WEBVTT

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In this video i want to talk a little bit about Newton's first law of motion

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This is a translation from Newton's principia from latin into english

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So the first law

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every body persists in its state of being at rest

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or moving uniformly straight forward

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except insofar as it is complled to change its state by force impressed

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so another way to rephrase what theyre saying

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is that if theres something, every body persists

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so everything will stay at rest or moving at constant velocity unless it is compelled to change its state

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by force

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unless its acted on by a force, especially an unbalanced force

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and ill explain that in a second

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so completely at rest

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so i have a, this is something we've seen before

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so i have a, let's say that i have a rock

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someplace

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and its layuing on a field of grass

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on a field of grass, and i can keep observing that rock

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and it is unlikely to move assuming that nothing happens to it

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theres no force applied to that rock, that rock will just stay there

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so ther first part is pretty obvious

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if a body, "every body persists in a state of being at rest"

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im not gonna do the second part

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except insofar there is a force being applied to it

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so clearly a rock will be at rest

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unless theres some force applied to it

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what's less intuitive about the first law is the second part

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"every body persists in either being at rest or moving uniformly straight forward"

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except insofar it is compelled to change its state by force impressed

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so this Newton's law

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and i think i should do a little side here

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this right here is newton

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and if this is newtons first law

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why do i have this huge picture of this guy over here?

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well the reason is because Newton's first law is

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really just a restatement of this guy's law of inertia

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and this guy, another titan of civilization really,

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is Galileo Galilei

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and he is the first person to formulate the law of inertia

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and newton just rephrased it a little bit

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and packaged it with his other laws

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but he did many many other things

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so you really have to give galileo credit for newtons first law

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but i was in the mist of a thought

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so we understand

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if something is at rest, it will stay at rest

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unless theres some force that acts on it

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and in some definitions you'll see if theres some unbalanced force

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and the reason that they say unbalanced is that

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if you have two forces that act on something

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they might balance out

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for example, i could push on this side of the rock

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with a certain amount of force

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and if you push on this side of the rock with the exact

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same amount of force

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the rock wont move

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and the only way it will move is

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if theres a lot more force on one side than on the other side

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so if you have an unbalanced force

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so if you have a ton of

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and maybe the rock is a bad analogy

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lets take ice, cuz ice is easier to move

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ice on ice

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so theres ice right here

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and i have another block of ice sitting on top of that ice

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so once again, we're familiar with the idea

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that is theres no force acting on it

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the ice wont move

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but what happens if i have if im pushing on the ice with

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a certain amount of force on that side and you're pushing on the ice

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on that side with the same amount of force

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the ice will still not move

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so this right here would be a balanced force

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a balanced force

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so the only way for the ice to change its condition

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to change its restful condition

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is if the force is unbalanced

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so if we add a little bit of force on this side

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so it more than compensates the force pushing it this way

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then youre going to see the ice block start to really accelerate

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in that direction

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but i think this part is obvious

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something that is in rest will stay at rest

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unless it is being acted on by an unbalanced force

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what's less obvious is the idea that

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something moving uniformly straight forward

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which is another way of saying something has a constant velocity

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constant velocity

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what he's saying is is that something

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that has a constant velocity

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will continue to have that constant velocity indefinately

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unless it is acted on by an unbalanced force

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and that is less intuitive

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because everything in our human experience

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even if i were to push this block of ice

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eventually it will stop

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it wont just keep going forever

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even assuming this ice field is infinitely long

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that ice will eventually stop

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or if i throw a tennis ball

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that tennis ball will eventually stop

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even if it doesnt, it will eventually grind to a half

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or if i roll a bowling ball, or if i, anything

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we've never seen, in our human experience

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it looks like everything will eventually stop

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so this is a very unintuitive thing to say that something

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in motion will just keep going in motion indefinitely

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everything in human intuition says that if you want

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something to keep going in motion, you have to keep putting more force

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more energy

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into it

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your car wont keep going forever, unless you keep, unless the engine

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keeps burning fuel and to drive and consuming energy

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all of these examples

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and i think this is actually a pretty brilliant insight

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from all of these fellows

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is that all of these things would have gone on forever

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the ball would have gone on forever

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this ice block would keep going on forever

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except for the fact that there are

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unbalanced forces

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acting on them to stop them

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so in the case of ice

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even though ice on ice doesnt have a lot of friction

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there is some friction between these two

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so in this case the force of friction is going to be acting against

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the direction of the movement of ice

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and friction really comes from

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at an atomic level

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if you have the water molecules in a lattice structure

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in the ice cube

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and then here the water molecules and the lattice structure

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on the ice sea of ice that its travelling on

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they do kind of bump and grind into each other

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although they are both smooth

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their imperfections they bump and grind

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they generate a little bit of heat

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and theyll essentially be working against the movement

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so theres a force of friction thats being applied to here

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and thats why stopping

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not only force of friction, you also have some air resistance

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the ice block is going to be bumping into all sorts of

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air particles, it might not be noticeable at first

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but its definately going to keep it from going

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on forever

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same thing with the ball being tossed to the air

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obviously at some point it hits the ground

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because of gravity

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so thats one force acting on it

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but even once it hits the ground,

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it doesnt keep rolling forever

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once again because of the friction

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because of the friction

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especially if theres grass here

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the grass is going to stop it from going

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and even while its in the air, its going to slow down

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its not going to have constant velocity, because you have all

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of these air particles

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that are going to bump into it

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and exert a force to slow it down

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so what is really brilliant about these guys is that they could imagine

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a reality where you didnt have gravity

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you didnt have air

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slowing things down

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and they could imagine that if in that reality something

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would just keep persisting in its motion

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and the reason why Galileo was good at thinking about that

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was that he studied the orbits of planets

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and he could, or he's probably theorized

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"maybe theres no air out there, maybe thats why these planets can just keep going"

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round and round in orbit, and their speed never slows down

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theres nothing in the space to actually slow them down

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so anyway

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hopefully you found that as fascinating as i do

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because on some level its super duper obvious

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but on another level its completely not obvious

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especially this moving uniformly straight forward

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and just to make the point clear

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if gravity disappeared and you had no air

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you throw a ball

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that ball would literally keep going in that direction forever

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unless some other unbalanced force acted to stop it

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and this is another example you might see in everyday life

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is if im in a airplane

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thats going at a completely constant velocity

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and theres absolutely no turbulence

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so if im sitting in the airplane right over here

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and there

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its going at a constant velocity

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completely smooth

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no turbulence

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theres really no way for me to tell whether

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that airplane is moving without looking our the window

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so assuming theres no windows on the airplane

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its going at a constant velocity

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and theres no turbulence

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lets say i cant hear the engines

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theres no way for me to sense the plane is moving

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because from my frame of reference

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it looks completely identical to if i was in the same plane that was resting on the ground

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thats its actually very intuitive

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that theyre similar states

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moving a constant velocity

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or being at rest

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you really cant tell

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whether youre one or other