Neil deGrasse Tyson Explains Force vs. Pressure

[Music] jack hey i got another one for you all right i'm ready maybe i got more explaining to do you explain it toto lucy yeah so here it is so what i like doing the point of these explainer videos just if it's not otherwise obvious is i want to just disentangle some things you might have thought you knew or thought you knew well but not well enough were there's some nuance to it and i just want to sort of pump people's enlightenment regarding the natural world around them that's all i i i always like the little plot twist in these okay all right that's what's cool about it okay okay so today i want to talk about force and pressure gotcha okay okay so i'm not talking about sort of emotional pressure okay that's what i'm talking about right right you know my job has gotten me under so much pressure i'm talking about physics pressure and physics force all right um by the way another way we use those words in everyday life we say um how much force are you showing on the battlefield so that's another cultural usage of those two terms each of those words has a precise definition in physics not to mention space force yes that's in there too okay they don't call it space pressure no there's a space force so a force is what you think it is right you push on something and you create a force that might set it into motion okay and if it's something that doesn't move but it's still fragile you put enough force on it you might break it nice yeah okay so forces make things happen and when we say happen we mean something changes about the object typically it's set into motion and isaac newton first wrote down an equation about this okay he said force equal equals the mass of the object times the acceleration it'll get if you put that force on that object that's it okay so you use that formula you say well here's an object i'm going to put a certain amount of force and it has to be like a net force so in other words if you put a force exactly opposite mind then the forces cancel and then there's no net force there's nothing accelerates right so if everything is in balance you can have very high forces operating but nothing's gonna happen right but if there's a slight imbalance then there will be motion and didn't long ago we talk about this like at the gym why is it that the person spotting for someone else does not have to be as muscle bound as the person lifting the weights have you ever thought about that um every time i go to the gym okay somebody will say hey man give me a spot and it's always a dude who's eight times bigger than i am and he's lifting on a building he's actually lifting a building i just stand there in case i drop it right and he goes hey buddy can you give me a spot and i'm just like no what am i supposed to do when you're lifting like and you're struggling you're not only trying if you're struggling and there's a point where you can't lift it anymore you want me to come help you right you want me to then take over okay here's why that works okay because if all forces are balanced then any force will move it no matter how small so watch what happens so i'm there pump i'm on the bench bench just a bench press typically right because the weight is above the person's neck right so this is dangerous you don't need a spotter if you do a bent over sort of rowing lift no because you could just drop the weight just you just trump that's no big deal when it's over your windpipe it's like hey chuck can you spot me and i'm like hey man you want to die it's okay you get my skinny ass to prevent you from dying all right so watch so here i am i'm lifting and that's getting harder and harder all right and now there's a point where i get it half way and i can't get it any further and i say chuck help me out here in that moment my upward force equals the downward force of those weights and right force on earth from gravity is called your weight so the weight equals the force pushing up on it if they're equal now the thing is just stopped moving okay it has stopped moving so now you come along and say here you go and then you lift and you lift you can probably use one hand to do this you lift it back up onto the rack because the forces were balanced whereas previously the the person's force was greater than the weight of the weights right and so if it's greater i'm in control here and i can push the thing away from earth away from earth's urges to try to bring it back when we're in balance then you break that tie basically and put it over the hump that's why that works that's very cool okay so we're team we're teaming up on the weights basically like you're teaming up right and it doesn't make a difference how strong i am i could take two fingers and just whatever little bit i'm doing now you're provided provided that he's not losing that battle okay if the weight is on its way down you're gonna need it's not balanced you have to counteract that right and then put in a little more to get the thing back up to the to the stack and that's when i stand over top of him and go sorry man you're going to die you sound like this has happened before all right so just to get a sense of what forces are okay that's and oh so with regard to acceleration if there's a net force then the object's motion will continue to increase in speed you have an acceleration all right so there you have it one last thing just in detail if all forces are balanced it can still be in motion it just won't be accelerating all right okay so you can have no motion or constant motion if there's a net force it will accelerate that's the point that's going on here all right so you're in your car and your foot is on the accelerator pedal and you're sticking to 60 miles an hour 55 miles an hour well what does it mean if your foot is on the accelerator pedal but you're not increasing in speed you're not accelerating oh well the force the accelerator pedal is trying to put in the car is exactly balanced by the friction of the tires on the road and the air resistance all of that is balanced and you're maintaining constant speed if you want to take it out of balance you press the pedal even harder to overcome that balance and now you can pass the car on the right by accelerating up to 70 you pass him and then you slow it back down again right so that's what's going on with force and everybody learns this in like physics 101 the first 10 days okay so now what is pressure pressure is when you have been dating for four years and she goes what are we doing here okay seriously how many times can i take you home for thanksgiving and explain to my parents that you know we're not ready yet what i mean okay so that's pressure you tell me that's pressure okay that's not the kind of pressure i'm talking about okay okay okay that's dating pressure how about that right right so we're talking about physics pressure so pressure intimately needs force to be what it is but it's not the same thing uh-oh okay okay it's not the same thing so if you want to find out what it is you got to look at the equation for pressure okay okay have you ever seen the equation of pressure i don't think i have all right let me before i get to that let me tell you a few things that are affected by pressure for example uh your knife set how sharp are your knives that is all about pressure all about pressure okay uh if you are you gonna fall through the ice on that pond as you walk across it that's all about pressure and stupidity let's do it right all right so let's talk about this here you go pressure is force divided by area oh okay and i didn't even know that equation but that makes perfect sense it makes perfect sense so watch so watch so if i'm walking out onto a frozen pond and i don't want to fall through if i have tiny itty-bitty ass feet then the area of the bottom of my feet is small but what happens if you have a small number in the denominator of a fraction the value of that goes higher right so if pressure is forced divided by area and that area gets smaller and smaller the pressure gets higher and you punch through that ice and you die you need clown shoes you need clown shoes [Laughter] get the biggest ass shoes you can find so that force is spread over the largest area possible right so when you have a big area the force divided by a big area makes a low pressure and so with low pressure now you can get across the ice without sinking through you improves your chances of of of not breaking the ice this is what snowshoes are what what are snowshoes they're like the the the the you know the mountain man snow equivalent of clown shoes all right because that the snowshoe is this big it's a uh it's like a big net and it attaches to the bottom of your feet and when you walk on it your body weight is now spread over a larger area and you don't plunge down through deep snow you still sink a little bit but not as much as you would have and then you can actually walk have you ever seen the width of the paws of a polar bear they're huge oh my god it's like oh my god because there's some big mofos and they don't want to sink through the snow okay they spend a lot of their time on ice but this matters okay and so what about your knives when you go to cut something you apply a force how do you make that force as effective as possible to cut you want the lowest possible area over which you're applying that force so that you have the highest possible pressure okay you get pressure for free so when you so what what what is a dull knife you look at it under a microscope it's it's a it's all chewed up it's flat it's like so your pressure let's say you put 10 pounds of pressure on it is spread over this long area over the length of the blade and you try to cut something with it with it and you mangle the food you have to press even harder to get it through a perfectly sharpened blade what's the area of a blade edge tell me that the area of a of a sharpened blade edge it is so tiny that even the mildest force of that knife will cut through the food and that's why chefs are always sharpened in their knives because they want to increase the pressure on their food because they don't want to have to increase their force to get the pressure they want they're reducing the area to get the pressure they want sweet so this is this is force versus pressure and uh i don't know how many people internalize this feel it think about it but this distinction between force and pressure manifests everywhere everywhere and by the way it's why a tornado can explode your house wow okay you say oh because the wind is high here's what's happening all right it's very low pressure in the middle of a tornado okay really really low pressure and inside your house you have slightly higher pressure than that tornado now suppose that pressure difference is like one pound per square inch difference let's say okay it might be a little high for this example like a tenth of a pound per square inch i don't care a tenth of a pound okay so inside the house the air has not equilibrated with the outside of the house yet the tornado comes it sits on your house oh my gosh every square inch of your wall is feeling a tenth of a pound pressing outward so ten square inches feels how much a hundred no ten it's a tenth of a pound so ten square inches is a pound okay a hundred square inches is just 10 inches by 10 inches that's 10 pounds right your wall is probably bigger than 10 inches by 10 inches square you keep adding this up and that pressure you get a thousand pounds of pressure oh my god that's more than the kool-aid guy actually exerts exactly get through a wall to say oh yeah so what i didn't i didn't say it right so the the it's a thousands of pounds of total force right spread across that wall but the whole wall is only built to handle you leaning on it or or or to hold up the house it's not enough to prevent the tornado from exploding your house and all the walls blow out take a look at video footage of homes they don't collapse they're turned into matchsticks that's matchsticks and they explode outwards that is pressure at its most deadly wow and so you know there you have it now see this is what i'm talking about when i say plot twist no one would ever think that you just talk about force and pressure and we end up right here that's right and by the way it's how bombs work what is a bomb it sets a pressure wave high temperature uh expansion of the air because there's some like an explosion is a very high um temperature abrupt ins uh device right but it happens to happen rapidly so that it's like like a bullet firing it's a rapid expansion of gas which shoves the bullet out but if it's a bomb there's no bullet it's just the expanding air right sometimes you can put in shrapnel but air will do this and the expanding air comes out and now you have air pressure too high on one side of the wall versus the other and that'll blow the wall inward rather than outward or if the bomb is inside the house it'll blow the house up instead of it right so this is pressure on the wall spread over the area and by the way if all of that force were in one spot it would just puncture a hole through the wall right that's so cool oh my god so why can't we find a way everybody's always trying to figure out a way to predict where a tornado will go which is almost impossible why not just have like a tornado airbag well you would die no it's going to say wait chuck chuck wait chuck you don't need tools to tell you where the freaking tornado is just look that's true you're trying to see airbags exploding oh there must be a tornado somewhere here right yeah exactly i'm over thinking you're overthinking that one totally check overthinking yeah okay chuck we're done there that's pressure versus uh force that's very cool not to mention very very cool song under pressure oh yeah yeah the uh queen yeah very good very good all right we gotta go okay good to have you chuck tweeting at chuck nice comic i follow you thank you thank you all right i follow you too we're done here i've been your host neil degrasse tyson you're a personal astrophysicist and as always keep looking up you

Channel: StarTalk

Views: 187,279

Rating: 4.9644246 out of 5

Keywords: startalk, star talk, startalk radio, neil degrasse tyson, neil tyson, science, space, astrophysics, astronomy, podcast, space podcast, science podcast, astronomy podcast, niel degrasse tyson, physics, Chuck Nice, force, pressure, force explained, pressure explained, force vs pressure, tornadoes, explosives

Id: R2gpl3dMa80

Length: 17min 34sec (1054 seconds)

Published: Tue Jan 26 2021