Newton's First Law of Motion

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,
"Every body persists

in a state of being at
rest, or moving uniformly

straightforward, 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--
every body 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-- and
this is something

that we've seen before.

Let's say that I have a rock.

Let's say that I
have a rock someplace

and it's laying on
a field of grass,

I can keep observing that rock.

And it is unlikely
to move, assuming

that nothing happens to it.

If there's no force
applied to that rock,

that rock will just stay there.

So the first part
is pretty obvious.

So, "Every body persists in
a state of being at rest"--

I'm not going to do
the second part--

"except insofar as 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 roll
it or do something to it.

What's less intuitive about the
first law is the second part.

"Every body persists in,"
either, "being in a state

of rest or moving
uniformly straight forward,

except insofar as
it is compelled

to change its state
by force impressed."

So this Newton's
first law-- and I

think I should do a
little aside here,

because, 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 of civilization

really, this 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,
many other things.

So you really have
to give Galileo

credit for Newton's first law.

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 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-- 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'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.

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 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, 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
straightforward, which

is another way of
saying something

having a constant velocity.

What he's saying is,
is that something

that has a constant
velocity will continue

to 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--

even 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 a tennis ball.

That tennis ball
will eventually stop.

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 indefinitely.

Everything in human intuition
says if you want something

to keep going 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 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 all

of these fellows is
that-- 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 an atomic level--

so if you have the actual
water molecules in a lattice

structure in the ice
cube, and then here are

the water molecules in a
lattice structure on the ice,

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,
there are imperfections here.

They bump and grind.

They generate a
little bit of heat.

And they'll, essentially, be
working against the movement.

So there's a force of friction
that's being applied to here.

And that's why it's stopping.

Not only a 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 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,
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 and exert

force to slow it down.

So what 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 the orbits of planets.

And he could, or at
least he's probably

theorized that, hey, maybe
there's no air out there.

And that maybe that's why
these planets can just

keep going round
and round in orbit.

And I should say their speed,
because their direction is

changing, but their
speed never slows down,

because 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 super-duper obvious.

But on a whole other level,
it's completely not obvious,

especially this moving
uniformly straightforward.

And just to make
the point clear,

if gravity disappeared,
and you had no air,

and you threw 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-- and this

is an example that you might
see in everyday life-- is,

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 it's going at a constant
velocity, completely smooth,

no turbulence.

There's really no way for me to
tell whether that airplane is

moving without looking
out the window.

Let's assume that there's
no windows in that airplane.

It's going at a
constant velocity.

And there's no turbulence.

And let's say, 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 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.

And you really can't
tell whether you

are one or the other.