Contact Forces | Dynamics | AP Physics 1 | Khan Academy

- [Instructor] There are
a lot of different types

of forces in physics,

but for the most part,

all forces can be
categorized as either being

a contact force or a long range force.

So contact forces as the name suggests

requires the two objects

that are exerting a force on each other

to be touching or in contact.

So tension, the normal
force, frictional forces,

these are all common everyday
examples of contact forces.

So, you know, this wire from this crane

can exert a contact force
i.e. attention force

on the wrecking ball but that wire

can only exert that tension
force on the wrecking ball

if the wire is actually connected to

i.e. touching the wrecking ball,

if you forgot to tie the
wire to the wrecking ball,

that wire is not gonna exert any tension

on the wrecking ball.

So these contact forces are
to be distinguished from

long range forces.

Sometimes these are called
action at a distance forces

because they can be exerted

on objects that are far
away from each other,

so gravity is a common example,

the earth can exert a
gravitational force on the moon

even though the earth and
the moon aren't touching.

So that's a group long range force.

Similarly, the electric force
can exert a repulsive force

on two charges if they're not touching,

so not a contact force,

and magnets can attract each other

even if they're not touching,

so those are all long range or
action at a distance forces.

But I'll be honest with you here,

this distinction is not
nearly as fundamental

as it might seem at first.

All of these forces that
we call contact forces

are really just an enormous
number of long range forces

in disguise.

In other words, these contact
forces, tension, normal force

and friction are all arising
microscopically due to

a bunch of long range forces

acting over really short distances.

So just cause they're
called long range forces

doesn't mean they can't exert
force over small distances,

and in fact, all those forces arise,

you know, cause all these forces to arise.

So let me go through and explain
how all these come about.

So we'll start with tension here.

Where does tension come from?

Well, tension is the
force exerted by a wire

or a cable or a string,
something like that.

And so these strings,

they're made out of atoms and molecules,

I'm trying to represent that over here.

Your string is probably
more than three atoms wide,

but I didn't wanna have to
draw an enormous number here,

so I imagine you've got
a certain number of atoms

and molecules in your string,

well, these atoms and molecules
are all bonded together,

chemically bonded together,

those are all electromagnetic bonds here,

they don't wanna move away from
their equilibrium position,

they have a position and if
they get displaced from there,

they wanna go back to that spot.

So that's what it means
to be in a solid here.

So this wire, if you connect
to a heavy load to it

like a wrecking ball,

that wrecking ball is gonna
try to rip these atoms

and molecules apart,

it's gonna try to pull
them away from each other,

but they don't wanna move
away from each other,

in other words, they try
to restore themselves

as this distance between these atoms

and molecules gets bigger,

and it does, you'll stretch
your string or your wire

sometimes imperceptibly, but a little bit,

these distances get bigger,

that force holding them
together gets bigger,

so more tension force occurs

and this is the microscopic
origin of that tension force.

These atoms and molecules
wanna restore themselves

to their previous length and

to do that they have to
pull harder and harder.

Now, this won't last forever,

you hang a heavy enough load over here,

you'll overwhelm these
electromagnetic bonds

and you'll rip these molecules apart

and that's what happens
when your string breaks.

So that's the microscopic
origin of tension

but you don't have to draw an
Avogadro's number of arrows,

we just represent the
tension with one arrow up,

it turns out you can pretty much summarize

all of those microscopic
electromagnetic chemical bonds

with one arrow that we call tension.

So how about the normal force,
where does that come from?

Well, this is kind of the opposite.

Tension is a pulling force,

the normal force is the force
that tries to prevent two

objects from getting
smashed into each other.

So now instead of the atoms and molecules

trying to get ripped apart,

the atoms and molecules
in this green box here

due to its weight are
trying to get shoved into

the atoms and molecules this table,

so I've tried to represent that here,

again, the box and the
table are made out of more

than these number of atoms and molecules,

but you've got your atoms
molecules of the box,

atoms and molecules of the table,

they won't get moved into each other,

there's gonna be an electron cloud

around these atoms and
molecules of the box

and similarly for the table,

there's gonna be an
electromagnetic repulsion

when they try to overlap

and other quantum mechanical effects,

it turns out it's surprisingly
complicated to explain

why matter is solid and it
can't penetrate each other,

but the enormous number of
electromagnetic interactions

and other quantum mechanical
effects between these

atoms and molecules are
the microscopic origin

of the normal force.

So again, it's, you know,
action at a distance

over a small scale,

which really bugs people out,
they're like, wait a minute,

are two things ever actually touching?

You know, as you sit in a chair,

do the atoms and molecules of your pants

actually physically touch?

Hard to actually define
what it means touching here,

you know, you've got these
amorphous electron clouds,

how do you define
whether they're touching?

Hard to do, but good news,
we don't have to do it,

we can actually just
summarize microscopically

all of these microscopic interactions

as one big normal force and that helps us

both calculationally and conceptually

knock it to at last here.

Now, you might be disturbed here,

you might be like, wait a minute,

this whole video is about contact forces,

you're telling me,

we don't even know if two
surfaces are in contact,

well, I'm saying it's hard to define

but here's a good way to define it,

your pants, atoms and molecules

are contacting the seat's
atom molecules as soon as

you notice that force

preventing them from
moving into each other.

So as soon as you could
detect this normal force,

that's as good a way as any

to define two surfaces
as being in contact.

So let's look at some other forces.

So how about the frictional force?

What are the microscopic
origins of the frictional force?

Well, you know, the frictional
force is the force that

resist two surfaces from being
dragged across each other.

Why is there a resistive force?

Well, if you zoomed in on these surfaces,

a table, no matter how smooth it looks,

even if you just wiped it down,

if you zoomed in close enough,

you'd be shocked at all the little

crevices and cracks and valleys involved,

the whole world you don't know about

unless you look at it microscopically,

and similarly for this purple
box, maybe it's cardboard,

if you zoomed in microscopically,

again, it's astonishing how
not smooth those surfaces are.

So obviously, if you tried
to drag one across the other

and these are bumping into each other,

these hills and valleys are
running into each other,

that's gonna be a problem

that's gonna cause a resistive force.

You might break this, you
know, yellow hill off,

sometimes they just bust off,

yep, that's gonna be a
resistive cause of friction.

Sometimes they don't bust off,

maybe they just like bend and bounce back

but even if they do,

still gonna cause a frictional force

and add to this friction,

and it's not just that
but sometimes even like

the atoms and molecules
in the surface over here,

look, this pot doesn't look too bad,

it looks like they could
slide across each other

pretty well,

but there can be adhesion
like molecular bonds that form

between those atoms and molecules
that are near each other,

that can also contribute to friction.

So again, astonishingly complicated,

there's actually lots of
questions to still be answered

in studying friction,

and the study of friction
is called tribology.

Shockingly, a lot of
questions to this day but,

the good news is you can summarize

all of those microscopic
interactions as one force

we call friction that resists

the two surfaces from
sliding over each other.

So you don't have to do
a lot of calculations

and microscopically
zoom in on the surface,

we can pretty much account for all of it

by simply drawing it as
one big resistive force

of friction backwards.

So recapping, contact
forces are those forces

that require the two objects
interacting to be touching

for that force to occur,

but we've seen that these
contact forces are actually due

to a mind bogglingly large
number of long range forces

all acting over a very short distance,

but you can summarize all
those long range forces

as a single contact force

when doing most introductory
physics problems.