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