Newton's first law | Middle school physics | Khan Academy

- [Narrator] Suppose
you have a puck at rest

on a concrete floor and you
don't do anything to it,

what will happen?

It will continue to be at rest, isn't it?

But why?

Not because there are
no forces acting on it.

I mean, there are forces acting on it.

For example, there is the force
of gravity acting downwards,

but there's also an upward
normal force that the ground

or the concrete is
putting back on the puck

as a result of which
the forces are balanced,

which means the net force
acting on the puck is zero.

So yeah, the puck's motion is
not changing, it's at rest,

it'll continue to be at rest.

But now let's bring a stick and hit it.

What's going to happen?

Well, we know what's going to happen.

If you hit it gently, it might, you know,

travel for some distance
and come to a stop.

The big question is, why did it stop?

Well, let's think about this.

Our intuition might say that,

"Hey, that's probably because, you know,

puck naturally tends to stop.

Objects naturally tend to come to a rest."

Or maybe you might think that,

"Hey, you know, it ran
out of the force that,

you know, we applied,"
but that's not true.

Let think about what's really going on.

Let's slow down things a little bit, okay?

So when we hit the puck, we apply a force.

In fact, we apply an
unbalanced force on the puck.

Because it's an unbalanced
force, the puck starts moving,

and this force lasts as
long as there is contact.

But the moment the puck loses
the contact with the stick,

the force will disappear,

and yet the puck
continues to move forward,

even though the force
disappears immediately

after the contact is lost.

But now the big question
is, why does the puck stop?

Well, the reason it stops is not

because it's natural tendency
is to come to a stop.

No, no, no.

But because there are forces

that are pushing it in
the opposite direction.

Which are these forces?

Well, there's air that's pushing it

in the opposite direction.

There's also the force of friction

between the concrete
and the puck's surface.

Because these forces are acting
in the opposite direction,

it's these forces that make it stop.

So let's look at the
animation carefully now.

The puck will start moving, and look!

Look, there are these resistive forces,

we call them, frictional
forces, air resistances.

These are the forces that make them stop.

So let's look at it one more time.

You have the stick that hits it,

it applies an unbalanced
force, it gets the puck moving,

but then these resistive
forces start applying force

in the opposite direction.

They start pushing them
in the opposite direction,

making it stop.

But now here's the question.

What if we could somehow
reduce those resistive forces,

reduce the amount of
friction, for example,

then we would expect the
puck to move much longer

before coming to a stop,

because the resistive forces
are much smaller, isn't it?

And that's exactly what happens.

If we were to carry this out on say, ice.

The friction between the puck
and the ice is very tiny,

much smaller compared
to that on the concrete.

And so now, if you hit it

with exactly the same
force in both the cases,

what will we find?

The puck moves much farther on the ice

compared to the concrete, even
though the force is the same!

Why?

Well, that's because the resistive force,

that's the one that was smaller over here,

because of which it moved
a much farther distance

before coming to a stop.

And now comes the question,

what if we could somehow make
that resistive force zero?

What if there was absolutely
no friction or air resistance

or anything like that?

What if we carried this
out in outer space,

deep intergalactic space

where there's absolutely no
other forces, for example?

Well, now, once you hit the puck,

that puck will keep moving forever.

This means, look, you don't
need an unbalanced force

to keep an object in motion.

Object which is in motion
will just keep moving

with that same velocity

as long as there is no
net force acting on it.

And if this sounds unbelievable,

then just think about the moon

that's going around the
earth, or for example,

the earth that's been
going around the sun.

They've been doing that...

They've been at it for billions of years.

What's making them move?

There are no forces acting in
the direction of the motion.

Then why is it moving?

They just keep moving because
they're already moving,

and more importantly,
because there's nothing

slowing them down,

there's no force acting
in the opposite direction

to slow them down.

Okay, we can put it all together

in what is called the Newton's First Law,

which says that if the net force

acting on an object is zero,

the object's motion will not change.

This means if you had an object at rest,

and again, if there is no
net force acting on it,

then that object will
continue to be at rest.

On the other hand, if you
have an object that's moving

and there are no net forces acting on it,

it'll continue to move

with that same constant velocity forever.

This is the non-intuitive part,

because on earth there are
always some resistive forces,

always some unbalanced forces
that tend to make things stop.

But in the absence of them,

objects in motion would
just continue to move

with that same velocity forever.

And by the way, whether
something is moving

at a constant velocity
or something's at rest

purely depends upon
reference frames, right?

For example, if this puck was in a train

moving at a constant velocity,

then the people inside the
train will see the puck

to be at rest.

But people who are outside on the ground

will see the puck moving
at a constant velocity.

But regardless of which reference
point you're dealing with,

the fact is, if there are no other forces

acting on the puck,

then the people who see
the puck to be at rest,

they will continue to see
the puck to be at rest,

and the people who see
the puck to be in motion,

they will continue to see that puck

to be moving at a constant velocity.

The fact is, as long as not net force

is acting on an object,

the object's motion will not change.

In other words, the object
will not accelerate.