Newton's third law of motion Video Lecture - Class 11

We're now ready for Newton's
third law of motion.
And something, once again,
you've probably heard,
people talk about.
But in this video, I want to
make sure we really understand
what Newton is talking
about when he says--
this is a translation of
the Latin version of it--
to every action, there's
always-- and just
to be clear--
Newton was English,
but he wrote it in Latin
because at that point in time,
people wrote things
in Latin because it
was viewed as a more
serious language.
But anyway-- to every
action, there is always
an equal and opposite
reaction, or the forces
of two bodies on each
other are always equal
and are directed in
opposite directions.
So what Newton is saying
is that you can't just
have a force acting
on some object
without that object also
having an opposite force acting
on the thing that's
trying to act on it.
And just to make it clear,
let's say that we have a--
and we'll talk about these
examples in the second.
Let's say that I have some
type of block right over here.
And that I move, and
I press on the block
and I try to push it forward.
So this is my hand.
This is my hand trying
to press on the block
and exert a force, a net
force in that direction.
So that the block
moves to the right.
Maybe this block is
sitting on some type of ice
so that it can move.
So let's say that I have some--
that doesn't look like ice--
I'll give it a more
ice-like color.
So the block is sitting on,
maybe, some ice like that.
So Newton's third law
is saying, look, I
can press on this
block, and sure,
I'll exert a net
force on this block
and that net force will
accelerate the block assuming
that I can overcome
friction, and if it's on ice
I can do that.
But that block is going to exert
an equal and opposite force
on me.
And for direct evidence--
this is something,
even though it might not
be so intuitive, when
it's said-- this equal
and opposite force.
But direct evidence that it's
exerting an equal and opposite
force-- is that my hand
will get compressed.
I could actually feel the
block exerting pressure on me.
Take your hand right now and
push it against your desk
or whatever you have
nearby and you are clearly
exerting a force on the desk.
So let me draw-- so let's
say I have a desk right here.
And if I try to push on the
desk-- so once again that's
my hand right here,
pushing on the desk.
If I push on the
desk, and I'm actually
doing it right now while
I record this video.
You'll see.
So I'm clearly exerting
a force on the desk,
if I do it hard
enough, I might even
get the desk to shake
or tilt a little bit.
And I'm actually
doing that right now.
But at the same time, you'll
see that your hand is getting
compressed, the palm of your
hand is being pressed down.
And that's because
the desk is exerting
an equal and opposite
force on you.
If it wasn't, you
actually wouldn't even
feel it, because you wouldn't
even feel the pressure.
Your hand would be
completely uncompressed.
Another example of that-- say
you're walking in the beach,
and you have some
sand right here.
If you were to step on the sand.
So let's say that
this is your shoe.
I'll do my best
attempt to draw a shoe.
So this is the shoe.
If you were to step on
the sand, clearly you
are exerting a
force on the sand.
The force that you're
exerting on the sand
is the force of your weight.
The gravitational attraction
between you and the Earth.
You are exerting
that on the sand.
The sand is also-- and
another evidence of that
is that the sand is
going to be displaced.
You're going to
create a footprint.
The sand is going to
move out of the way
because it's being
pushed down so hard.
So clearly you are exerting
a force on the sand.
But the sand is also exerting
an equal and opposite force
on you.
And what's the evidence of that?
Well, if you believe,
Newton's second law--
if you have this
gravitational force on you,
you should be
accelerating downwards
unless there is some other
force that balances it out.
And the force that
balances it out
is the force that the beach,
or the sand is exerting on you
upwards.
And so when you
net them out there
is a zero net force on you.
And that's why you
get to stay there.
Why you don't start
accelerating down
towards the center of the Earth.
Other examples of this- this is
maybe the most famous example
of Newton's third law--
is just how rockets work.
When you're in a
rocket, either trying
to escape the atmosphere,
or maybe you're in space,
there's nothing to push off
of, nothing to push off,
that lets you accelerate.
So what you do is you keep stuff
to push off in your fuel tanks,
and when you allow the
proper chemical reactions
or the proper combustion
to take place,
what it does is it expels gases
at ultra high velocities out
the back of your rockets.
And each of those
particles you're
exerting a force on them.
Enough force even though they're
super small mass for each
of them, they're going
at super high velocity.
So they're being
accelerated tremendously.
So there's an equal
and opposite force
on the rocket, the thing that
is actually expelling the gas.
And so that's what
allows a rocket
to accelerate even when there's
nothing in this direct vicinity
to push off of.
It just expels a bunch
of things or accelerates
a bunch of things at
a super fast rate.
It exerts a force on
all these particles,
and that allows an
equal and opposite force
to accelerate the rocket ahead.
And another example of
this is, if you ever
find yourself drifting in space.
And this is an actually
useful example,
so that you don't end up
drifting in space forever.
Let's say, we don't ever
want this to happen.
This astronaut,
by some chance he
loses his connection
to this little tool arm
right here in the space shuttle,
and he starts drifting away.
What can that
astronaut do to change
the direction of his motion
so that he drifts back
to the space shuttle?
Well you look around, there's
nothing to push off of.
He doesn't have any
wall to push off of.
Let's just assume he
doesn't have any rocket jets
or anything like that.
What could he do?
Well the one thing
you could do--
and this is the situation if
you're ever drifting in space--
is you should find
the heaviest or I
should say the most
massive thing on you--
and we'll explain the difference
between mass and weight
in a future video-- but
you should find the most
massive thing that you can
carry, that you can take off
of you, that you could throw.
And you should throw it in a
direction opposite yourself.
So let me put it this way.
If I throw-- let's say I'm
in space and I'm floating.
I'll make it look like the glove
of a-- so let's say that this
the glove of the astronaut.
There you go.
That's his hand, that's
the astronaut's hand
right over here.
And let's say he find some
piece of equipment on his,
or she finds some piece
of equipment on them
that they can throw.
They can take off
of their tool set.
And they could find
the most massive object
that they could throw.
So what's going to happen
is-- for some period of time,
while they push
this object away,
they will be exerting a force
on that object for some period
of time while they have
contact with the object.
And that entire
time, that object,
while it is accelerating--
while the astronaut is exerting
a force on it-- will be exerting
an equal and opposite force
on the hand of the astronaut,
or on the astronaut itself.
So the object will
accelerate in that direction,
and while the
astronaut is pushing,
the astronaut will
accelerate in this direction.
So what you do is you throw
in the opposite direction
and that'll allow the astronaut
to accelerate towards the space
shuttle and hopefully
grab on to something.