Newton's First Law of Motion Concepts

Now that we know a little
bit about Newton's First Law,

let's give ourselves
a little quiz.

And what I want you
to do is figure out

which of these statements
are actually true.

And our first statement is,
"If the net force on a body

is zero, its velocity
will not change."

Interesting.

Statement number two, "An
unbalanced force on a body

will always impact
the object's speed."

Also an interesting statement.

Statement number
three, "The reason

why initially
moving objects tend

to come to rest in
our everyday life

is because they are being
acted on by unbalanced forces."

And statement four, "An
unbalanced force on an object

will always change the
object's direction."

So I'll let you
think about that.

So let's think about these
statement by statement.

So our first statement
right over here,

"If the net force
on a body is zero,

its velocity will not change."

This is absolutely true.

This is actually
even another way

of rephrasing
Newton's First Law.

If I have some type
of object that's

just traveling through
space with some velocity--

so it has some speed
going in some direction,

and maybe it's deep space.

And we can just,
for purity, assume

that there's no
gravitational interactions.

There will always be
some minuscule ones,

but we'll assume no
gravitational interactions.

Absolutely no
particles that it's

bumping into, absolute
vacuum of space.

This thing will
travel on forever.

Its velocity will not change.

Neither its speed nor its
direction will change.

So this one is absolutely true.

Statement number two, "An
unbalanced force on a body

will always impact
the object's speed."

And the key word right
over here is "speed."

If I had written "impact
the object's velocity," then

this would be a true statement.

An unbalanced force
on a body will always

impact the object's velocity.

That would be true.

But we wrote "speed" here.

Speed is the
magnitude of velocity.

It does not take into
account the direction.

And to see why this
second statement is false,

you could think about
a couple of things.

And we'll do more
videos on the intuition

of centripetal acceleration
and centripetal

forces, inward forces,
if this does not

make complete intuitive sense
to you just at this moment.

But imagine we're looking at
an ice skating rink from above.

And you have an ice skater.

This is the ice skater's head.

And they are traveling
in that direction.

Now imagine right
at that moment,

they grab a rope that is
nailed to a stake in the ice

skating rink right over there.

We're viewing all of this from
above, and this right over here

is the rope.

Now what is going to happen?

Well, the skater
is going to travel.

Their direction is
actually going to change.

And they could hold
on to the rope,

and as long as they
hold on to the rope,

they'll keep going in circles.

And when they let
go of the rope,

they'll start going
in whatever direction

they were traveling
in when they let go.

They'll keep going
on in that direction.

And if we assume very,
very, very small frictions

from the ice skating
rink, they'll

actually have the same speed.

So the force, the inward
force, the tension

from the rope pulling on the
skater in this situation,

would have only changed
the skater's direction.

So and unbalanced force
doesn't necessarily

have to impact the
object's speed.

It often does.

But in that situation, it
would have only impacted

the skater's direction.

Another situation like
this-- and once again,

this involves centripetal
acceleration, inward forces,

inward acceleration--
is a satellite in orbit,

or any type of thing in orbit.

So if that is some
type of planet,

and this is one of the
planet's moons right over here,

the reason why it stays in orbit
is because the pull of gravity

keeps making the object
change its direction, but not

its speed.

Its speed is the
exact right speed.

So this was its
speed right here.

If the planet wasn't
there, it would just

keep going on in that
direction forever and forever.

But the planet right
over here, there's

an inward force of gravity.

And we'll talk more about the
force of gravity in the future.

But this inward
force of gravity is

going to accelerate this object
inwards while it travels.

And so after some
period of time,

this object's velocity
vector-- if you

add the previous velocity
with how much it's

changed its new velocity vector.

Now this is after its traveled
a little bit-- its new velocity

vector might look
something like this.

And it's traveling at
the exact right speed

so that the force
of gravity is always

at a right angle to
its actual trajectory.

It's the exact right speed so it
doesn't go off into deep space

and so it doesn't
plummet into the earth.

And we'll cover that
in much more detail.

But the simple answer is,
unbalanced force on a body

will always impact its velocity.

It could be its speed,
its direction, or both,

but it doesn't have to be both.

It could be just the speed
or just the direction.

So this is an
incorrect statement.

Now the third
statement, "The reason

why initially
moving objects tend

to come to rest in
our everyday life

is because they are being
acted on by unbalanced forces."

This is absolutely true.

And this is the example we gave.

If I take an object,
if I take my book

and I try to slide
it across the desk,

the reason why it
eventually comes to stop

is because we have the
unbalanced force of friction--

the grinding of the
surface of the book

with the grinding of the table.

If I'm inside of a
pool or even if there's

absolutely no
current in the pool,

and if I were to try to
push some type of object

inside the water,
it eventually comes

to stop because of all of the
resistance of the water itself.

It's providing an unbalanced
force in a direction

opposite it's motion.

That is what's slowing it down.

So in our everyday
life, the reason

why we don't see these
things go on and on

forever is that we have
these frictions, these air

resistants, or the friction
with actual surfaces.

And then the last statement, "An
unbalanced force on an object

will always change the
object's direction."

Well, this one actually is
maybe the most intuitive.

We always have this situation.

Let's say I have a
block right over here,

and it's traveling with some
velocity in that direction--

five meters per second.

If I apply an unbalanced
force in that same direction--

so that's my force
right over there.

If I apply it in
that same direction,

I'm just going to accelerate
it in that same direction.

So I won't
necessarily change it.

Even if I were to act against
it, I might decelerate it,

but I won't necessarily
change its direction.

I could change its direction
by doing something like this,

but I don't necessarily.

I'm not always
necessarily changing

the object's direction.

So this is not true.

An unbalanced force on
an object will not always

change the object's direction.

It can, like these
circumstances, but not always.

So "always" is what makes
this very, very, very wrong.