A LEVEL PHYSICS REVISION: NEWTON'S THIRD LAW EXPLAINED

hey guys welcome back to another one of these a-level physics mechanics videos today we're going to be talking all about the Newton's third law of motion now first of all we should type what Newton's third lawyers and you do need to memorize this Newton's third law simply states that any action has an equal and opposite reaction so what does this mean in practice well what it means in practice is that anytime a force is exerted you're going to have an equally opposite Force so any Force that acts on an object uh causes a force of equal magnitude to act from the object in the opposite so what does this mean exactly well let's say we have a box okay now let's say I'm here and I'm going to push this box the force of 10 Newtons now this box when I push it has to exert a force of the same type in the exact opposite direction with the exact same magnitude now you might be thinking so then why do things accelerate right because we have these two equal and opposite forces well we have to keep in mind that I'm pushing on the box which makes the Box accelerate and the box is pushing on mute makes me accelerate away okay which kind of does make sense whenever you were to push something you do get pushed in opposite directions here's a little like for example if you and your friends pushed each other or you push one of your friends you get pushed back slightly right even if they don't push you back another way to actually uh look at this is if you were on roller skates right or ice skates and so let's put this is my little ice rink and again I'm standing here and you're standing opposite me right and I push you with the force of 20 newtons that'll result in you automatically pushing me back with the force of 20 newtons now again I'm not saying you literally pushed me but the fact that I'm in contact with you and push we would both actually move apart from one another in opposite directions because we both have forces pushing us away and this is actually why things in space can get really really tricky right because if you try and touch something you can you end up pushing yourself away from that thing so the key things to remember here is and this is also why you don't fall through the floor right so if I'm standing on on the floor right and I weigh approximately well I'll make 75 kilograms so let's say 750 Newtons let's say 770 Dunes right now if I draw just this diagram what should happen well there's an overall result force on the system right f equals m a which means I should accelerate downwards now I'm sitting in my chair right now I'm not accelerating downwards thankfully otherwise I'll be crashing through the floor of my house so the only way that this can resolve itself is if there's an equal and opposite Force pushing me back up from the ground or in this case my chair back into me right so I'm exerting 770 newtons that should be 70. so I just realize I didn't write 50. 770 Newtons down on my chair the chair is pushing up 770 Newtons so overall there is no acceleration right which makes sense I'm not falling downwards okay now there are a few caveats to this is that they have to the forces have to be the same pair so sometimes again I'm going to use the classic I'm pushing a box right so I'm pushing a box 10 Newtons here and then there's going to be a force this way which is friction right and what some people say is okay well this is the equal and opposite Force friction no because these aren't the same type of force so when we say there's a force of equal magnitude in the opposite direction it's actually the exact same Force so if I call this a pushing force when I push something it pushes me back with the exact same um right now this does actually lead to some interesting things for example when I jump right if I'm standing on the earth which I am and I jump up I exert a force downwards and the Earth exerts forcing me upwards and so I get thrown up but the Earth technically actually gets pushed down ever so slightly so why is it two different forces well let's say let's say I can jump pretty well okay now let's say that is a 700 Newton jump right so if I put this into f equals m a for me 700 Newtons equals again I weigh approximately 75 kilograms times a so a would equal 700 over 75. for now let's say that that is 10 meters per second squared right so I'd accelerate upwards at 10 meters per second squared yeah now let's do it for the earth now that same 700 Newton force is equal to the mass of the Earth which is a lot it's something on the order of I think it's something along the lines of six 1.6 times 10 to the 26 kilograms times a which means a is equal to 700 over 1.6 times 10 to the 26. so that's going to be you know a billion billion billionth of a meter every second squared so technically speaking when we jump up the Earth does get pushed down it accelerates downwards but it's such a tiny tiny tiny tiny amount of like acceleration that it's not noticeable but it does technically exist okay so the key thing to note here is that if they asked you um you push a box with 10 Newtons of force what's the kind of reaction force or what force does act on you or label all of the forces you need to label a reaction force going the other way you have to otherwise you don't get the mark So if they give you if they ask for a fully labeled diagram you need to make sure you're including these equal and opposite forces because of Newton's third law and hopefully you can also see why it's although it's an equal and opposite reaction it doesn't mean that every Force cancels each other out because keep in mind we're talking about forces acting on an object right so if I just look at the forces acting on the box it's 10 Newtons to the right if I look at the overall force on the system it's 10 Newtons to the right and then 10 Newtons back but it doesn't mean that they will cancel out things can still happen okay so hopefully that makes a bit of sense now one of the questions that they'd love to ask you and this comes up in thermodynamics as well it comes up in the kinetic model for gases it comes up again and again and again and it also comes up in just general mechanics of course is they give you a situation and then they say explain with reference to Newton's Laws of Motion why displacing and then why whatever right so it could be anything from why does pressure exist using Newton's um using the kinetic model of gases and news laws of motion it could be anything but they ask you the same question and all you have to do is apply it now it should be noted that just for some exam technique you need to use all three laws of motion okay so you get one mark for every law of motion you could correctly relate to the situation and then you get a final Mark just for I'm applying it to this um so that in this case why this force causes the system to decelerate okay and again this comes up a lot they actually ask explain with reference to Newton's Laws of Motion a hell of a lot so it's a really good kind of technique to be able to apply it so I like to just think okay I like to work backwards funnily enough so I start with the third law and then I work my way back because it's the initial part so let's read the actual situation parachute opens during The spacecraft's Descent through the atmosphere so we've got something falling Ticket 2 shows the parachute spacecraft system with the open parachute displace in the atmospheric gas causing the system to decelerate okay so what you can imagine is you have a little bunch of these little gas particles right and they're hitting the parachute and that also causes them to for the parachute to actually exert a force on the gas particles in general so what you can say is gas particles collide with the parachute okay due to Newton's third law and again I'm going to bullet point or this causes the parachute to exert a force on the gas particles okay so that'd be my first bullet point next bullet point well that's Newton's third law done so what does that mean in terms of Newton's Second Law right or in this case Newton's third first law Newton's got down first law states that if uh a reaction force sorry resultant Force acts on a system it will cause an acceleration right so Newton's first law says that object in constant motion will stay in that motion unless acted on by an outside force well we wrote here that a force is being exerted which means due to Newton's first law there has to be some kind of acceleration some kind of change of motion okay now from Newton's second law second law this acceleration or this acceleration slash force is proportional to the mass of the in this case gas displaced right in this case the mass of the gas displaced so this causes the parachutes and spacecraft to slow down slash decelerate and that would be a full full marks Okay so again the kind of logic behind this is that you just need to make reference to all three Newton's laws in this context and then finish off with this and it causes it to slow down on decelerate mainly because there's a collision between the gas particles and the parachute and because there's a collision there are equal and opposite reaction forces so if you want to say the parachute hits the gas and therefore the gas exerts an equal and opposite force on the parachute that's equally fine and because of the first thought this means that there has to be an acceleration and this acceleration is proportional to the mass of the gas displaced which in this case if you're falling through the atmosphere you're going to be hitting a lot of air or atmosphere very very quickly so that acceleration isn't negligible at all so hopefully that makes sense again we will come back to this when we do kind of things like thermodynamics because there are a bunch of questions that basically ask you with reference to Newton's Laws of Motion but hopefully that was a good enough kind of overview for now