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Anderson Video - Force

Professor Anderson
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Let's talk about forces. This is the next sort of physics concept that we want to talk about. And forces are rather important. But let me start by asking you guys a question. When you hear that word force what comes to mind? What do you think of when you hear the word force? Angela? >> (student speaking) Gravity. >> Gravity. Gravity is certainly a force. Gravity is something we're all familiar with, right? I'm holding this pen, if I drop it, it accelerates because of gravity, right? Probably one of the forces that we are very familiar with. Now, let me ask you a followup question. Is there gravity in outer space? What do you think, Angela? >> (student speaking) Yes. >> Yes, absolutely, right? If there was no gravity in outer space then the International Space Station wouldn't stay in orbit. The moon wouldn't stay in orbit. The Earth wouldn't stay in orbit around the sun. So there's certainly gravity in outer space, okay? Probably a little less than here on the surface of the Earth. But there is certainly gravity in outer space. But a lot of people think there's no gravity in outer space. Why do they think that there's no gravity in outer space? >> (student speaking) Because you float. >> What's that? Ian is it? Yes, because you float. Have you been in outer space? >> (student speaking) No, but it's that cliche thing, astronauts floating around [inaudible]. >> Astronauts floating around, right? Specifically what do astronauts feel when they are in outer space? There's a word, right, what's that word? >> (student speaking) Weightless. >> Weightless, they feel weightless, okay? And when you feel weightless you feel like there's no gravity acting on you, right? But, of course, there is gravity acting on you. So what do we really mean by weightless? What we mean, maybe you have a thought Doug, or Sean, sorry. Sean, what do you think? >> (student speaking) Um, because it's a constant falling. >> Ah, because you are constantly falling, that's exactly right. The astronauts in outer space are, in fact, constantly falling, right? They're moving around the Earth in a circle. They are constantly falling towards the center of the Earth. If you're moving in a circle we know that there is centrifugal acceleration. So those astronauts are, in fact, accelerating. But gravity is pulling on every part of them with the exact same force. It's pulling on their toes, it's pulling on their heads, it's pulling on their hands, it's pulling on all parts of them. And there's nothing pushing back, okay? The walls of the space station aren't pushing back on them. There's no scale that they can put underneath them that would push back on them. And it's that pushing back that gives you a feeling of weight. So the reason that I feel like I have weight right now is because the ground is pushing back up on me. If I put a scale underneath me it would read my weight, okay? 172.8 pounds last time I checked, not that I'm worried about that, right? There is a force pushing back up on me which gives me this illusion of weight. So gravity is pulling me down. The floor is pushing me back up. If I remove the floor, if I jump into a hole, then I am also weightless. There's nothing pushing back up on me. And typically when humans do that they feel a little nauseous, right, because your stomach which is usually kind of down all of a sudden goes up into your abdomen and you feel a little queasy and your sense of balance goes off. And so astronauts have to be trained to deal with living in those conditions. All right. So gravity is certainly one of the ones that we are familiar with. When I say force what else do you think about? Yeah, Chris? >> (student speaking) Star Wars. >> Star Wars, right. >> (student speaking) No, but electromagnetic force I would say. >> No, I think of Star Wars. I just watched it the other day with my kids. The force is what binds us and penetrates us. Binds the galaxy together or something like that. Obi Wan, I have to review, but I think it's something similar to that. Yeah, they talk about force, and they give it this mystical power, this force. >> (student speaking) I think of Newtons. >> You think of Newtons? >> (student speaking) Yeah. >> Okay, like Fig Newtons? >> (student speaking) No, like the -- >> Like the unit Newtons? >> (student speaking) Yeah. I just imagine a big N next to it. >> Okay, good. That is the right units for SI units. I would say most people in America at least probably don't think of Newtons but think of pounds, right? All right, let's talk about some of this stuff. Let's talk about the force, all right? The force F. That's how we're going to write it. And the force F is a vector. You notice I put a vector sign on top of it. And so we have to add forces according to our vector rule. We always have to be careful about taking into account not only the magnitude but the direction, okay? The units like we just mentioned for force are Newtons. Okay, we write it with a capital N. And in America most people are familiar with pounds. It turns out that one Newton is about a quarter pound. Okay. What can we say about forces? There are a couple classes, general classes of forces that you can think about. One of those is contact forces. And contact forces are where things are actually in contact with each other, okay? So things like friction. If I rub my hands together my hands are in contact with each other. And if I do it long enough my hands will heat up according to friction. Now, when you get further on in physics you're going to look at this word and say contact, what does that really mean, all right? What does that really mean when I say my hands are in contact with each other? What we mean is I can feel one hand pushing on the other hand. But we know what hands are made of. Hands are made up of atoms. And we know what atoms are made up of. Atoms are made up of electrons and protons and neutrons, of course. And they're in these orbits. And so when I put my hands together what am I saying by contact? What do we really mean by contact? Do I mean that those electrons have bumped into each other? And the protons have bumped into each other? No, that's not what happens. Those things never bump into each other. Yeah, Sean? >> (student speaking) The feeling is the repulsion of the magnetic force from the electrons? >> Yeah, exactly right. It's actually a manifestation of the electromagnetic force. The electrons start to get close to the other electrons and they push back on each other. And the protons start to get close to the other protons and they push back on each other. And if you have a different orientation maybe they get attracted together, okay? And so all those things are pushing and pulling really not in contact with each other, really at a distance. But it's nice to think about forces in these sort of general categories. All right, so friction is certainly a contact force. The normal force is a contact force. What do we mean by normal force? What mean is the following: If I take an object and I put it on the table I know that gravity is pulling it down. But there is some normal force from the table pushing it up. And that normal force only happens when those things are in contact with each other. If I lift the pen off the table there's no normal force anymore, okay? So these are a couple contact forces. Springs can exhibit contact forces. If I tie something to a spring then that spring can pull on the object, but it has to be contact with it. It has to be touching it. And, of course, tension. If I hang something from a wire that wire has to be physically connected to my object to apply any force. All right, so those are contact forces. Which, quite frankly, later on you're going to learn what are really just field forces. But it's nice to categorize them this way for now. All right. Field forces, these are things that act at a distance. So one of them that we just mentioned, gravity. That's a good one. Gravity is pulling on us, pulling on the international space station, pulling on the moon. The Earth is being pulled towards the sun by gravity to keep it in orbit. What else? We had just mentioned another one, the electromagnetic. Really a combination of the electric force which is two charges and the magnetic force which is little current loops all combined into one called the electromagnetic. And then we have two more that we need to worry about. The weak nuclear which is responsible for radioactive decay. And then we have the strong nuclear which is what keeps the nucleus together. Keeps all those protons and neutrons packed into the nucleus. These are the four field forces that you have to worry about. And really the two that you need to worry about in this course are gravity and the electromagnetic force. Now, you're not going to learn all about the electromagnetic force just yet. But it is, in fact, responsible for all of the friction, the normal force, springs, tension. Those are all manifestations of the electromagnetic force. All right, let's continue this discussion