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Let's say that I have a huge, maybe frozen over lake,
or maybe it's a big pond.
So I have a huge surface of ice over here-- my best attempt
to draw a flat surface of ice-- and I'm
going to put two blocks of ice here.
So I'm going to put one block of ice
just like this, one block of ice right over here.
And then I'm going to put another block of ice right
over here.
And then another block of ice right over here.
And these blocks of ice are identical.
They're both 5 kilograms.
They are both 5 kilograms-- let me write this down.
So they are both 5 kilograms.
Or both of their masses, I should say, are 5 kilograms.
And the only difference between the two
is that relative to the pond, this one
is stationary-- this one is stationary--
and this one is moving with a constant velocity--
constant velocity.
Constant velocity in the right-wards direction.
And let's say that its constant velocity
is at 5 meters per second-- 5 meters per second.
And the whole reason why I made blocks of ice on top of ice
is that we're going to assume, at least for the sake
of this video, that friction is negligible.
Now what does Newton's First Law of Motion
tell us about something that is either not in motion--
or you could view this as a constant velocity of 0--
or something that has a constant velocity?
Well Newton's First Law says, well
look, they're going to keep their constant velocity
or stay stationary, which is the constant velocity of 0,
unless there is some unbalance, unless there
is some net force acting on an object.
So let's just think about it here.
In either of these situations, there
must not be any unbalanced force acting on them.
Or their must not be any net force.
But if you think about it, if we're
assuming that these things are on Earth,
there is a net force acting on both of them.
Both of them are at the surface of the Earth,
and they both have mass, so there
will be the force of gravity acting downwards
on both of them.
There is going to be the downward force of gravity
on both of these blocks of ice.
And that downward force of gravity, the force of gravity,
is going to be equal to the gravitational field
near the surface of the Earth, times-- which
is a vector-- times the mass of the object.
So times 5 kilograms.
This right over here is 9.8 meters per second squared.
So you multiply that times 5.
You get 49 kilogram meter per second squared, which
is the same thing as 49 newtons.
So this is a little bit of a conundrum here.
Newton's First Law says, an object at rest
will stay at rest, or an object in motion
will stay in motion, unless there is some unbalanced,
or unless there is some net force.
But based on what we've drawn right here,
it looks like there's some type of a net force.
It looks like I have 49 newtons of force pulling this thing
downwards.
But you say, no, no no, Sal.
Obviously this thing won't start accelerating downwards
because there's ice here.
Its resting on a big pool of frozen water.
And so my answer to you is, well, if that's your answer,
then what is the resulting force that cancels out
with gravity to keep these blocks of ice,
either one of them, from plummeting down
to the core of the Earth?
From essentially going into free fall,
or accelerating towards the center of the Earth?
And you say, well, I guess if these things would be falling,
if not for the ice, the ice must be
providing the counteracting force.
And you are absolutely correct.
The ice is providing the counteracting force
in the opposite direction.
So the exact magnitude of force, and it
is in the opposite direction.
And so if the force of gravity on each of these blocks of ice
are 49 newtons downwards it is completely
netted off by the force of the ice on the block upwards.
And that will be a force 49 newtons upwards in either case.
And now, hopefully, it makes sense
that Newton's First Law still holds.
We have no net force on this in the vertical direction,
actually no net force on this in either direction.
That's why this guy has a 0 velocity
in the horizontal direction.
This guy has a constant velocity in the horizontal direction.
And neither of them are accelerating
in the vertical direction.
Because you have the force of the ice on the block,
the ice is supporting the block, that's
completely counteracting gravity.
And this force, in this example, is called the normal force.
This is the normal force-- it's 49 newtons upwards.
This right here is the normal force.
And we'll talk more about the normal force in future videos.
The normal force is the force, when
anything is resting on any surface that's
perpendicular to that surface.
And it's going to start to matter a lot when
we start thinking about friction and all the rest.
So what we'll see in future videos, when you have something
on an incline, and let's say I have a block on an incline
like this.
The normal force from the, I guess
you could say, this wedge on the block,
is going to be perpendicular to the surface.
And if you really think about what's happening here,
it's fundamentally an electromagnetic force.
Because if you really zoomed in on the molecules of the ice
right over here, even better the atoms of the ice here.
And you really zoomed in on the atoms or the molecules
of the ice up here, what's keeping this top block of ice
from falling down is that in order
for it to go through its molecules would have to kind
of compress against, or I guess it would have to get closer
to, the water molecules or the individual atoms
in this ice down here.
And the atoms, let me draw it on an atomic level
right over here.
So maybe, let me draw one of this guy's molecules.
So you have an oxygen with 2 hydrogens
and it forms this big lattice structure.
And we can talk about more of that in the chemistry playlist.
And let's talk about this ice as one of these molecules.
So maybe it looks something like this.
And it has its 2 hydrogens
And so what's keeping these guys from getting compressed, what's
keeping this block of ice from going down further,
is the repulsion between the electrons in this molecule
and the electrons in that molecule.
So on a macro level we view this is kind of a contact force.
But on a microscopic level, on an atomic level,
it's really just electromagnetic repulsion at work.
コツ：単語をクリックしてすぐ意味を調べられます！

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B1 中級 米

Normal force and contact force | Forces and Newton's laws of motion | Physics | Khan Academy

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Sophia Chung 2020 年 7 月 2 日 に公開

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