We'll email you at these times to remind you to study
You can set up to 7 reminders per week
We'll email you at these times to remind you to study
Does Mass and Density are Different? If yes How?
Scientists say that there is no air in space.Then how do u say that there is friction out there? There is no vaccum in space
We're now ready for Newton's third law of motion.
And something - once again, you've probably heard
you know, that people talk about
but in this video I want to make sure that
we really understand what Newton's talking about
when he says - and this is a translation
of the Latin version of it -
To every action - just to be clear,
Newton was English, but he wrote it in Latin
because people - at the time - wrote things in Latin
and [Latin] was viewed as a 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 says is that
you can't just have a force on some object
without that object 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 a second. Let's say that
I have a...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, 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 we...maybe the block is sitting on some
type of ice, so it can move.
So let's say that I have some - that doesn't look like
ice! A little 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 then the net force will
accelerate the block, assuming the block can overcome
friction - and since it's on ice, I can do that, but
the block is going to exert an equal and opposite force
on me!" An equal and opposite force on me.
And for direct evidence - this is something, that might
not be so intuitive when it's said -
this equal and opposite force, but direct evidence
says that about exerting equal and opposite force is
that I can feel my hand getting compressed! I
can feel the block pushing on me. Take your hand right now
and push it against your desk, or whatever you have near
by, 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 I try to push on the desk - so once again, this is
my hand right here, pushing on the desk, if
I push on the desk - and I'm actually doing this right now
as I record this video, you'll see
you're 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 - I'm actually doing that right now, but
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, you
wouldn't even feel the pressure - it would feel -
your hand would be completely uncompressed.
Another example of that. Say you're walking
in the beach. Say you're walking on the beach. And
you have some sand right here. If you were to step
on the sand; so let's say this is your shoe - do my
best attempt to draw a shoe - so this is a shoe; if
you were to step on the sand, clearly you are exerting
a force on the sand. You are exerting a force on the
sand. The force you are exerting on the sand is
the force of your weight, the gravitational attraction
between you and the Earth. You're exerting that on
the sand. The sand is also - and another evidence of that
is that the sand is going to be displaced - it's 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 - is also exerting an equal and opposite
force on you. And what's the evidence of that? Well,
if you believe, if you believe Newton's second law,
if you have this gravitational force on you, you should
be accelerating downwards unless there's 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's a zero net force
on you. And that's why you get to stay there; 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, you know,
trying to escape the atmosphere, or maybe you're in
space, there's nothing to push off of, nothing to push
off of to let 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, 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
masses for each of them, its super high velocities.
So they're being accelerated tremendously.
So there's an equal and opposite force on the rocket.
The thing 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, it accelerates a bunch of things
at a super fast rate. It exerts a force on all these
particles and that allows it to exert 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 actual - useful - example,
so that you don't end up drifting in space forever.
Let's say you don't ever want this to happen this
astronaut by some chance, he loses his connection to
this little tool arm right here, connected to the space
shuttle, and he starts drifting away. 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, if you look around, there's nothing to push off of.
He doesn't have any wall to push off of, and let's just
assume that he doesn't have any rocket jets, or anything
like that. What could he do? Well, the one thing he
could do, and this is for the situation when you're
ever drifting in space, is you should find the heaviest,
or should I say, the most massive thing on you, and we'll
explain the difference between mass and weight in a
future video, you should find the most massive thing
you can carry, that you can take off of you, that
you can 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 just
show - I'll just make it look like the glove of a -
so let's say this is, this is the glove of the astronaut
uh...there you go, there's his hand, that's the astronaut's
hand, right over here, and let's just say he finds some
equipment, on his - or she finds some piece of equipment
on them, that they can throw, they can take off of
their toolset and that they could find the most massive
object that they could throw. So what's going to happen
is that for some period of time while they push the
object away, they will be exerting a force on that
object, they will be exerting a force on that object
for some period in 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 the astronaut itself. So the
object accelerates in that direction, and while the
astronaut is pushing, the astronaut will accelerate,
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
and hopefully, grab onto something.
Log in to save your progress and obtain a certificate in Alison’s free Physics - Motion, Speed and Time online course
Sign up to save your progress and obtain a certificate in Alison’s free Physics - Motion, Speed and Time online course
Please enter you email address and we will mail you a link to reset your password.