WEBVTT

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Let's say that I have a
huge, maybe frozen over lake,

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or maybe it's a big pond.

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So I have a huge surface of
ice over here-- my best attempt

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to draw a flat surface
of ice-- and I'm

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going to put two
blocks of ice here.

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So I'm going to put
one block of ice

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just like this, one block
of ice right over here.

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And then I'm going to put
another block of ice right

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over here.

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And then another block
of ice right over here.

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And these blocks of
ice are identical.

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They're both 5 kilograms.

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They are both 5 kilograms--
let me write this down.

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So they are both 5 kilograms.

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Or both of their masses, I
should say, are 5 kilograms.

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And the only difference
between the two

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is that relative to
the pond, this one

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is stationary-- this
one is stationary--

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and this one is moving
with a constant velocity--

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constant velocity.

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Constant velocity in the
right-wards direction.

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And let's say that
its constant velocity

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is at 5 meters per second--
5 meters per second.

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And the whole reason why I made
blocks of ice on top of ice

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is that we're going to
assume, at least for the sake

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of this video, that
friction is negligible.

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Now what does Newton's
First Law of Motion

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tell us about something that
is either not in motion--

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or you could view this as
a constant velocity of 0--

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or something that has
a constant velocity?

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Well Newton's First
Law says, well

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look, they're going to keep
their constant velocity

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or stay stationary, which is
the constant velocity of 0,

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unless there is some
unbalance, unless there

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is some net force
acting on an object.

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So let's just think
about it here.

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In either of these
situations, there

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must not be any unbalanced
force acting on them.

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Or their must not
be any net force.

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But if you think
about it, if we're

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assuming that these
things are on Earth,

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there is a net force
acting on both of them.

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Both of them are at the
surface of the Earth,

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and they both have
mass, so there

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will be the force of
gravity acting downwards

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on both of them.

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There is going to be the
downward force of gravity

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on both of these blocks of ice.

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And that downward force of
gravity, the force of gravity,

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is going to be equal to
the gravitational field

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near the surface of the
Earth, times-- which

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is a vector-- times
the mass of the object.

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So times 5 kilograms.

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This right over here is 9.8
meters per second squared.

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So you multiply that times 5.

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You get 49 kilogram meter
per second squared, which

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is the same thing as 49 newtons.

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So this is a little bit
of a conundrum here.

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Newton's First Law
says, an object at rest

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will stay at rest, or
an object in motion

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will stay in motion, unless
there is some unbalanced,

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or unless there
is some net force.

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But based on what
we've drawn right here,

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it looks like there's
some type of a net force.

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It looks like I have 49 newtons
of force pulling this thing

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downwards.

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But you say, no, no no, Sal.

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Obviously this thing won't
start accelerating downwards

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because there's ice here.

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Its resting on a big
pool of frozen water.

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And so my answer to you is,
well, if that's your answer,

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then what is the resulting
force that cancels out

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with gravity to keep
these blocks of ice,

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either one of them,
from plummeting down

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to the core of the Earth?

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From essentially
going into free fall,

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or accelerating towards
the center of the Earth?

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And you say, well, I guess if
these things would be falling,

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if not for the ice,
the ice must be

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providing the
counteracting force.

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And you are absolutely correct.

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The ice is providing
the counteracting force

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in the opposite direction.

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So the exact magnitude
of force, and it

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is in the opposite direction.

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And so if the force of gravity
on each of these blocks of ice

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are 49 newtons downwards
it is completely

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netted off by the force of
the ice on the block upwards.

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And that will be a force 49
newtons upwards in either case.

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And now, hopefully,
it makes sense

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that Newton's First
Law still holds.

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We have no net force on this
in the vertical direction,

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actually no net force on
this in either direction.

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That's why this guy
has a 0 velocity

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in the horizontal direction.

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This guy has a constant velocity
in the horizontal direction.

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And neither of them
are accelerating

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in the vertical direction.

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Because you have the force
of the ice on the block,

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the ice is supporting
the block, that's

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completely
counteracting gravity.

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And this force, in this example,
is called the normal force.

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This is the normal force--
it's 49 newtons upwards.

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This right here is
the normal force.

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And we'll talk more about the
normal force in future videos.

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The normal force
is the force, when

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anything is resting
on any surface that's

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perpendicular to that surface.

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And it's going to start
to matter a lot when

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we start thinking about
friction and all the rest.

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So what we'll see in future
videos, when you have something

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on an incline, and let's say
I have a block on an incline

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like this.

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The normal force
from the, I guess

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you could say, this
wedge on the block,

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is going to be perpendicular
to the surface.

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And if you really think
about what's happening here,

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it's fundamentally an
electromagnetic force.

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Because if you really zoomed
in on the molecules of the ice

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right over here, even better
the atoms of the ice here.

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And you really zoomed in on
the atoms or the molecules

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of the ice up here, what's
keeping this top block of ice

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from falling down
is that in order

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for it to go through its
molecules would have to kind

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of compress against, or I guess
it would have to get closer

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to, the water molecules
or the individual atoms

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in this ice down here.

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And the atoms, let me
draw it on an atomic level

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right over here.

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So maybe, let me draw one
of this guy's molecules.

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So you have an oxygen
with 2 hydrogens

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and it forms this big
lattice structure.

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And we can talk about more of
that in the chemistry playlist.

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And let's talk about this ice
as one of these molecules.

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So maybe it looks
something like this.

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And it has its 2 hydrogens

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And so what's keeping these guys
from getting compressed, what's

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keeping this block of ice
from going down further,

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is the repulsion between the
electrons in this molecule

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and the electrons
in that molecule.

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So on a macro level we view
this is kind of a contact force.

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But on a microscopic
level, on an atomic level,

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it's really just electromagnetic
repulsion at work.