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Main content
Current time:0:00Total duration:9:32
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Video transcript

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 everybody persists in a state of being at rest so state of being at rest or moving uniformly straight forward 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 everybody 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 a I have this is something that we've seen before let's say that I have a I would say that I have a rock let's say that I have a rock someplace I have a rock someplace and it's laying on a field of grass on a field of grass I can keep observing that rock and it is unlikely to move assuming that nothing happens to it there's no force applied to that rock that rock will just stay there so the first part is pretty obvious if a body so everybody persists in a state of being at rest I'm not going to do the second part except in so far 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 or roll it or do something to it what's less intuitive about the first law is the second part everybody persist in either being in a state of rest or moving uniformly straightforward except insofar as it is compelled to change its state by force impressed so the Newton's first law and I I think I should do a little aside here because this right here is Newton 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 civilization really this is Galileo Galilei Galileo and he is the first person to formulate the law of inertia and Newton just rephrased a little bit and packaged with this other laws but he did many many many other things so you really have to give Galileo credit for Newton's first law and 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 that's 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 if you 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 have 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 a balanced force so the only way for the ice to change its 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 over it oh 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 that 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 straight forward which is another way of saying something having a constant velocity constant velocity what he's saying is is that something that has a constant velocity will continue to have 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 if I even if I were to push 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 if I throw a if I throw a tennis ball that tennis ball will eventually stop even if even if it doesn't even if it's 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 undef initely everything in human intuition says if you want something to keep going in tumult 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 and 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 for all of these fellows is that they're the 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 a very at an atomic level so if you have the actual water molecules in a lattice structure in the ice cube and then here the water molecules in a lattice structure in the on the ice on the 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 their imperfections here they bump and grind they generate a little bit of heat and they'll essentially be be working against the movement so there's a force of friction that's being applied to here and that's why stopping not only a force of friction you also have some air resistance the ice block is going to be jumping 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 because of the friction you know 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 are going to bump into it and exert an exert force to slow it down so it 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 he studied the orbits of planets and he could or at least he's probably theorized that hey maybe maybe there's no air out there and that maybe that's why these plants can just keep going round and round in orbit and there they're there I should say their speed because their direction is changing but their speed never slows down because there's no 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 SuperDuper obvious but on a whole other level it's completely not obvious especially this moving uniformly straight forward and just to make the point clear if if gravity disappeared and you had no air you threw her throw 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 another way to think about and this is an example that you might see in everyday life is if I'm in a if I'm in a airplane 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 there it's going at a constant velocity so constant velocity completely smooth no turbulence there's really no way for me to whether that airplane is moving without looking out the window let's assuming that there's no windows in that airplane it's going at a constant velocity and there's no turbulence and let's say and let's say I have I 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 it 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 you really can't tell whether you're one or the other