Newton's Third Law of Motion Every action has an equal and opposite reaction
Newton's Third Law of Motion
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- 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
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