a straight line at 60 mph, what's the net force on your car? What do you think? Samantha? >> (student speaking) Zero? >> Zero. Why is it zero? >> (student speaking) Because force is mass times acceleration, so if you're not accelerating, then there's no force. >> That's exactly right. Force is mass times acceleration. If you're moving at constant V, what is your acceleration? Zero. So then, the net force is equal to zero. You know, well, wait a minute. That doesn't make sense, right? I know that when I step on the gas, I drive at 60 miles per hour. I keep my foot on the gas. Right? If I take my foot of, the gas, I slow down. Isn't that applying a force? I mean, I'm pushing on the accelerator, okay, and I'm not directly that force that drives the wheel, of course. That collects more gas into the carburetor which goes into the pistons, fires up, creates these little mini explosions, right? Piston gets pushed down, cranks on the crankshaft, [inaudible] the wheels, pushes on the wheels. There's friction between the rubber and cement, coefficient of friction of about 1.0 for the rubber, right? All that translates into a forward force driving your car. But we've accepted that that force is zero, so what's going on? Yeah, Sean? >> (student speaking) There's a constant force of the friction on the road and the air resistance pushing against the car? >> Ah, ha -- We have to include all the forces. Good. So let's draw a picture of our car. Here's our car, okay? I don't know what brand of car that is, but that's fine. And now we're heading down the road at constant V. What are the forces that are acting on this car? Well, the way we picture a force diagram is with something called a free-body diagram. I change my car to a dot, and now I've identified the forces that are acting on the car. There is a force pushing it forward, which is the force of the engine acting on the drive shaft, acting on the wheels, acting on all that stuff. And so, this is the force, just called the force from the car itself. But Sean said there's also air resistance, which is the opposite direction. Right? When you drive your car down the road, you have to push a lot of air out of the way, and you don't get to do that for free. It takes a lot of force to do that. The more streamlined your car is, the easier it is to do that. If you have a big truck with a big flat front on it, like a semitruck, you push a lot of air out of the way. That force can be very big. Is that it? Are those the only two forces acting on the car? What else do we have? >> (student speaking) Gravity. >> We have gravity. Gravity you already know points down. And is that it? Christian, what do you think? >> (student speaking) You have the contact path creating friction from the rubber to the road. >> Okay, we have some other fictional coefficient, which is going to oppose our motion. This is friction; we'll make that with a lower case F. There's some friction in the rolling motion of the tires, right? Ball bearings are in there that are actually sliding between metal plates, and those things have some internal friction. Is that it? Is there anything else acting on the car? >> (student speaking) Normal force? >> Yeah, in the back? >> (student speaking) The ground pushing up? >> The ground pushing up, right? And that's what we call the normal force N. So our very simple question here, the net force is zero, is actually pretty complicated. These are the forces that are acting on the car. And if we had them all up, they're going to add up to zero. And this is the concept that we are going to develop in this chapter and the next chapter, is this idea of adding up forces which are vectors and making sure that their sum is equal to the mass times the acceleration, and then we can solve for things like the acceleration.