-
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