Neil deGrasse Tyson Explains the Difference Between Mass, Weight, and Density

[Music] good to see you again good to see you what's happening what do we got what's going on i can't wait to hear i'm still at it you know i i i lose sleep at night thinking i gotta tell this to chuck okay just so you know i'm thinking i don't i don't believe that i don't believe that for one second but i'll take it i will take it i just got to straighten some things out okay i think they some people not everybody but some people i think have this all confused and i want to straighten it out all right it's the difference between mass weight and density all right well okay now for without a doubt there are there's a lot of people that get that confused i just want okay so generally if you go on a diet you want to lose weight right absolutely so what that's not what actually what's actually happening it is but it's not the root of what's happening what you're really doing is you're losing mass you want there to be less of you tomorrow than there is today if only i could keep the parts that i want if if you consume fewer calories today then you burned calories today you will lose weight okay it's that simple okay i i've joked about this a a weight loss book written by a physicist would have one sentence consume calories at a slower rate than you burn them that's the entire that's it but so if tomorrow you want less of you than there is today you're losing mass okay the mass is the sum of all the particles that comprise chuck okay that's your mass i could take you and put you on the moon you'll weigh less but you still have all of your particles okay then that is where i need to move when can we do this so if you weigh 180 pounds i don't know how much you weigh let's call you 180 pounds on earth okay on the moon you weigh one-sixth of that right so you weigh 30 pounds so chuck you want to lose 150 pounds your nutrition will say no chuck you'll die no i'm just going to go to the moon for a day weigh myself i'm 150 so your weight is not itself the measure of how much mass you have okay all right you also weigh a little less at the top of a mountain because you're farther away from the center of the earth than you do in death valley where you're at a low point relative to other places on earth so if you get on a scale at the top of everest you're going to weigh less than you would on that same scale in death valley in death correct okay less okay people at the equator weigh a little less than they would if they're visiting santa claus oh that's there's a lot of sun down there and they got to stay in bikini shape all year long that's fine right that's really what it's about so oh for another we do another explainer video why there's actually very little sun on the equator okay it's it's one of the that fact that's where you find your greatest rain forest um sun is not happening on the equator but that's a whole lot of conversation it's a that's a climactic conversation we will have i promise all right so so but i get your sentiment it's it's south it's south and it's warm it's warm all right so so the equator is the speed at which you are moving is faster than at other latitudes people on equator moving about a thousand miles an hour we here in new york we're moving about 800 miles an hour so that centrifugal force makes them lighter than we are gotcha so all of this is affecting your weight but it's not affecting your mass okay so if you want to lose weight in any zone then lose mass all right just to be clear about that all right so weight loss programs are actually mass loss programs okay and so weight then is is about forces then it's not it's not about it's the definition of weight wait is the force of gravity on you well who's gravity are we talking about earth or this or or the sun or venus or the moon so you find out what is what are the conditions of that force of gravity on you in that place not only among planets but even different places on earth and then you've got your weight and that weight is related to how many molecules of truck exist but in the end of the day if you want to weigh less what you really mean is you want less mass you want less chuck tomorrow than you have today so just want to put that out there something better right everybody can agree on that let's check tomorrow [Laughter] we don't we don't show you the the mails we the mail we get um when will chuck disappear we will hold those from you right okay so now we talk about how heavy something is okay okay so when we think of something floating on water generally the idea of it being heavy is not that's not the same thought correct no of course if it's heavy it sinks okay so now let's take let's take uh you know uh you know let's go to who's the guy who splits wood sections into firewood uh whatever that is okay go take one of these bunion guys paul bunyan cylinders of wood okay it's gonna be really heavy we know this take it plunk it down in a swimming pool what happens to it it floats it floats even though it's super heavy okay it is heavy but it floats right okay so how do you get around that what do we let's let's give more examples of that okay um ivory soap nobody who's what okay yes ivory soap floats right and as far as i know it's the only soap that floats i did an experiment when i was younger but i haven't checked lately but i did test this yeah ivory soap floats all the others so same okay okay so why that should be a selling point i don't know but it was in its day it was a selling point and then it was it was 88 100 percent purer right and it was like pure what right exactly you know i i remember thinking as a kid but any every so long predates me but i remember thinking all that all right so heavy cream is lighter than skim milk heavy cream is lighter than skim milk and you know why because it floats on the skim milk that's how you get skim milk there you go the cream floats between rises to the top you skim it off you got some heavy cream so we say it's heavier but we don't really mean heavier heavier to me is the absolute weight how heavy is it can i pick it up what we mean is thinner with with no it's not thinner well it's more less viscous less dense okay this viscosity is no i should have added that we'll get to viscosity in a minute okay heavy cream is less dense than water than skim milk right if you are less dense than something else you float there you go there it is nothing else nothing else matters right okay you will float so heavy cream is we would say it's lighter than skim milk but that's a little deceptive if you want to be precise you say it's less dense than skim milk and therefore it floats okay so this is the fight i was in a fight the encounter i had in a pasadena coffee shop when i ordered hot chocolate with whipped cream and the hot chocolate came and there's no whipped cream anywhere to be found and i said um server where is the whipped cream they said oh we put it on i said where is it oh it must have sunk to the bottom at which point i said either you are lying or the laws of physics that apply to the rest of the universe do not apply in your coffee shop and so he got all gruffy and he went by and brought cream with him okay and then plunked it down and it took one bob and instead of and it floated i said thank you for my whipped cream mm-hmm now i don't know if that became like a story you know because i'm sure wait staff tell stories right but that's what happened so he's thinking it's heavy it probably fell to the bottom and would choose to lie and rather lie about it thinking that i didn't know what the hell i was talking about and when you took it away you looked at him and you went science you just got scienced you just got scienced science so a couple more things about weight mass and density so um you realize that logs float all right right if you're gonna say if i want it but i don't float very well all right so if i want to float on the water i'm going to use a log so why don't i get a big log and then hollow it out okay okay these are the first canoes right okay the canoes are made of wood you hot and you know the canoe is going to float i'm going to get in it and we're good to go well why don't you make a boat out of steel well i can't do that because the steel will sink that's not a boat that's not going to work no let's be clever about it okay by the way if you make a boat out of steel is then impervious to the weapons of other boats oh my gosh what a military advance this would be this in the 19th century i think it was where the first people figured out i can make a boat out of things that are impervious to cannonballs right and so whereas if you make a boat out of wood then it just busts through the wood right so what matters is not the density of the material what matters is the density of everything that is sitting below water so if i create a hull only the outer edge of the hole is made of steel what's everything else inside the hull made out of something else no uh it's air air air it's a it's like a ballast okay no no ballast has a different purpose right because you want to stabilize okay so you put some other you put some heavy things just at the very bottom of the hull and that keeps okay position and that keep that keeps the that keeps it in positions that would be ballast okay and a lot of the cobblestone in lower manhattan that built the roads was balanced in ships that came from europe okay and then they offload that and build built roads with it sweet yeah yeah so a lot of it no and other things as well but point is you so now the the density is what is the total mass divided by the total volume okay so that's grams per cubic centimeter or pounds per cubic inch that's that's so those are the units of density so let me get some units out there so we've got the units of mass or grams and kilograms and i'm not going to tell you the unit of mass for the english system okay and the unit he's supposed to say why not tell me why not no so seriously what is why wouldn't you give out the uh okay the unit of mass in the english system is slug no yes that's why i told you i told you not to ask me so yeah so so the mass grams kilograms this sort of thing volume is the cubic of some of some length so cubic inch cubic centimeter cubic meter those are flames density is the mass divided by the volume mass divided by volume okay so just see how that works so if i have a certain amount of mass and i put it in a smaller and smaller volume it becomes more and more dense yeah yeah so what happens that if the denominator gets smaller then the number gets bigger bigger so we get higher and higher density because you're cramming all this into a smaller volume right the volume gets bigger and bigger and bigger for the same amount of mass right then the density gets lower and lower and lower you get beach balls another right you're spreading it out spreading out so what i've done with the steel is i've put air in between the two steel sides of the boat so now the density the effective density is the mass of the steel plus the mass of the air divided by the volume and when i do that i get something that's less dense than water and the whole thing floats so then it goes it will only go down a certain amount in the water certain amount that's it that's it and it but it will float that's how you make boats out of steel and that's how military ships were first put together i mean cool the the the indestructible type right so then you needed a weapon that could destroy the steel and then you have the the eternal contest of warring arms rays arms race exactly exactly so so that there you have it chuck so so we have mass density so anything that floats on water is simply less dense than water right it doesn't matter how much it weighs or what it is it just has to be less dense than water does nothing else matters wow that's cool and it could weigh a zillion pounds and it'll just it'll still flow which is why my uncle edward floats that's a big man that's a fat brother i'm telling you oh he's a big fat buddy well so so fat is less dense than than water and bones and muscle is more dense so if you float every time you just have way more fat than muscle and bone well there you have it good for you uncle eddie that's why it's no accident that that marine mammals have a lot of fat right okay the blubber like as they call it right so there's a lot of fat there and that gives them buoyancy in the water yes all right oh by the way what's an ice cube made out of water but why do they float because they have air inside of them i don't know no well sometimes they do but that's not oh no they expand they expand it's less dense right so it's less dense the water the water it used to be now takes up more volume right so bigger vine makes it less than some ice floats there you go you know we should do an explainer on ice expanding we should do that we should do that not now not now we're going to oh why not well no i guess we're done we're done here we're done here all right well looking forward to ice expanding there okay thanks for doing this with me chuck of course always a pleasure neil degrasse tyson here for star call keep looking up you

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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, mass explained, density eplained, weight explained, Moon

Id: 3s9AYGYm_iI

Length: 15min 15sec (915 seconds)

Published: Mon Nov 02 2020