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

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

So the first law

every body persists in its state of being at rest

or moving uniformly straight forward

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

so another way to rephrase what theyre saying

is that if theres something, every body persists

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

by force

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

and ill explain that in a second

so completely at rest

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

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

someplace

and its layuing on a field of grass

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

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

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

so ther first part is pretty obvious

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

im not gonna do the second part

except insofar there is a force being applied to it

so clearly a rock will be at rest

unless theres some force applied to it

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

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

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

so this Newton's law

and i think i should do a little side here

this right here is newton

and if this is newtons first law

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

well the reason 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,

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 other things

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

but i was in the mist of a thought

so we understand

if something is at rest, it will stay at rest

unless theres some force that acts on it

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

and the reason that they say unbalanced is that

if you have two forces that act on something

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 wont move

and the only way it will move is

if theres 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

lets take ice, cuz ice is easier to move

ice on ice

so theres ice right here

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

so once again, we're familiar with the idea

that is theres no force acting on it

the ice wont move

but what happens if i have if im 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 would be a balanced force

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 youre going to see the ice block start to really accelerate

in that direction

but i think this part is obvious

something that is in rest will stay at rest

unless it is 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 has a constant velocity

constant velocity

what he's saying is is that something

that has a constant velocity

will continue to have that constant velocity indefinately

unless it is acted on by an unbalanced force

and that is less intuitive

because everything in our human experience

even if i were to push this block of ice

eventually it will stop

it wont just keep going forever

even assuming this ice field is infinitely long

that ice will eventually stop

or if i throw a tennis ball

that tennis ball will eventually stop

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

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

we've never seen, 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 that if you want

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

more energy

into it

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

keeps burning fuel and to drive and consuming energy

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 have gone on forever

this ice block would keep 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 doesnt have a lot of friction

there is some friction between these two

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

the direction of the movement of ice

and friction really comes from

at an atomic level

if you have the water molecules in a lattice structure

in the ice cube

and then here the water molecules and the lattice structure

on the ice sea of ice that its travelling on

they do kind of bump and grind into each other

although they are both smooth

their imperfections they bump and grind

they generate a little bit of heat

and theyll essentially be working against the movement

so theres a force of friction thats being applied to here

and thats why stopping

not only 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 its definately 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 thats one force acting on it

but even once it hits the ground,

it doesnt keep rolling forever

once again because of the friction

because of the friction

especially if theres grass here

the grass is going to stop it from going

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

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

of these air particles

that are going to bump into it

and exert a force to slow it down

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

a reality where you didnt have gravity

you didnt have air

slowing things down

and they could imagine that if in that reality something

would just keep persisting in its motion

and the reason why Galileo was good at thinking about that

was that he studied the orbits of planets

and he could, or he's probably theorized

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

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

theres nothing in the space to actually slow them down

so anyway

hopefully you found that as fascinating as i do

because on some level its super duper obvious

but on another level its completely not obvious

especially this moving uniformly straight forward

and just to make the point clear

if gravity disappeared and you had no air

you throw a ball

that ball would literally keep going in that direction forever

unless some other unbalanced force acted to stop it

and this is another example you might see in everyday life

is if im in a airplane

thats going at a completely constant velocity

and theres absolutely no turbulence

so if im sitting in the airplane right over here

and there

its going at a constant velocity

completely smooth

no turbulence

theres really no way for me to tell whether

that airplane is moving without looking our the window

so assuming theres no windows on the airplane

its going at a constant velocity

and theres no turbulence

lets say i cant hear the engines

theres no way for me to sense the plane is moving

because from my frame of reference

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

thats its actually very intuitive

that theyre similar states

moving a constant velocity

or being at rest

you really cant tell

whether youre one or other