 there we go. Force. There's a fancy schmancy math physics definition, but a definition of a force that's easy for us to wrap our brain around. A force is a push or a pull. And then I wrote down here vector or scalar. Well, what do you think? Do forces have direction? When you push or pull something, does it matter which direction you push or pull? Does that make a difference as to which way it ends up moving? Force most definitely has direction. It is a vector. Okay? I'll convince you right now. Vlad, can you stand up right here, please? Stand right here. Stand on this side of Taylor. Put your hands against his back. I'm going to push here. Push. Right now, both of us are pushing. Push. Keep pushing. Both of us are pushing in opposite directions. Yes? Now, stop. Come around here to this side. I'm going to push with two hands here. You're going to push with two hands here with exactly the same force as you did before. Yes? Go. Oh, did it make a difference to how Vlad reacted? Force definitely has direction. In the first situation, Taylor and I were pushing in such a way that we were cancelling each other out. In fact, you could argue that Taylor was pushing positively and I was pushing negatively, or vice versa. Second direction are forces. We're combining. Definitely, force is a vector. Symbol for force, capital letter F. Although now, Nathan, I should put a little vector bar above it. If you don't, I won't take marks off, but I'm going to try and be fussy, except for myself to remind myself, hey, I better remember the direction then. Units, force is measured in newtons. Figure out who gets it next. You have to stand up. There you go. Force is measured in newtons. Named after a scientist whose last name was? Newton. Surrounded Newton. So remember Taylor and I pushing on Vlad. Here's the same idea. Example one says, find the net or combined force in the following diagrams. What do you think the net force is here? Take a guess. Seven newtons. But what do you think the net force is here? Yep. Now, I said force is a vector. So you know what I should have said? Seven newtons, right. And Paige, you know what, you're going to tell me for part B? Three newtons, what? Or we could have called that east if we put a compass on here, or depends on, I haven't told you what this is. So I'll just assume to the right. What's my net force in C? Alexis, I think you're right. What? Zero. That was Taylor and I each pushing on Vlad from opposite sides. Right now, all of you are experiencing a net force of zero. You know how I know? Because you're not accelerating. Just like Vlad was not accelerating. Does that mean there's no forces acting on you? Absolutely not. There is a force acting on you, gravity. What it means is there's another force exactly the same size as gravity, pushing in the opposite direction, your chair. Your chair pushes up and exactly cancels out gravity. You know how I know that, Marcus? Because if I could pull a magic dumbledore trick and make the chair vanish out of thin air, what would happen to you instantly? You would accelerate again because you're an unbalanced force. D, nine is incorrect. Good thought though. As a matter of fact, D is 12. Most of physics 11 is going to be keeping things in straight lines. We'll let left be positive and right be negative, or vice versa, but we'll keep things linear. Physics 12, the first half of it is physics 11 done over again, but in two dimensions. Oh, and Jordan, if you take first year university physics, that's actually physics well done over again, but in three dimensions because we live in a three dimensional world, but there you really need calculus to do the math. Everybody out seeing plan? All works? Good. Newton's laws of motion. Put your pencils down for a second. I got to go off on a bit of a rant here. One of the reasons we have so much respect for Sir Isaac Newton was at the ripe young age of around 20 or 21. He rewrote almost everything. He wasn't a particularly gifted high school student. He didn't stand out, but he was from the gentry. He was reasonably wealthy. And so like anyone who was from the gentry when he reached university age, he could afford to go to university. So he did. Went to Cambridge or Oxford. I think Cambridge, but when he was in his first or second year of university, I can't remember which the black plague came to London, the black death as it was called. People are dying like crazy. If you were in any way wealthy, you got out of town. You went and lived in the countryside because although back then they didn't exactly know what was causing the black death. They did know if you hung around people who were sick, you got sick. So he went and for about a year he lived as a farmer. Now when I say farmer, he lived as a gentleman farmer, which meant he told other people what to do and he wandered the fields, but he never really did much breaking or hoeing or anything like that. But Taylor in that one year of him wandering around, you could make an argument that no single human being has thought as much or accomplished as much intellectually in a single year as he did. Maybe Einstein came close, but even I think most people would still give the nod to Newton. In that one year, he first of all invented the calculus. We put a duck in front of it to make it sound really impressive. He invented an entirely new branch of mathematics. He completely came up with the equations to describe gravity. In that year he realized that light, that white light is actually not white. It's made up of many colors. You've all seen a picture or tried putting light through a prism and you see the rainbow that you get. And he completely overturned two and a half thousand years of motion physics. He came up with motion laws. He summarized anything that moves and he did it wonderfully. He did it with exactly three rules. Gotta give him credit. And in doing so, he actually challenged two and a half thousand years of science. It would be like you John, challenging me right now on 9.8 and saying, no, I don't think that's actually correct, which by the way is what Einstein did, because I haven't quite told you actually correct. Gravity is not quite how we've described it. It's warping space time. But Newton did the same thing for things that move. Let me explain to you what they thought at the time. And let me plug my laptop back in and I'll pause the video so that people who are watching at home don't get totally bored. But that also means that they miss out on this lovely little demo. Newton's first law, which is sometimes called the law of inertia. That's not quite right, but I'm not going to be fussy enough to say that it's wrong right now. Eventually, if you take third year physics at university, you'll discuss what mass is and you'll talk about the Higgs boson field and whatever. Here's what he said. Now this is the English version and then I'm going to give you a simpler Mr. Dewitt easier to remember nerdy version. In the absence of an unbalanced net force, an object will either remain at rest or move at a straight line in a constant speed. When Vlad was standing and Taylor and I were balancing out our forces, what was Vlad's acceleration? Zero. In fact, he was remaining at rest. That's not that radical. Ah, but what Newton said is, or if Vlad were in outer space, he could have been going in a straight line at a constant speed. In fact, here's what Newton said. If you have no net force, if you have no net force, it's not your speed that's zero. It's your acceleration that's zero, which means Rob, either you're standing still or you're going in a straight line without your speed changing because that's your acceleration being zero. Now what that also means is, remember when I rolled the tennis ball, that means Newton said, oh, and by the way, the reason it's stopping, something's pushing it to a stop. We'll get to that in a second, but Newton said it's stopping not because it's losing life energy, it will keep going in a straight line at a constant speed forever and because it's not going in a straight line at a constant speed forever, something has to be stopping it. That's part two of this. If an object is accelerating, changing its speed or direction, there has to be an unbalanced, I typed wrong. It's not an unbalanced four, it's an unbalanced what? Four, sorry guys, I keep meaning to fix that. Someone at the end of class remind me to fix that, acting on the object. So what brings that ball to a stop? Yeah, I'm going to say no because gravity this way can't exert a force this way. There is a way that it can do it indirectly, but we haven't got there yet, so I'm going to say good thought, but no, yes. Turns out it's mostly air resistance. As that ball is moving, it's bumping into air molecules and air molecules have mass. You know how I know they have mass? Because they can exert a force on you. Did you feel a push ripple? Not much mass, which is why a ball will roll straight for quite a while. Oh, and we're doing one more thing. We're assuming this floor is level. You can see there seems to be a divot right there. It started to curve and roll. Ignore that part of things where our magic idealized physics work. You've all been in a car and the driver makes a sudden left turn. So if you're in a car and someone is turn, if you're in the passenger seat and someone is making a quick left turn, what do you feel? Well, for your own experience, you feel like you're being pushed against the door. That's not what's happening. You want to continue going in a straight line. The car is actually turning underneath you and you run into the door with your right shoulder and the door has to push you that way in order to get you to turn. Part of the problem here also with forces is our bodies are backwards accelerometers. Our bodies are backwards accelerometers. When you're in a car and someone hits the brakes fairly quickly, which way do you feel like you're being pushed forwards? You feel like, yes, but you all answered this correctly on the test when you're braking. Which way are you accelerating backwards? Have we been on the elevator before? When it first launches upwards, which way do you feel like your stomach is staying down? But which way are you clearly accelerating up? And when you're clearly accelerating down, which way do you feel like you're being pushed up? Our built-in accelerometers, unfortunately, are backwards. I've learned over the years to translate them that way. When I'm on a ride and I feel like I'm getting pushed this way, I know really I'm accelerating that way. It's a pretty easy thing to figure out when you're recognizing. You've got to be aware of it. So contrary to what your common sense might tell you, you're not being pushed to the right at all. The driver steers the car to the left. Your body wants to carry on on a straight line. That's Newton's first law, the law of inertia. The door of the car is moving left with the rest of the car. You feel as if you're being pushed against the door by a force aimed towards the right door. But in fact, your body is trying to continue on its original path and the car is pushing you to the left. Yep. Yep. Nope. I'm going to give you what the equivalent is in physics 11, but it's the wrong definition and any third-year prof would freak out on me. But I'll let you, when you get to the thing, we're going to use it interchangeably with something here in physics 11, even though technically it's wrong. Is that okay? In fact, if you look at the next page, it says this and Vlad, your definition is going to come down right there. Thank you for anticipating where we're going. Oh, I forgot to give about candies for the shuffle. Can you make sure I don't forget towards the end of class? As a general rule, and this is what Newton pondered and realized, an object tends to continue moving in whatever speed and direction it presently has. The flip side of that railing is, if you see it speed or direction changing, there's an outside unbalanced force. There has to be. This can include zero speed, of course, at rest. When your driver accelerates the car, if you're in the car and someone floors it, which way do you feel like you're being pressed? Backwards, but which way are you clearly accelerating? Forward. Your body wants to stay where it is because your body possesses inertia. Inertia Vlad is that which resists a force. That's not the definition we're going to write there, but inertia is that thing which resists a force. This tendency that all massive bodies have to resist change in their states of motion is given a special name, inertia. You may recognize the root word inert. What are the inert gases? They don't react with any inertia. It resists reaction. It wants to stay as is. You all possess inertia. Every object in the universe that has mass has this property of inertia. What we're going to do, Vlad, is we're going to use mass and inertia interchangeably because the more mass you have, the more inertia you have, but they're technically not the same thing, but I'll let you, when you get to second or third, your physics go into the quantum details of what they really are. What object has more inertia, a bicycle, or a logging truck? What does that mean? Because it has so much more inertia, a logging truck is more difficult to get moving because it wants to resist a change, more difficult to stop because it wants to resist a change, and more difficult to turn it a corner even if you're not changing the speed. It doesn't want to change direction. How do we measure inertia? Mass, and I'm telling you that's wrong, but for now, good enough. The more atoms an object has, the more inertia it possesses. How would Newton's first law help explain how seat belts save lives? You've all grown up and you probably didn't know why it's called that. What do you yell out when you're running for the car and you want to sit in the front seat? Alexis, what do you yell out? Shotgun. Do you know where that came from? It came from the 50s. We called that the shotgun seat, the four seat belts from mandatory because in a car accident, the person in that seat was the first one out to inspect the damage. That was the joke we made. They went through the windshield, but we made light of it. We said, oh, you're the first one out to inspect the damage. You're the shotgun. That's where it comes from. So having said that, how does Newton's first law help explain how seat belts save lives? If there was not a seat belt and you hit a head-on collision, what would your body want to do? Newton's first law says it wants to keep going in a straight line at a constant speed, and the only way it can do that is to go through the windshield or go slamming into the dashboard. What does the seat belt do? More specifically, in terms of forces, what does a seat belt do? It does what Taylor did for me to black. It provides the balanced force in the opposite direction, and if your forces are balanced, well, then what's your acceleration going to be? So if the car stopped, so will you. Seat belt provides force in opposite direction. All right, Mr. Dewick's party pants. Then why don't they put seat belts on motorcycles? Okay, let's think about it in terms of physics. Why don't they put seat belts on motorcycles? Because they don't. I just want to come back to that. Mr. Dewick has a good video example of Newton's first law and a good demo too, and I'll tell you my car accident story in a second. So what would the world look like if there was no force that brought you to a stop? When you're in a car and you hit the brakes, what force is stopping you? Do you guys know? Here's a letter F. You may have heard it already. Friction. That's why on ice it's tougher to stop. It has nothing to do with how good a driver you are. Less friction. In outer space, no friction, and part of what was stopping the ball was air friction and ground friction. What would the world be like if there was no friction? We don't deal with it well. So this is a video clip. This has happened a couple of years ago in the winter. There was a street in Portland that iced over. Now ice still does have some friction, but the slant of the hill was just enough so that gravity canceled out that tiny bit of extra friction. And people had some trouble with it. Hang on. Okay, I'm trying to ask it to do too much. This is why I'm psyched about the new computer. Let's first of all close this. And I bet you I'm going to have to pause the video so those of you that are at home too bad. Back to my question. Why don't we put seatbelts on motorcycles? Yeah. We don't want you attached to the wreckage in a motorcycle because there, that has a small enough mass to tumble on top of and with you. Cars rarely roll over. Motorcycles do. In fact, what we would prefer if you've ever watched the racing, the racing motorcyclists on TV when they have an accident. First of all, they're wearing leather because they want the road to apply force to the uniform, not to their skin. They're going to get stopped by road friction. If they're not wearing leather, we call it road rash. You probably heard the expression, right? But they want to skid to a stop on the road. Ideally, you'll get bruised, maybe a broken bone, but that's a better trade off than the alternative. Okay. No, no, no, no, no. I got one more here, I think. Oh, by the way, Maggie, what'd I get with Maggie? You've seen the magicians pull a tablecloth out from underneath the dish. That's exactly how it's done. The only key is you have to make very sure you yank down on the tablecloth so that when it bends around the table, it gets pulled exactly sideways. If you accidentally lift up, you're changing the equation and the dishes will all shatter. Inertia. What's the heaviest part of your body? What's the heaviest organ in your body? This weighs more than anything. Lots of protection, big skull for that heavy brain. It will resist the force. It will want to keep going in a straight line at a constant speed. Beautiful physics. Bad planning, but beautiful physics. Beautiful physics. Okay. The law of inertia. Newton's second law, turn the page, or next page over. Newton's second law is actually the equation, and we're going to be using this equation over and over, even though this one is on your formula sheet, memorize it. But you know what? I'm not going to say memorize it. You all will because you're naturally lazy. This is one of the ones that even years later, my physics kids will remember, it's this. Force equals mass times acceleration. That actually Sir Isaac Newton phrased the equation a bit differently. He was really interested in acceleration. He got the A by itself, but everyone remembers F equals MA. F equals MA. So if your mass is bigger, bigger force, that's why if I hit something with a sledge hammer, it does more damage than if I hit it with a tiny hammer. Or, and if your A is bigger, bigger force, that's why a bullet can kill someone because it's got such a high velocity and it's go to a zero velocity and about this distance, that's a huge acceleration even though it's a small mass. Right? The worst are big mass and big acceleration. The 9-11 twin towers, the planes that hit them, the conspiracy theorists who think that there's no way those planes could have knocked those buildings down have no physics background whatsoever. If you crunch the numbers both for forces and for energy and for momentum, they're staggering. It's not surprising that the buildings went down. I'm stunned they stayed up for as long as they did. In fact, if they hadn't fallen, then I'd be wondering about government conspiracies somehow. Sadly, my little brother's a bit of a conspiracy knuckle. Oh no, the hijackers didn't bring them down. There's no way planes, oh, easily, easily. Huge mass and huge speed coming to a stop very, very fast, which means huge acceleration, humongous force. Note, this also means that one Newton equals, what do I measure mass in? What do I measure mass in? Kilograms? What do I measure acceleration in? One Newton is one kilogram meter per second squared and they wrote that for a number of years and then about a hundred years ago they decided to name that a Newton. Malcolm has a mass of 65 kilograms. What's the magnitude of the gravitational force acting on them? What mass, what gravitational acceleration? Now, you know what, we're going to say this, 9.8 and then we'll add the direction, what direction, John? No, what direction down? What is 65 times 9.8? It's going to be just below 650 because 65 times 10 is 650. What'd you get? Sorry? It's going to be really close to 650, like 646 or something? 637? 637 what? Force, what do you measure it in? By the way, I think I've told you it's worth memorizing units because it cuts down on your thinking, yes? So you'll now know force. Newton's named after a scientist whose last name was Newton. Okay, let's do some rocket science. I looked this up, the space shuttle, oh sad, it says has a mass had, they're not using the space shuttle anymore. It did its last mission this year. Let's pretend it's still present tense, let's live in the good old days. Space shuttle has a mass of 2.03 times 10 to the 6 kilograms, big mass. If each engine is capable of exerting a force of 12.5 mega Newtons, A, what's the maximum acceleration of the space shuttle? Note there are two engines. Okay, what's this question asking me to find? Not F. Oh, get the A by itself for me, John. What's the M doing to the A mathematically? Pardon me? Yeah, so how am I going to move the M over? Okay, so here's your second equation, are you going to memorize it? No, you'll derive it in your head, I think is the best way. A equals F over M. Now, the mass is 2.03 times 10 to the 6th. Yes, you're going to be dealing with some scientific notation, so this is the time for you to figure out your scientific notation button again. What's the force? Well, it's two engines, so I'm going to put a two times. What's one engine? What's mega? Does someone have their formula sheet? I can't remember what mega is. 10 to the what? 12.5 times 10 to the then 6th. Okay, good. So 2 times 12.5 times 10 to the 6th power divided by, okay, let's go to my calculator here. What's the maximum acceleration that the space shuttle can do? And that's ignoring air resistance. Scientific notation button 6 divided by 2.03 scientific notation button 6. See if you get the same as me, I'm getting 12.315, I'll probably go 12.3. Yes, the BOD, 12.3 what? Too much thinking, look up, look up. 12.3 what? Can you say that in English? Instead, we never said mth square, what do we, what a measure acceleration in? We just did a test on it folks, what a measure acceleration in? Meters per second squared, right? So kilograms, mass, acceleration, meter second squared, force, kilogram meters per second squared, Newton, what did you just say? B, this is just for us to think and do the rocket science. So initially on the launch pad, the shuttle can exert an acceleration of 12.3 meters per second squared, about a G and a half. Why does the maximum acceleration change as the engines burn? John, why would the mass change? Ah, mass decreases, so a increases. To solve that, when you have a changing mass and a changing acceleration, that's what you need calculus for. You can't do it without calculus. Calculus, Newton invented that to deal with more than one thing changing at the same time. Turn the page. How would Newton's second law help explain why a sprinter might want to wear lightweight shoes? You know what, did we just turn the page? Yeah, and does this say Newton's second law? Why don't we write out Newton's second law? What, how about right here, F equals MA. How would that explain why a sprinter would want to wear shoes that are lightweight? Is that a handout? Oh, I thought that was given a high five to Sebastian Hesin-Likey. No, okay. I have to disagree with you, you're on the right track, but I don't think it's about the force being bigger. You already said, he said, if you have less, I agree, if you have less mass, here's what I think. If you're an Olympic sprinter, have you lifted weights? Yes. Have you trained? Yes. I'm going to argue you've maxed out your force. You can't exert any bigger force because your muscles are as big as they can get without steroids. Okay, so force is maxed out. What can I control? Mass of my shoes because if this, if this is as big as I can get it, but John, this gets smaller, what will happen to my acceleration? It gets bigger and that's what a sprinter wants, right? Faster you accelerate, the quicker you're at a head start, ideally, might he be able to accelerate all the way to the finish line. So as M gets smaller, A increases. It's actually the same reason why the space shuttle's acceleration increases. When you have a force that's fixed that you can't change, and I'm sure every Olympic sprinter can't get any stronger, they've maxed their body out. What can you change? Lighter shoes. And you'll notice still in college or in high school athletics, track and field athletes, they wear shorts and tank tops. Do they wear shorts and tank tops in the Olympics? They wear those body suits, lighter mass and less air resistance because there's even that little bit of fluttering with your shorts, that slows you down at that low. You pause here, I've got something cool to show you though.