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

The force that keeps a block of ice from falling towards the center of the earth. Created by Sal Khan.

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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.