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Get excited, because in this video, we're going to talk about one of the most important laws in all of science, and that is the law of conservation of energy. You'd be amazed by how much of the universe we can infer based on the law of conservation of energy, and you'll be amazed by how many holes you can poke in science fiction plots based on the law of conservation of energy. Let's just start with the language that you might typically see, and then we'll try to understand it a little bit deeper. So it tells us that the total energy of an isolated system is constant. Energy is neither created nor destroyed. It can only be transformed from one form to another or transferred from one system to another. I pretty much underlined the whole thing because it's so important. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So it tells us that the total energy of an isolated system is constant. Energy is neither created nor destroyed. It can only be transformed from one form to another or transferred from one system to another. I pretty much underlined the whole thing because it's so important. Now, to understand this, let's just think about the types of energy that we have studied. We have studied things like kinetic energy, which is the energy due to an object's motion. We have talked about potential energy, which you could view as energy due to an object's position, and that would be the case of mechanical potential energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
I pretty much underlined the whole thing because it's so important. Now, to understand this, let's just think about the types of energy that we have studied. We have studied things like kinetic energy, which is the energy due to an object's motion. We have talked about potential energy, which you could view as energy due to an object's position, and that would be the case of mechanical potential energy. If you were to combine these two types of energy together, they're known as mechanical energy. So let me put a little box around it. That is mechanical energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
We have talked about potential energy, which you could view as energy due to an object's position, and that would be the case of mechanical potential energy. If you were to combine these two types of energy together, they're known as mechanical energy. So let me put a little box around it. That is mechanical energy. And when you're first learning physics, these are the types of energies that we focus on, but there are other types of energy. There's thermal energy. You have nuclear energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
That is mechanical energy. And when you're first learning physics, these are the types of energies that we focus on, but there are other types of energy. There's thermal energy. You have nuclear energy. You have chemical energy. And so these aren't the only forms. So when we talk about the law of conservation of energy, things like kinetic energy could be transformed into chemical energy, but we're not gonna talk about those other types of energies in this video. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
You have nuclear energy. You have chemical energy. And so these aren't the only forms. So when we talk about the law of conservation of energy, things like kinetic energy could be transformed into chemical energy, but we're not gonna talk about those other types of energies in this video. So to start to appreciate this, let's first think about how mechanical energy can be conserved. So you can almost view this as a law of conservation of mechanical energy, but then we're gonna make things a little bit more complicated and see if we can trip ourselves up and see if we can somehow defy the law of conservation of energy, and be very skeptical of anyone who claims that they can defy the law of conservation of energy. So let's start with a system that contains all of the earth and a ball. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So when we talk about the law of conservation of energy, things like kinetic energy could be transformed into chemical energy, but we're not gonna talk about those other types of energies in this video. So to start to appreciate this, let's first think about how mechanical energy can be conserved. So you can almost view this as a law of conservation of mechanical energy, but then we're gonna make things a little bit more complicated and see if we can trip ourselves up and see if we can somehow defy the law of conservation of energy, and be very skeptical of anyone who claims that they can defy the law of conservation of energy. So let's start with a system that contains all of the earth and a ball. So let's just call this the earth-ball system. Earth-ball system. And when you're dealing with the law of conservation of energy, it's important to specify your system. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So let's start with a system that contains all of the earth and a ball. So let's just call this the earth-ball system. Earth-ball system. And when you're dealing with the law of conservation of energy, it's important to specify your system. And we're gonna assume that it's an isolated system, that it's not interacting much with other outside systems, things like the sun or whatever else. And so we have, I've drawn the earth here in this kind of grass flat thing. And then let me draw my ball. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And when you're dealing with the law of conservation of energy, it's important to specify your system. And we're gonna assume that it's an isolated system, that it's not interacting much with other outside systems, things like the sun or whatever else. And so we have, I've drawn the earth here in this kind of grass flat thing. And then let me draw my ball. And let's say my ball is held above the earth just like that. So while the ball is stationary, and we'll assume that there's no air here. So while the ball is stationary like that, then we have all potential energy, we could call it gravitational potential energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And then let me draw my ball. And let's say my ball is held above the earth just like that. So while the ball is stationary, and we'll assume that there's no air here. So while the ball is stationary like that, then we have all potential energy, we could call it gravitational potential energy. So it's all, and the symbol for potential energy we tend to use is u. And we could say this is gravitational potential energy. And we could say there's no kinetic energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So while the ball is stationary like that, then we have all potential energy, we could call it gravitational potential energy. So it's all, and the symbol for potential energy we tend to use is u. And we could say this is gravitational potential energy. And we could say there's no kinetic energy. No kinetic energy. If we thought about a broader system, if we talk about the solar system or something like that, then the earth is orbiting around the sun, the sun is orbiting around the central of the galaxy. But that's why we're specifying, this is the earth-ball system. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And we could say there's no kinetic energy. No kinetic energy. If we thought about a broader system, if we talk about the solar system or something like that, then the earth is orbiting around the sun, the sun is orbiting around the central of the galaxy. But that's why we're specifying, this is the earth-ball system. But what would happen if I were to let go of the ball? And especially, what is the energy profile of the ball right as it touches the ground right over here? And I'm assuming it's just going to hit the ground and just not bounce in any way. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
But that's why we're specifying, this is the earth-ball system. But what would happen if I were to let go of the ball? And especially, what is the energy profile of the ball right as it touches the ground right over here? And I'm assuming it's just going to hit the ground and just not bounce in any way. That would complicate things. Well, in that situation, all of a sudden you have no, no gravitational potential energy. But right as it touches it, not when it's stationary, right as it touches it, it's going to have a lot of kinetic energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And I'm assuming it's just going to hit the ground and just not bounce in any way. That would complicate things. Well, in that situation, all of a sudden you have no, no gravitational potential energy. But right as it touches it, not when it's stationary, right as it touches it, it's going to have a lot of kinetic energy. So all, all kinetic energy. And so what we saw, what we see here is that that potential energy all gets transferred into kinetic energy right as that ball touches the ground. Now I know what you're thinking. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
But right as it touches it, not when it's stationary, right as it touches it, it's going to have a lot of kinetic energy. So all, all kinetic energy. And so what we saw, what we see here is that that potential energy all gets transferred into kinetic energy right as that ball touches the ground. Now I know what you're thinking. But what about right after that moment? If that ball, especially if it doesn't bounce, if it just sits there, it looks like we have no energy anymore. It looks like energy has been destroyed. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
Now I know what you're thinking. But what about right after that moment? If that ball, especially if it doesn't bounce, if it just sits there, it looks like we have no energy anymore. It looks like energy has been destroyed. So my question to you is where did that energy go? Pause the video and try to think about it. So some of you might say, hey, once the ball has just, it's at rest there, well, maybe we found a case where we have defied the law of conservation of energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
It looks like energy has been destroyed. So my question to you is where did that energy go? Pause the video and try to think about it. So some of you might say, hey, once the ball has just, it's at rest there, well, maybe we found a case where we have defied the law of conservation of energy. And remember, I told you to be skeptical if anyone ever tells you that. Where the energy has gone, it's actually, it's been dissipated. It has gone into heat. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So some of you might say, hey, once the ball has just, it's at rest there, well, maybe we found a case where we have defied the law of conservation of energy. And remember, I told you to be skeptical if anyone ever tells you that. Where the energy has gone, it's actually, it's been dissipated. It has gone into heat. It would have been converted into thermal energy. So the ball and the ground would actually get that much warmer because that kinetic energy, right as it touches the ground, would be turned into thermal energy. So once again, we have not defied the law of conservation of energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
It has gone into heat. It would have been converted into thermal energy. So the ball and the ground would actually get that much warmer because that kinetic energy, right as it touches the ground, would be turned into thermal energy. So once again, we have not defied the law of conservation of energy. Now another thing you might say is, well, okay, imagine a world that there is air. So let me draw a bunch of air particles right over here. And we know that if, as a ball falls down and it goes through the air, you can consider that air resistance, some people would call that the friction due to air, well, that would slow that ball down. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So once again, we have not defied the law of conservation of energy. Now another thing you might say is, well, okay, imagine a world that there is air. So let me draw a bunch of air particles right over here. And we know that if, as a ball falls down and it goes through the air, you can consider that air resistance, some people would call that the friction due to air, well, that would slow that ball down. So maybe it would not have as much kinetic energy when it gets down here. And so it seems like energy would be destroyed in that situation. And once again, I would tell you no, the energy has not been destroyed. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And we know that if, as a ball falls down and it goes through the air, you can consider that air resistance, some people would call that the friction due to air, well, that would slow that ball down. So maybe it would not have as much kinetic energy when it gets down here. And so it seems like energy would be destroyed in that situation. And once again, I would tell you no, the energy has not been destroyed. As the ball falls down, it's going to heat up the ball and the air around it. And so once again, that air resistance, that is a dissipative force. It's going to result in the generation of thermal energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And once again, I would tell you no, the energy has not been destroyed. As the ball falls down, it's going to heat up the ball and the air around it. And so once again, that air resistance, that is a dissipative force. It's going to result in the generation of thermal energy. And if we wanted to write this in terms of equations, there's a couple of ways to write this. We could write, if we're just writing the law of conservation of mechanical energy, and we're not talking about dissipative forces, we could say that the initial kinetic energy plus the initial potential energy is going to be equal to, is going to be equal to your final kinetic energy, your final kinetic energy plus your final potential energy. Now, another way to write this exact same thing is to say that the change in kinetic energy plus the change in potential energy is going to be equal to zero, assuming we don't have any dissipative forces, and assuming that we're not converting into some of these other forms of energy, like chemical energy or thermal energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
It's going to result in the generation of thermal energy. And if we wanted to write this in terms of equations, there's a couple of ways to write this. We could write, if we're just writing the law of conservation of mechanical energy, and we're not talking about dissipative forces, we could say that the initial kinetic energy plus the initial potential energy is going to be equal to, is going to be equal to your final kinetic energy, your final kinetic energy plus your final potential energy. Now, another way to write this exact same thing is to say that the change in kinetic energy plus the change in potential energy is going to be equal to zero, assuming we don't have any dissipative forces, and assuming that we're not converting into some of these other forms of energy, like chemical energy or thermal energy. But if you wanted to include dissipative forces, dissipative forces do something called non-conservative work. They do actually negative work, because the force of, say, friction is always acting opposite in the direction of motion. So to factor that in, we could rewrite these equations. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
Now, another way to write this exact same thing is to say that the change in kinetic energy plus the change in potential energy is going to be equal to zero, assuming we don't have any dissipative forces, and assuming that we're not converting into some of these other forms of energy, like chemical energy or thermal energy. But if you wanted to include dissipative forces, dissipative forces do something called non-conservative work. They do actually negative work, because the force of, say, friction is always acting opposite in the direction of motion. So to factor that in, we could rewrite these equations. We could write that your initial kinetic energy plus your initial potential energy plus any work done by non-conservative forces, this would be like air resistance or friction, and this would be negative right over here if we're talking about, say, friction. That is going to be equal to your final kinetic energy plus your final potential energy. Or this one right over here, we could write your change, change in kinetic energy plus change in potential energy is going to be equal to the work done by dissipative forces. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So to factor that in, we could rewrite these equations. We could write that your initial kinetic energy plus your initial potential energy plus any work done by non-conservative forces, this would be like air resistance or friction, and this would be negative right over here if we're talking about, say, friction. That is going to be equal to your final kinetic energy plus your final potential energy. Or this one right over here, we could write your change, change in kinetic energy plus change in potential energy is going to be equal to the work done by dissipative forces. And remember, if we're talking about friction, dissipative forces, this right over here is going to be negative. Another way that you could have thought about this is we could have put in thermal energy. We could say that our change in kinetic energy plus our change in potential energy plus our change in thermal energy is going to be equal to zero. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
Or this one right over here, we could write your change, change in kinetic energy plus change in potential energy is going to be equal to the work done by dissipative forces. And remember, if we're talking about friction, dissipative forces, this right over here is going to be negative. Another way that you could have thought about this is we could have put in thermal energy. We could say that our change in kinetic energy plus our change in potential energy plus our change in thermal energy is going to be equal to zero. Or you can include the work done by dissipative force. And so, for example, if you saw a situation where your total change in mechanical energy right over here was negative, you're not defying the law of conservation of energy. It's not that energy was destroyed. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
We could say that our change in kinetic energy plus our change in potential energy plus our change in thermal energy is going to be equal to zero. Or you can include the work done by dissipative force. And so, for example, if you saw a situation where your total change in mechanical energy right over here was negative, you're not defying the law of conservation of energy. It's not that energy was destroyed. It's that you had this negative work done by those dissipative forces. And where did that energy go? It gets converted to thermal energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
It's not that energy was destroyed. It's that you had this negative work done by those dissipative forces. And where did that energy go? It gets converted to thermal energy. So let's do a couple of other examples just to appreciate this. So let's do a earth pendulum system here. So here, that's the earth. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
It gets converted to thermal energy. So let's do a couple of other examples just to appreciate this. So let's do a earth pendulum system here. So here, that's the earth. And then I have some type of tower. And let's say I have a pendulum here. I have a pendulum. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So here, that's the earth. And then I have some type of tower. And let's say I have a pendulum here. I have a pendulum. And at the low point, the ball just goes right over there and then it goes back up to that point. And let's say the difference in height between this point right over here and this point where it is right over there, it is equal to h. So, and let's say this is the highest point that the end of the pendulum will get to. So at this point, we are at maximum potential energy. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
I have a pendulum. And at the low point, the ball just goes right over there and then it goes back up to that point. And let's say the difference in height between this point right over here and this point where it is right over there, it is equal to h. So, and let's say this is the highest point that the end of the pendulum will get to. So at this point, we are at maximum potential energy. And right as it's about to change direction, we have no kinetic energy. It's gonna be stationary for just a moment. But then the pendulum's going to swing back. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
So at this point, we are at maximum potential energy. And right as it's about to change direction, we have no kinetic energy. It's gonna be stationary for just a moment. But then the pendulum's going to swing back. And when it gets right over here, all of that potential energy's going to be converted to kinetic energy, assuming we don't have any dissipative forces like friction slash air resistance. And then all of that kinetic energy gets converted back into potential energy. Another example that we could look at that would complicate this a little bit more is to think about an Earth spring ball system. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
But then the pendulum's going to swing back. And when it gets right over here, all of that potential energy's going to be converted to kinetic energy, assuming we don't have any dissipative forces like friction slash air resistance. And then all of that kinetic energy gets converted back into potential energy. Another example that we could look at that would complicate this a little bit more is to think about an Earth spring ball system. So that's the Earth. And let's say there's a spring right over here. And we have a ball that starts stationary. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
Another example that we could look at that would complicate this a little bit more is to think about an Earth spring ball system. So that's the Earth. And let's say there's a spring right over here. And we have a ball that starts stationary. So up here, it's all potential energy, gravitational potential energy. So all gravitational potential energy. Assuming we have no air resistance, we let go. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
And we have a ball that starts stationary. So up here, it's all potential energy, gravitational potential energy. So all gravitational potential energy. Assuming we have no air resistance, we let go. Right before it touches the spring, when we have maximum velocity, here it is going to be all kinetic energy, all kinetic energy. But then it's going to compress the spring. And assuming we don't have any thermal energy generated, you actually will always in the real world have some thermal energy generated. | Law of conservation of energy Work and energy AP Physics 1 Khan Academy.mp3 |
There are a lot of different types of forces in physics, but for the most part, all forces can be categorized as either being a contact force or a long-range force. So contact forces, as the name suggests, requires the two objects that are exerting a force on each other to be touching or in contact. So tension, the normal force, frictional forces, these are all common everyday examples of contact forces. So, you know, this wire from this crane can exert a contact force, i.e. a tension force, on the wrecking ball, but that wire can only exert that tension force on the wrecking ball if the wire is actually connected to, i.e. touching, the wrecking ball. If you forgot to tie the wire to the wrecking ball, that wire's not gonna exert any tension on the wrecking ball. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So, you know, this wire from this crane can exert a contact force, i.e. a tension force, on the wrecking ball, but that wire can only exert that tension force on the wrecking ball if the wire is actually connected to, i.e. touching, the wrecking ball. If you forgot to tie the wire to the wrecking ball, that wire's not gonna exert any tension on the wrecking ball. So these contact forces are to be distinguished from long-range forces. Sometimes these are called action-at-a-distance forces because they can be exerted on objects that are far away from each other. So gravity's a common example. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
If you forgot to tie the wire to the wrecking ball, that wire's not gonna exert any tension on the wrecking ball. So these contact forces are to be distinguished from long-range forces. Sometimes these are called action-at-a-distance forces because they can be exerted on objects that are far away from each other. So gravity's a common example. The Earth can exert a gravitational force on the Moon even though the Earth and the Moon aren't touching. So that's a long-range force. Similarly, the electric force can exert a repulsive force on two charges if they're not touching, so not a contact force. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So gravity's a common example. The Earth can exert a gravitational force on the Moon even though the Earth and the Moon aren't touching. So that's a long-range force. Similarly, the electric force can exert a repulsive force on two charges if they're not touching, so not a contact force. And magnets can attract each other even if they're not touching. So those are all long-range or action-at-a-distance forces. But I'll be honest with you here, this distinction is not nearly as fundamental as it might seem at first. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Similarly, the electric force can exert a repulsive force on two charges if they're not touching, so not a contact force. And magnets can attract each other even if they're not touching. So those are all long-range or action-at-a-distance forces. But I'll be honest with you here, this distinction is not nearly as fundamental as it might seem at first. All of these forces that we call contact forces are really just an enormous number of long-range forces in disguise. In other words, these contact forces, tension, normal force, and friction, are all arising microscopically due to a bunch of long-range forces acting over really short distances. So just because they're called long-range forces doesn't mean they can't exert force over small distances. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
But I'll be honest with you here, this distinction is not nearly as fundamental as it might seem at first. All of these forces that we call contact forces are really just an enormous number of long-range forces in disguise. In other words, these contact forces, tension, normal force, and friction, are all arising microscopically due to a bunch of long-range forces acting over really short distances. So just because they're called long-range forces doesn't mean they can't exert force over small distances. And in fact, all those forces arise, cause all these forces to arise. So let me go through and explain how all these come about. So we'll start with tension here. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So just because they're called long-range forces doesn't mean they can't exert force over small distances. And in fact, all those forces arise, cause all these forces to arise. So let me go through and explain how all these come about. So we'll start with tension here. So where does tension come from? Well, tension's the force exerted by a wire or a cable or a string, something like that. And so these strings, they're made out of atoms and molecules so I'm trying to represent that over here. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So we'll start with tension here. So where does tension come from? Well, tension's the force exerted by a wire or a cable or a string, something like that. And so these strings, they're made out of atoms and molecules so I'm trying to represent that over here. Your string is probably more than three atoms wide, but I didn't wanna have to draw an enormous number here, so imagine you've got a certain number of atoms and molecules in your string. Well, these atoms and molecules are all bonded together, chemically bonded together. Those are all electromagnetic bonds here and they don't wanna move away from their equilibrium position. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
And so these strings, they're made out of atoms and molecules so I'm trying to represent that over here. Your string is probably more than three atoms wide, but I didn't wanna have to draw an enormous number here, so imagine you've got a certain number of atoms and molecules in your string. Well, these atoms and molecules are all bonded together, chemically bonded together. Those are all electromagnetic bonds here and they don't wanna move away from their equilibrium position. They have a position and if they get displaced from there, they wanna go back to that spot. So that's what it means to be in a solid here. So this wire, if you connect a heavy load to it, like a wrecking ball, that wrecking ball's gonna try to rip these atoms and molecules apart. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Those are all electromagnetic bonds here and they don't wanna move away from their equilibrium position. They have a position and if they get displaced from there, they wanna go back to that spot. So that's what it means to be in a solid here. So this wire, if you connect a heavy load to it, like a wrecking ball, that wrecking ball's gonna try to rip these atoms and molecules apart. It's gonna try to pull them away from each other. But they don't wanna move away from each other. In other words, they try to restore themselves. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So this wire, if you connect a heavy load to it, like a wrecking ball, that wrecking ball's gonna try to rip these atoms and molecules apart. It's gonna try to pull them away from each other. But they don't wanna move away from each other. In other words, they try to restore themselves. As this distance between these atoms and molecules gets bigger, and it does, you'll stretch your string or your wire, sometimes imperceptibly, but a little bit. As these distances get bigger, that force holding them together gets bigger, so more tension force occurs. And this is the microscopic origin of that tension force. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
In other words, they try to restore themselves. As this distance between these atoms and molecules gets bigger, and it does, you'll stretch your string or your wire, sometimes imperceptibly, but a little bit. As these distances get bigger, that force holding them together gets bigger, so more tension force occurs. And this is the microscopic origin of that tension force. These atoms and molecules wanna restore themselves to their previous length, and to do that, they have to pull harder and harder. Now, this won't last forever. You hang a heavy enough load over here, you'll overwhelm these electromagnetic bonds and you'll rip these molecules apart, and that's what happens when your string breaks. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
And this is the microscopic origin of that tension force. These atoms and molecules wanna restore themselves to their previous length, and to do that, they have to pull harder and harder. Now, this won't last forever. You hang a heavy enough load over here, you'll overwhelm these electromagnetic bonds and you'll rip these molecules apart, and that's what happens when your string breaks. So that's the microscopic origin of tension, but you don't have to draw an Avogadro's number of arrows. We just represent the tension with one arrow up. It turns out you can pretty much summarize all of those microscopic electromagnetic chemical bonds with one arrow that we call tension. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
You hang a heavy enough load over here, you'll overwhelm these electromagnetic bonds and you'll rip these molecules apart, and that's what happens when your string breaks. So that's the microscopic origin of tension, but you don't have to draw an Avogadro's number of arrows. We just represent the tension with one arrow up. It turns out you can pretty much summarize all of those microscopic electromagnetic chemical bonds with one arrow that we call tension. So how about the normal force? Where does that come from? Well, this is kind of the opposite. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
It turns out you can pretty much summarize all of those microscopic electromagnetic chemical bonds with one arrow that we call tension. So how about the normal force? Where does that come from? Well, this is kind of the opposite. Tension's a pulling force. The normal force is the force that tries to prevent two objects from getting smashed into each other. So now, instead of the atoms and molecules trying to get ripped apart, the atoms and molecules in this green box here, due to its weight, are trying to get shoved into the atoms and molecules of this table, so I've tried to represent that here. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Well, this is kind of the opposite. Tension's a pulling force. The normal force is the force that tries to prevent two objects from getting smashed into each other. So now, instead of the atoms and molecules trying to get ripped apart, the atoms and molecules in this green box here, due to its weight, are trying to get shoved into the atoms and molecules of this table, so I've tried to represent that here. Again, the box and the table are made out of more than these number of atoms and molecules. But you've got your atoms and molecules of the box, atoms and molecules of the table. They won't get moved into each other. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So now, instead of the atoms and molecules trying to get ripped apart, the atoms and molecules in this green box here, due to its weight, are trying to get shoved into the atoms and molecules of this table, so I've tried to represent that here. Again, the box and the table are made out of more than these number of atoms and molecules. But you've got your atoms and molecules of the box, atoms and molecules of the table. They won't get moved into each other. There's gonna be an electron cloud around these atoms and molecules of the box, and similarly for the table, there's gonna be an electromagnetic repulsion when they try to overlap, and other quantum mechanical effects. It turns out it's surprisingly complicated to explain why matter is solid and it can't penetrate each other. But the enormous number of electromagnetic interactions and other quantum mechanical effects between these atoms and molecules are the microscopic origin of the normal force. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
They won't get moved into each other. There's gonna be an electron cloud around these atoms and molecules of the box, and similarly for the table, there's gonna be an electromagnetic repulsion when they try to overlap, and other quantum mechanical effects. It turns out it's surprisingly complicated to explain why matter is solid and it can't penetrate each other. But the enormous number of electromagnetic interactions and other quantum mechanical effects between these atoms and molecules are the microscopic origin of the normal force. So again, it's action at a distance over a small scale, which really bugs people out. They're like, wait a minute, so are two things ever actually touching? As you sit in a chair, do the atoms and molecules of your pants actually physically touch? | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
But the enormous number of electromagnetic interactions and other quantum mechanical effects between these atoms and molecules are the microscopic origin of the normal force. So again, it's action at a distance over a small scale, which really bugs people out. They're like, wait a minute, so are two things ever actually touching? As you sit in a chair, do the atoms and molecules of your pants actually physically touch? Hard to actually define what it means touching here. So you got these amorphous electron clouds. How do you define whether they're touching? | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
As you sit in a chair, do the atoms and molecules of your pants actually physically touch? Hard to actually define what it means touching here. So you got these amorphous electron clouds. How do you define whether they're touching? Hard to do, but good news, we don't have to do it. We can actually just summarize macroscopically all of these microscopic interactions as one big normal force, and that helps us both calculationally and conceptually not get too lost here. Now you might be disturbed here. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
How do you define whether they're touching? Hard to do, but good news, we don't have to do it. We can actually just summarize macroscopically all of these microscopic interactions as one big normal force, and that helps us both calculationally and conceptually not get too lost here. Now you might be disturbed here. You might be like, wait a minute, this whole video is about contact forces. You're telling me we don't even know if two surfaces are in contact? Well, I'm saying it's hard to define, but here's a good way to define it. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Now you might be disturbed here. You might be like, wait a minute, this whole video is about contact forces. You're telling me we don't even know if two surfaces are in contact? Well, I'm saying it's hard to define, but here's a good way to define it. Your pants, atoms and molecules, are contacting the seats, atom and molecules, as soon as you notice that force preventing them from moving into each other. So as soon as you can detect this normal force, that's as good a way as any to define two surfaces as being in contact. So let's look at some other forces. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Well, I'm saying it's hard to define, but here's a good way to define it. Your pants, atoms and molecules, are contacting the seats, atom and molecules, as soon as you notice that force preventing them from moving into each other. So as soon as you can detect this normal force, that's as good a way as any to define two surfaces as being in contact. So let's look at some other forces. So how about the frictional force? What are the microscopic origins of the frictional force? Well, you know, the frictional force is the force that resists two surfaces from being dragged across each other. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
So let's look at some other forces. So how about the frictional force? What are the microscopic origins of the frictional force? Well, you know, the frictional force is the force that resists two surfaces from being dragged across each other. Why is there a resistive force? Well, if you zoomed in on these surfaces, a table, no matter how smooth it looks, even if you just wiped it down, if you zoomed in close enough, you'd be shocked at all the little crevices and cracks and valleys involved, the whole world you don't know about unless you look at it microscopically. And similarly, for this purple box, maybe it's cardboard, if you zoomed in microscopically, again, it's astonishing how not smooth those surfaces are. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Well, you know, the frictional force is the force that resists two surfaces from being dragged across each other. Why is there a resistive force? Well, if you zoomed in on these surfaces, a table, no matter how smooth it looks, even if you just wiped it down, if you zoomed in close enough, you'd be shocked at all the little crevices and cracks and valleys involved, the whole world you don't know about unless you look at it microscopically. And similarly, for this purple box, maybe it's cardboard, if you zoomed in microscopically, again, it's astonishing how not smooth those surfaces are. So obviously, if you tried to drag one across the other, these are bumping into each other, these hills and valleys are running into each other, that's gonna be a problem. That's gonna cause a resistive force. You might break this yellow hill off. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
And similarly, for this purple box, maybe it's cardboard, if you zoomed in microscopically, again, it's astonishing how not smooth those surfaces are. So obviously, if you tried to drag one across the other, these are bumping into each other, these hills and valleys are running into each other, that's gonna be a problem. That's gonna cause a resistive force. You might break this yellow hill off. Sometimes they just bust off. Yep, that's gonna be a resistive cause of friction. Sometimes they don't bust off. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
You might break this yellow hill off. Sometimes they just bust off. Yep, that's gonna be a resistive cause of friction. Sometimes they don't bust off. Maybe they just like bend and bounce back, but even if they do, still gonna cause a frictional force and add to this friction. And it's not just that, but sometimes even like the atoms and molecules in the surface over here, look, this spot doesn't look too bad. Looks like they could slide across each other pretty well, but there can be adhesion, like molecular bonds that form between those atoms and molecules that are near each other that can also contribute to friction. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Sometimes they don't bust off. Maybe they just like bend and bounce back, but even if they do, still gonna cause a frictional force and add to this friction. And it's not just that, but sometimes even like the atoms and molecules in the surface over here, look, this spot doesn't look too bad. Looks like they could slide across each other pretty well, but there can be adhesion, like molecular bonds that form between those atoms and molecules that are near each other that can also contribute to friction. So again, astonishingly complicated. There's actually lots of questions to still be answered in studying friction. The study of friction is called tribology. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
Looks like they could slide across each other pretty well, but there can be adhesion, like molecular bonds that form between those atoms and molecules that are near each other that can also contribute to friction. So again, astonishingly complicated. There's actually lots of questions to still be answered in studying friction. The study of friction is called tribology. Shockingly, a lot of questions to this day, but the good news is you can summarize all of those microscopic interactions as one force we call friction that resists the two surfaces from sliding over each other. So you don't have to do a lot of calculations and microscopically zoom in on the surface. We can pretty much account for all of it by simply drawing it as one big resistive force of friction backwards. | Contact Forces Dynamics AP Physics 1 Khan Academy.mp3 |
That's my drawing of a hard, flat, frictionless surface. And on that, I have a block. And that block is not accelerating in any direction. It is just sitting there. And let's say we know the weight of that block. It is a 10 newton block. So my question to you is, pause this video and think about what are all of the forces that are acting on this block? | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
It is just sitting there. And let's say we know the weight of that block. It is a 10 newton block. So my question to you is, pause this video and think about what are all of the forces that are acting on this block? All right, now let's work through this together. And to do it, I'm going to draw what's known as a free body diagram to think about all of the forces. And the reason why it's called a free body diagram is that we just focus on this one body and we don't draw everything else around it and we just draw the forces acting on it. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
So my question to you is, pause this video and think about what are all of the forces that are acting on this block? All right, now let's work through this together. And to do it, I'm going to draw what's known as a free body diagram to think about all of the forces. And the reason why it's called a free body diagram is that we just focus on this one body and we don't draw everything else around it and we just draw the forces acting on it. And there's actually two typical ways of drawing a free body diagram. I'll do them both. So first, I could draw it this way. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And the reason why it's called a free body diagram is that we just focus on this one body and we don't draw everything else around it and we just draw the forces acting on it. And there's actually two typical ways of drawing a free body diagram. I'll do them both. So first, I could draw it this way. So this is my block here. Now, I told you that it weighs 10 newtons. The weight of an object, that's the force of gravity acting on that object and it would be downwards. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
So first, I could draw it this way. So this is my block here. Now, I told you that it weighs 10 newtons. The weight of an object, that's the force of gravity acting on that object and it would be downwards. So we have, from this 10 newtons right over here, we know that there is a downward force, the force of gravity acting on the mass of this object, of 10 newtons. It has a magnitude of 10 newtons and the force is acting downwards. We could say this is the magnitude of the force of gravity. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
The weight of an object, that's the force of gravity acting on that object and it would be downwards. So we have, from this 10 newtons right over here, we know that there is a downward force, the force of gravity acting on the mass of this object, of 10 newtons. It has a magnitude of 10 newtons and the force is acting downwards. We could say this is the magnitude of the force of gravity. And when you draw a free body diagram, it's typical to show your vectors originating out of the center of that object in your drawing. Now, my question to you is, is that the only force acting on this? If you think it is, what would happen to the object? | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
We could say this is the magnitude of the force of gravity. And when you draw a free body diagram, it's typical to show your vectors originating out of the center of that object in your drawing. Now, my question to you is, is that the only force acting on this? If you think it is, what would happen to the object? Well, it would start or it would be accelerating downwards. But I just said that this is not accelerating in any direction. So there must be something that counteracts this and there is. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
If you think it is, what would happen to the object? Well, it would start or it would be accelerating downwards. But I just said that this is not accelerating in any direction. So there must be something that counteracts this and there is. There's the normal force of this surface acting on the block. That surface is what's keeping the block from accelerating downwards. And I will do that with this vector right over here. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
So there must be something that counteracts this and there is. There's the normal force of this surface acting on the block. That surface is what's keeping the block from accelerating downwards. And I will do that with this vector right over here. So it's going to be going upwards and it's going to have the exact same magnitude just in the opposite direction. So I could say the magnitude of the normal force, the normal force's magnitude right over here is also going to be equal to 10 newtons, but it's going upwards. And we can see that these two are going to net out. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And I will do that with this vector right over here. So it's going to be going upwards and it's going to have the exact same magnitude just in the opposite direction. So I could say the magnitude of the normal force, the normal force's magnitude right over here is also going to be equal to 10 newtons, but it's going upwards. And we can see that these two are going to net out. And so you have no net force acting in this vertical dimension. And I have no forces. I haven't thought about any or drawn any in the horizontal direction. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And we can see that these two are going to net out. And so you have no net force acting in this vertical dimension. And I have no forces. I haven't thought about any or drawn any in the horizontal direction. And so that's why you have no net force in total and this thing isn't going to be accelerating. Now I mentioned that there's other ways to draw a free body diagram. Another way that you might see it is like this, where you see the body and from the outside of the body, you see the vectors being drawn. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
I haven't thought about any or drawn any in the horizontal direction. And so that's why you have no net force in total and this thing isn't going to be accelerating. Now I mentioned that there's other ways to draw a free body diagram. Another way that you might see it is like this, where you see the body and from the outside of the body, you see the vectors being drawn. So in this situation, you would have 10 newtons downward and you would have 10 newtons upward. This is another way that you might see free body diagrams drawn. Now what I wanna do is do something interesting to this block. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
Another way that you might see it is like this, where you see the body and from the outside of the body, you see the vectors being drawn. So in this situation, you would have 10 newtons downward and you would have 10 newtons upward. This is another way that you might see free body diagrams drawn. Now what I wanna do is do something interesting to this block. Let me redraw it. So I have my surface here, my hard frictionless surface, and it's flat, and I still have my block here. It's my 10 newton block. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
Now what I wanna do is do something interesting to this block. Let me redraw it. So I have my surface here, my hard frictionless surface, and it's flat, and I still have my block here. It's my 10 newton block. But now I'm gonna apply a force to it. I am going to apply a force like that is in this direction. It's in this direction and its magnitude, let's say its magnitude is 20 newtons. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
It's my 10 newton block. But now I'm gonna apply a force to it. I am going to apply a force like that is in this direction. It's in this direction and its magnitude, let's say its magnitude is 20 newtons. And just so that we know the direction, this angle right over here, let's say that that is 60 degrees. I'll say theta is equal to 60 degrees. The magnitude of this force is equal to 20 newtons. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
It's in this direction and its magnitude, let's say its magnitude is 20 newtons. And just so that we know the direction, this angle right over here, let's say that that is 60 degrees. I'll say theta is equal to 60 degrees. The magnitude of this force is equal to 20 newtons. So what would the free body diagram now look like? Well, it might be tempting to just draw the force right on one of these free body diagrams, something like that, something like that. And that would not be inaccurate, but we would have to watch out because this force is acting at an angle, so if we were to break it up into its horizontal and vertical components, some part of that force is acting downward. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
The magnitude of this force is equal to 20 newtons. So what would the free body diagram now look like? Well, it might be tempting to just draw the force right on one of these free body diagrams, something like that, something like that. And that would not be inaccurate, but we would have to watch out because this force is acting at an angle, so if we were to break it up into its horizontal and vertical components, some part of that force is acting downward. And so you're actually going to have a larger normal force to counteract that. And some other component is going to be working horizontally. And so what we wanna do is actually break things up because if you leave it in this angle, it gets very, very confusing. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And that would not be inaccurate, but we would have to watch out because this force is acting at an angle, so if we were to break it up into its horizontal and vertical components, some part of that force is acting downward. And so you're actually going to have a larger normal force to counteract that. And some other component is going to be working horizontally. And so what we wanna do is actually break things up because if you leave it in this angle, it gets very, very confusing. So what I wanna do is I wanna break up this new blue force into its horizontal and vertical components. And to do so, we just have to remember a little bit of our basic trigonometry. If I have a force like this, if I have a force like this, and it is acting at an angle theta right over here with the horizontal, and I wanna break it up into its horizontal and its vertical components, and its vertical components, if the magnitude of the hypotenuse is capital F, then the magnitude of the adjacent side to this angle, this comes straight out of Sohcah Toah from our right triangle trigonometry, it would be the magnitude of our hypotenuse times the cosine of this angle. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And so what we wanna do is actually break things up because if you leave it in this angle, it gets very, very confusing. So what I wanna do is I wanna break up this new blue force into its horizontal and vertical components. And to do so, we just have to remember a little bit of our basic trigonometry. If I have a force like this, if I have a force like this, and it is acting at an angle theta right over here with the horizontal, and I wanna break it up into its horizontal and its vertical components, and its vertical components, if the magnitude of the hypotenuse is capital F, then the magnitude of the adjacent side to this angle, this comes straight out of Sohcah Toah from our right triangle trigonometry, it would be the magnitude of our hypotenuse times the cosine of this angle. And the magnitude of the vertical component, that would be the magnitude of our hypotenuse times the sine of that angle. And you could think about it the other way as well. If the force was like this, where it's just going in the opposite direction, but once again, you have this angle theta, and now the components would look like this, where the vertical component would have the same magnitude, but now it would be pointing downwards, and the horizontal component would have the same magnitude, but now it is pointing to the left, it is the same idea. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
If I have a force like this, if I have a force like this, and it is acting at an angle theta right over here with the horizontal, and I wanna break it up into its horizontal and its vertical components, and its vertical components, if the magnitude of the hypotenuse is capital F, then the magnitude of the adjacent side to this angle, this comes straight out of Sohcah Toah from our right triangle trigonometry, it would be the magnitude of our hypotenuse times the cosine of this angle. And the magnitude of the vertical component, that would be the magnitude of our hypotenuse times the sine of that angle. And you could think about it the other way as well. If the force was like this, where it's just going in the opposite direction, but once again, you have this angle theta, and now the components would look like this, where the vertical component would have the same magnitude, but now it would be pointing downwards, and the horizontal component would have the same magnitude, but now it is pointing to the left, it is the same idea. If this force has magnitude F that's represented by the hypotenuse of this triangle, then the magnitude of our horizontal component is still going to be F cosine theta, the vector's not going in the other direction, and the magnitude of our vertical component here is going to be F sine theta. And so what about this scenario over here? Well, in this scenario, our vertical component is gonna look like this, and our horizontal component is going to look like this. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
If the force was like this, where it's just going in the opposite direction, but once again, you have this angle theta, and now the components would look like this, where the vertical component would have the same magnitude, but now it would be pointing downwards, and the horizontal component would have the same magnitude, but now it is pointing to the left, it is the same idea. If this force has magnitude F that's represented by the hypotenuse of this triangle, then the magnitude of our horizontal component is still going to be F cosine theta, the vector's not going in the other direction, and the magnitude of our vertical component here is going to be F sine theta. And so what about this scenario over here? Well, in this scenario, our vertical component is gonna look like this, and our horizontal component is going to look like this. And so what's the magnitude of our horizontal component? Well, it's going to be the magnitude of our hypotenuse times the cosine of the 60 degrees. So this is going to be 20 newtons times the cosine of 60 degrees. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
Well, in this scenario, our vertical component is gonna look like this, and our horizontal component is going to look like this. And so what's the magnitude of our horizontal component? Well, it's going to be the magnitude of our hypotenuse times the cosine of the 60 degrees. So this is going to be 20 newtons times the cosine of 60 degrees. And it's really helpful in both trigonometry and physics classes to know the values of your sines, your cosines, and your tangents at angles like zero degrees, 30 degrees, 60 degrees, 90 degrees, 45 degrees. You could use a calculator here, but it's useful to know that the cosine of 60 degrees is 1 1⁄2, so 20 times 1 1⁄2, this is going to be equal to 10 newtons. And if we wanna know the magnitude of our vertical component, well, this is going to be 20 newtons times the sine of 60 degrees. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
So this is going to be 20 newtons times the cosine of 60 degrees. And it's really helpful in both trigonometry and physics classes to know the values of your sines, your cosines, and your tangents at angles like zero degrees, 30 degrees, 60 degrees, 90 degrees, 45 degrees. You could use a calculator here, but it's useful to know that the cosine of 60 degrees is 1 1⁄2, so 20 times 1 1⁄2, this is going to be equal to 10 newtons. And if we wanna know the magnitude of our vertical component, well, this is going to be 20 newtons times the sine of 60 degrees. Once again, this is useful to know. It is square root of three over two. And so 20 divided by two is 10. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And if we wanna know the magnitude of our vertical component, well, this is going to be 20 newtons times the sine of 60 degrees. Once again, this is useful to know. It is square root of three over two. And so 20 divided by two is 10. So this is going to be 10 square roots of three newtons. And so we can use that information. We've broken up our original vector into two component vectors that if you took their sum, you'd get your original one. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
And so 20 divided by two is 10. So this is going to be 10 square roots of three newtons. And so we can use that information. We've broken up our original vector into two component vectors that if you took their sum, you'd get your original one. But what's useful now is that we've broken it up into vectors that are parallel or perpendicular to our surface, which will allow us to think about what nets against these things that I already have in place. So let me draw that. So actually, I'll first draw this type of free body diagram. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
We've broken up our original vector into two component vectors that if you took their sum, you'd get your original one. But what's useful now is that we've broken it up into vectors that are parallel or perpendicular to our surface, which will allow us to think about what nets against these things that I already have in place. So let me draw that. So actually, I'll first draw this type of free body diagram. So there is my block. And I have the force of gravity acting on it downwards. I will draw it right over here. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
So actually, I'll first draw this type of free body diagram. So there is my block. And I have the force of gravity acting on it downwards. I will draw it right over here. So that's 10 newtons. That is the force of gravity acting downwards. Now, is that the only thing that I have acting downwards? | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
I will draw it right over here. So that's 10 newtons. That is the force of gravity acting downwards. Now, is that the only thing that I have acting downwards? No, I also have the vertical component of this applied force. And so this is going downwards 10 square roots of three newtons and these aren't drawn perfectly to scale, but hopefully you get the idea. So this is 10 square roots of three newtons, just like that. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
Now, is that the only thing that I have acting downwards? No, I also have the vertical component of this applied force. And so this is going downwards 10 square roots of three newtons and these aren't drawn perfectly to scale, but hopefully you get the idea. So this is 10 square roots of three newtons, just like that. And now what is our normal force? Assuming that our surface is able to not be compressed in any way, that it is a hard, frictionless surface. Well, now our normal force is going to counteract both of these forces. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
So this is 10 square roots of three newtons, just like that. And now what is our normal force? Assuming that our surface is able to not be compressed in any way, that it is a hard, frictionless surface. Well, now our normal force is going to counteract both of these forces. Our normal force might look something like this. Once again, I haven't drawn it completely to scale, but this would be 10 plus 10 square roots of three newtons. And what about now in the horizontal direction? | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
Well, now our normal force is going to counteract both of these forces. Our normal force might look something like this. Once again, I haven't drawn it completely to scale, but this would be 10 plus 10 square roots of three newtons. And what about now in the horizontal direction? Well, I have this blue vector right here, and so that is going to the right with a magnitude of 10 newtons, 10 newtons. And so now you can hopefully appreciate why a free body diagram is really, really, really useful. Just looking at this, I can predict what's going to happen to my block. | Breaking down forces for free body diagrams AP Physics 1 Khan Academy.mp3 |
We should talk a little more about Newton's third law because there are some deep misconceptions that many people have about this law. It seems simple, but it's not nearly as simple as you might think. So people often phrase it as, for every action, there's an equal and opposite reaction, but that's just way too vague to be useful. So a version that's a little better says that for every force, there's an equal and opposite force. So this is a little better. The equal sign means that these forces are equal in magnitude. And this negative sign means that they're just different by the direction of the vector. | More on Newton's third law Forces and Newton's laws of motion Physics Khan Academy.mp3 |
So a version that's a little better says that for every force, there's an equal and opposite force. So this is a little better. The equal sign means that these forces are equal in magnitude. And this negative sign means that they're just different by the direction of the vector. So these are vectors. So this says that this pink vector F has the opposite direction, but equal in magnitude to this green vector F. But to show you why this is still a little bit too vague, consider this. If this is all you knew about Newton's third law, that for every force, there's an equal and opposite force, you might wonder, if you were clever, you might be like, wait a minute. | More on Newton's third law Forces and Newton's laws of motion Physics Khan Academy.mp3 |
And this negative sign means that they're just different by the direction of the vector. So these are vectors. So this says that this pink vector F has the opposite direction, but equal in magnitude to this green vector F. But to show you why this is still a little bit too vague, consider this. If this is all you knew about Newton's third law, that for every force, there's an equal and opposite force, you might wonder, if you were clever, you might be like, wait a minute. If for every force F, right, there's got to be a force that's equal and opposite, well, why doesn't that just mean that every force in the universe cancels? Shouldn't every force just cancel then at that point? Doesn't that just mean that there's no acceleration that's even possible? | More on Newton's third law Forces and Newton's laws of motion Physics Khan Academy.mp3 |
If this is all you knew about Newton's third law, that for every force, there's an equal and opposite force, you might wonder, if you were clever, you might be like, wait a minute. If for every force F, right, there's got to be a force that's equal and opposite, well, why doesn't that just mean that every force in the universe cancels? Shouldn't every force just cancel then at that point? Doesn't that just mean that there's no acceleration that's even possible? Because if I go and exert a force F on something, if there's gonna be a force negative F, doesn't that mean that no matter what force I put forward, it's just gonna get canceled? And the answer is no. And the reason it's no is because these two forces are exerted on different objects. | More on Newton's third law Forces and Newton's laws of motion Physics Khan Academy.mp3 |
Doesn't that just mean that there's no acceleration that's even possible? Because if I go and exert a force F on something, if there's gonna be a force negative F, doesn't that mean that no matter what force I put forward, it's just gonna get canceled? And the answer is no. And the reason it's no is because these two forces are exerted on different objects. So you have to be careful. So the reason I say that this statement of Newton's third law is still a little bit too vague is because this is really on different objects. So if this is the force on object A exerted by object B, then this force over here has to be the force on object B exerted by object A. | More on Newton's third law Forces and Newton's laws of motion Physics Khan Academy.mp3 |
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