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Video transcript

we're now ready for Newton's third law of motion and something once again you've probably heard it 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 this is a translation of the Latin version of it to every action there is always and just so you clear Newton was English but he wrote it in Latin because at that point in time people wrote in 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 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 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 even more ice like color so the block is sitting on maybe some ice like that so Newton's third law 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 an equal and opposite force on me and for direct evidence this is something even though it might not be so intuitive when it said this equal and opposite force but direct evidence that's 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 nearby and you are clearly exerting a force on the desk so let me let me draw so let's say I have a desk right here and if I if I try to push on the desk so once again this is my these are my 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 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 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 that this is your shoe 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 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 is also exerting an equal and opposite force on you and what's the evidence of that well if you believe if you believe if you believe Newton's second law if you have this gravitational force on you 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 when you net them out there is a zero net force on you and that's why you get to stay there while you don't start raining 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 at a rocket either you know trying to escape the atmosphere or maybe you're in space there's nothing to push off of nothing to push off that let you that 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 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 there's they're super small mass for each one 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 expelling the gas and so that's what allows a rocket to accelerate even when there's nothing in this directing city to push off of it just expels a bunch of things that it or it accelerates a bunch of things that a super fast rating put 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 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 this little tool arm right here on the Space Shuttle and he starts 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 you look around you're like well there's nothing to push off of he doesn't have any wall to push off of it 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 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 can throw and you should throw 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 shoot I'll make it look like the glove of a so let's say that this is this is the glove of the astronaut there you go that's his hand that's the astronauts hand right over here and let's say he finds 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 to their tool set and then they could find the most massive object that they can 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 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 it you are I'll the astronaut 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 will accelerate in this direction so what you do is you throw in the opposite direction and that allowed the astronaut to accelerate towards the space shuttle and hopefully grab on to something
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