pushing against the top-- right. --than when you're over here. - right. - and so guess what, gang? it's the pressure difference. the difference between the pressure pushing the bottom here and here, that difference is the same as the smaller pressure pushing here versus the even smaller pressure pushing here. so the difference in pressure is the same at any depth. but you can short-circuit the whole thing by just saying, hey, gang, the buoyant force is numerically equal to the weight of water displaced. it's displacing the same amount of water at every point. look, that level stays the same. it's not till i take it out that it falls, okay? so i'm displacing the same amount of water. ain't that nice? ain't that nice? okay. yeah. so as you're swimming and you're swimming deeper and deeper, does the buoyancy stay the same on you? i wonder how many people have drowned with that idea. [laughter] you know, ordinarily, when swimming-- you're swimming under the water, right? you're kinda swimming under the water. you're in a pool, just a few feet deep, right?

you're swimming, yeah. you kinda stop and you kinda come down like this, like you keep going out there, okay, and you stopped, and you kinda bob to the surface. isn't that true? that's because the buoyancy acting on you turns out to be a little bit greater than your weight. so the net force is up. but i often wondered what happens to someone who swims very, very deep, maybe closing their eyes 'cause the water's smarting their eye. and they're very, very deep. and they stop and they keep-- they just wait until they bob to the surface and they wait, they wait, they wait, they wait and clunk. oh, gosh. [laughter] then you're all out of breath. because it turns out when you get deeper and deeper, you don't get as much buoyancy acting on you. can anyone figure out why? really, really deep, the pressure gonna be more? more. do i change the volume of this with more pressure? no. not very much. how about like a balloon? very much. how about--aren't you a little bit like a balloon? you got lungs, honey? air in the lungs. can that air be compressed? - yeah. - huh?

so what happens to your volume as you go deeper and deeper? smaller. you get scrunched up, so the buoyancy gets more or less? - less. - less. and it might not be enough to bring you back to the surface. so if that baseball got squashed up, then the buoyancy would be less deeper. but if it stays the same size, then it doesn't. kinda neat, huh? got a little thought experiment, gang. a little thought experiment. see this great, big cubic block of lead right here? very, very heavy. see it? see this other cubic block over here of aluminum? they look the same, don't they? notice--both are painted. what color are they painted? green. again. green. green. yeah, they're painted green. can you see they're both the same shade of green? how many people can't see they're the same shade of green?

how many-- "i can't use my imagination?" [laughter] who? they're both green. in fact, they're the exact same-- they look exactly the same, don't they? but if you try to move them, you can find out which one is lead, yeah? now, i take these both identical volume blocks and i, boom, i push them into the water. [makes sounds] they sink. you got to tell me, which one got the greatest buoyant force? the lead, the humongously heavy lead, or the light aluminum? - same. - check the neighbor. how many say the buoyant force is greater on the lead? how many say it's the same on each? how many say, well-- how about it, gang? it turns out it's the same on each. you know why? same color. because they both have the same color. [laughter] all right. hey. same color, all right. anything with that shade of green will have the same buoyancy.

no, it's the same volume, yeah? 'cause the same volume displaces the same amount, okay? can we continue further? again, i've got the lead block. this block over here looks like it's aluminum painted green, right? but it's not. you know what it is? it's a styrofoam block. but you know what? it looks to you--doesn't it look to you to be the same? it has the same size, styrofoam, lead. i push them in the water. [makes sounds] one sinks. the other one doesn't. i got a question for you. upon which is the buoyant force greater? on the styrofoam or on the lead? check your neighbor.

okay, gang. how many say same, same? how many say there's more buoyant force on the lead? no, there's more buoyant force in the styrofoam. that's why it floats. none of the above. [laughter] how about it, gang? how about it? which one? lead. is it the lead? the styrofoam displaces the amount of its weight, same amount of water-- i got a question for you, gang. i got a question for you. you got a ship and the ship is loaded with iron. what's gonna happen to the way that ship floats? sink deeper. it's gonna sink deeper, yeah? how about if you got a ship loaded with styrofoam? - what happens? - it's gonna sink deeper too.

what's this with floating anyway? yeah. how come this piece of clay sinks? some would say, "because there's no buoyant force acting on it." true or false? - false. - false. there is a buoyant force acting on the clay that's submerged in this colored water, okay? but the buoyant force is how b compared to the weight? less. how many say, "when you drop it, ah, the weight is obviously greater than the buoyant force?" how many are starting to be able to think like that? or how many people say, "well, i will remember what is going on and try to regurgitate it as best i can?" no, no, no. you can see that, honey, the fact that it went down, the weight must have overpowered the buoyant force. how can i make the buoyant force bigger? by displacing more water. i suppose i take the clay. and now, i shape it such that when i put it in the water, it will displace more water.

let's see if it displaces more water. no, it doesn't. okay? not yet. the buoyant force is not big enough. i'd have to make the volume bigger. oh, i really can't make the volume bigger like styrofoam is all puffed up. but what i can do is i could shape it into the shape of a bowl and make out of it of guess what? begin with a b. - boats. - end with oat. okay? a boat. if you told people few centuries ago that you were gonna make a boat made out of iron, honey, they would look at you like you were bonkers, because iron doesn't float. people made boats out of wood. well, here's this clay. it has the same weight. watch this. oh, yeah. [laughter] yeah, look at it now, gang. [laughter] isn't that nice? now, it's being held up by what? - your fingers. - your hands. my fingers. you get the idea. more about his next time.

but what i wanna ask you next time for homework is this. i wanna ask you, if you have a ship that's loaded with styrofoam, will that ship float higher, lower or the same as if the ship were empty? so a ship loaded with a cargo of styrofoam will float higher in the water, the same level in the water, or lower in the water than the same ship with no cargo? that's your question for homework for next time. okidok? see you then. [music] captioning performed by aegis rapidtext