Newton's First Law of Motion, also known as the Law of Inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law, originally formulated by Galileo, is fundamental to understanding motion and forces. Created by Sal Khan.
Want to join the conversation?
- my physics textbook says that if a bus is moving then the people sitting or standing in the bus also move in the with the same speed in the same direction .but according to me how can the people also be moving? and further my book states that when the bus stops suddenly , the upper body still continues to move and the lower body tens to move turn with the bus which causes the passengers to fall forward.
but it isn't clear to me that how is the upper body in motion?
can someone explain!<its stated under the examples of inertia>(11 votes)
- The people are at rest but they are in motion with respect to the ground. If the people were not in motion how will they arrive at their destination?
It is your feet which keeps u in contact with the bus ..right. If the bus stops then your feet will also come to rest. but the tendency of inertia the upper body keeps moving to avoid that sudden jerk of coming at rest.(23 votes)
- At8:45when we were talking about the plane example......we said that if the plane was moving or at rest and we were inside it, we wont realize that whether the plane is movinr or is at rest.......However even if the plane was accelerating with we being inside it......wont we still feel that we are at rest? Isn't it that both our velocity and the acceleration w.r.t. plane zero?(15 votes)
- We actually cannot say, because there is no force acting on us. We have the same velocity as the plane, which is our plain of reference. Thus unless we have a window, we cannot say for certain.(4 votes)
- I know this video was posted a while ago and people probably won't notice it but at7:50, Sal corrected himself and said that the directions are changing. How are the directions changing? Is Earth changing direction? Someone, please answer I am genuinely confused on this subject.(5 votes)
- This a question, i have been holding for a while. In a Traffic collision, between a car and a truck, which is a way bigger than the car, both will feel the impact in the same way?(4 votes)
- Both will experience the same force. But that force will result in much more acceleration for the car than the truck, because the car's mass is much less.(3 votes)
- Does Newton's 1st law also apply to "reading a book in a car?" Like when the car is moving and you're staring at a book that is "still" and you all of sudden get motion sickness?(1 vote)
- If an object is moving with constant velocity, with no unbalanced force applied, i.e. 5 m/s, can I assume that after 2 seconds , or 5 seconds, or 10 seconds, when I measure its velocity, it would still come out to be 5 m/s?(3 votes)
- If there is no net force (same as no unbalanced force) on an object it will have a constant velocity and which means that for as long as there is not net force it will measure the same velocity.(4 votes)
- when is the force of friction maximum at rest or at motion.(3 votes)
- Max friction occurs at rest. When motion begins it turns from static to kinectic which is always less. At max Friction Force = the coefficient of friction x FN(4 votes)
- when the ice melts and starts to slide on the water, what is the unbalanced causing it to move?(3 votes)
- The ice will move because as it melts the frictional force will get smaller. This will cause an unbalanced force and allow the ice to move. You can calculate stuff like this by learning to draw Free Body Diagrams (FBD's) that show all the forces acting on the object (ice). The frictional force in this case would be the product of μ static which is the static friction coefficient and the Normal force which is the force pushing against the surface of the ice.(3 votes)
- I think first law should be as " A body continues its state of rest of motion unless acted by external net force t̲h̲a̲t̲ u̲n̲b̲a̲l̲a̲n̲c̲e̲s̲ t̲h̲e̲ f̲o̲r̲c̲e̲ o̲n̲ t̲h̲e̲ b̲o̲d̲y̲"(3 votes)
In this video, I want to talk a little bit about Newton's First Law of Motion. And this is a translation from Newton's Principia from Latin into English. So the First Law, "Every body persists in a state of being at rest, or moving uniformly straightforward, except insofar as it is compelled to change its state by force impressed." So another way to rephrase what they're saying is, that if there's something-- every body persists-- so everything will stay at rest, or moving with a constant velocity, unless it is compelled to change its state by force. Unless it's acted on by a force, especially an unbalanced force. and I'll explain that in a second. So if I have something that's at rest, so completely at rest. So I have-- and this is something that we've seen before. Let's say that I have a rock. Let's say that I have a rock someplace and it's laying on a field of grass, I can keep observing that rock. And it is unlikely to move, assuming that nothing happens to it. If there's no force applied to that rock, that rock will just stay there. So the first part is pretty obvious. So, "Every body persists in a state of being at rest"-- I'm not going to do the second part-- "except insofar as there's some force being applied to it." So clearly a rock will be at rest, unless there's some force applied to it, unless someone here tries to push it or roll it or do something to it. What's less intuitive about the first law is the second part. "Every body persists in," either, "being in a state of rest or moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." So this Newton's first law-- and I think I should do a little aside here, because, this right here is Newton. And if this is Newton's first law, why do I have this huge picture of this guy over here? Well, the reason is is because Newton's first law is really just a restatement of this guy's law of inertia. And this guy, another titan of civilization really, this is Galileo Galilei. And he is the first person to formulate the law of inertia. And Newton just rephrased it a little bit and packaged it with his other laws. But he did many, many, many other things. So you really have to give Galileo credit for Newton's first law. So that's why I made him bigger than here. But I was in the midst of a thought. So we understand if something is at rest, it's going to stay at rest, unless there's some force that acts on it. And in some definitions, you'll see unless there's some unbalanced force. And the reason why they say unbalanced is, because you could have two forces that act on something and they might balance out. For example, I could push on this side of the rock with a certain amount of force. And if you push on this side of the rock with the exact same amount of force, the rock won't move. And the only way that it would move if there's a lot more force on one side than on the other side, so if you have an unbalanced force. So if you have a ton of-- and maybe the rock is a bad analogy. Let's take ice, because ice is easier to move, or ice on ice. So there's ice right here. And then, I have another block of ice sitting on top of that ice. So once again, we're familiar with the idea, if there's no force acting on it that ice won't move. But what happens if I'm pushing on the ice with a certain amount of force on that side, and you're pushing on the ice on that side with the same amount of force? The ice will still not move. So this right here, this would be a balanced force. So the only way for the ice to change its condition, to change its restful condition is if the force is unbalanced. So if we add a little bit of force on this side, so it more than compensates the force pushing it this way, then you're going to see the ice block start to move, start to really accelerate in that direction. But I think this part is obvious. This, you know, something that's at rest will stay at rest, unless it's being acted on by an unbalanced force. What's less obvious is the idea that something moving uniformly straightforward, which is another way of saying something having a constant velocity. What he's saying is, is that something that has a constant velocity will continue to have that constant velocity indefinitely, unless it is acted on by an unbalanced force. And that's less intuitive. Because everything in our human experience-- even if I were to push this block of ice, eventually it'll stop. It won't just keep going forever, even assuming that this ice field is infinitely long, that ice will eventually stop. Or if I throw a tennis ball. That tennis ball will eventually stop. It'll eventually grind to a halt. Or if I roll a bowling ball, or if I, anything. We've never seen, at least in our human experience, it looks like everything will eventually stop. So this is a very unintuitive thing to say, that something in motion will just keep going in motion indefinitely. Everything in human intuition says if you want something to keep going in motion, you have to keep putting more force, keep putting more energy into it for it to keep going. Your car won't go forever, unless you keep, unless the engine keeps burning fuel to drive and consuming energy. So what are they talking about? Well, in all of these examples-- and I think this is actually a pretty brilliant insight from all of these fellows is that-- all of these things would have gone on forever. The ball would keep going forever. This ice block would be going on forever, except for the fact that there are unbalanced forces acting on them to stop them. So in the case of ice, even though ice on ice doesn't have a lot of friction, there is some friction between these two. And so you have, in this situation, the force of friction is going to be acting against the direction of the movement of the ice. And friction really comes from, at an atomic level-- so if you have the actual water molecules in a lattice structure in the ice cube, and then here are the water molecules in a lattice structure on the ice, on the actual kind of sea of ice that it's traveling on-- they do kind of bump and grind into each other. Although they're both smooth, there are imperfections here. They bump and grind. They generate a little bit of heat. And they'll, essentially, be working against the movement. So there's a force of friction that's being applied to here. And that's why it's stopping. Not only a force of friction, you also have some air resistance. The ice block is going to be bumping into all sorts of air particles. It might not be noticeable at first, but it's definitely going to keep it from going on forever. Same thing with the ball being tossed to the air. Obviously, at some point, it hits the ground because of gravity. So that's one force acting on it. But even once it hits the ground, it doesn't keep rolling forever, once again, because of the friction, especially if there's grass here. The grass is going to stop it from going. And even while it's in the air, it's going to slow down. It's not going to have a constant velocity. Because you have all of these air particles that are going to bump into it and exert force to slow it down. So what was really brilliant about these guys is that they could imagine a reality where you didn't have gravity, where you did not have air slowing things down. And they could imagine that in that reality, something would just keep persisting in its motion. And the reason why Galileo, frankly, was probably good at thinking about that is that he studied the orbits of planets. And he could, or at least he's probably theorized that, hey, maybe there's no air out there. And that maybe that's why these planets can just keep going round and round in orbit. And I should say their speed, because their direction is changing, but their speed never slows down, because there's nothing in the space to actually slow down those planets. So anyway, hopefully you found that as fascinating as I do. Because on some level, it's super-duper obvious. But on a whole other level, it's completely not obvious, especially this moving uniformly straightforward. And just to make the point clear, if gravity disappeared, and you had no air, and you threw a ball, that ball literally would keep going in that direction forever, unless some other unbalanced force acted to stop it. And another way to think about it-- and this is an example that you might see in everyday life-- is, if I'm in an airplane that's going at a completely constant velocity and there's absolutely no turbulence in the airplane. So if I'm sitting in the airplane right over here. And it's going at a constant velocity, completely smooth, no turbulence. There's really no way for me to tell whether that airplane is moving without looking out the window. Let's assume that there's no windows in that airplane. It's going at a constant velocity. And there's no turbulence. And let's say, I can't hear anything. So I can't even hear the engines. There's no way for me to sense that the plane is moving. Because from my frame of reference, it looks completely identical to if I was in that same plane that was resting on the ground. And that's another way to think about it. That it's actually very intuitive that they're similar states, moving at a constant velocity or being at rest. And you really can't tell whether you are one or the other.