some people say, if the polar icecaps melt, that 24 hours will be a little bit different than 24 hours because they think that when the polar icecaps up there melt, that they'll kind of spread out. what do you think? here are the polar icecaps up there and they melt and they spread out. what's gonna happen to the length of the day? check your neighbor. all right, gang, the mississippi river, mississippi river got a lot of dirt up there, and that dirt all flow southward and as it flow southward, what happens to the length of the day, to new orleans, okay? hey, so what? it turns out, the distribution of the earth, if we change the rotational inertia, what will happen to the spin rate? it will change also, decrease one, you increase the other, ain't that neat?

so you've got a whole lot of stars out there, and all these stars are all coming. they are slightly, slightly rotating, okay? and as they rotate, they gravitationally come in, and they get closer and closer and closer. now what do you suppose happens to the rotation rate of this new galaxy being formed as these stars all start to get closer to one another, take a guess. what's going to happen to the rotation rate? spread right out into a nice big disk honey and that's the way it is. so when you look out at astronomy, astronomers all the time interested in angular momentum because they know what's concerned. whatever angular momentum a galaxy has at one time, it will have that same angular momentum at another time, but if it's tighter and the "i" is smaller, then the speed will be greater. it works, gang. it's the rules of the universe everywhere. so we see a little sample of it right here.

question time. i've got here some water. i've got some water here gang. i get the water in the bucket. you guys know what i am going to be doing with this water in the bucket, because we're talking about centripetal/centrifugal force, aren't we, right? i'm going to take this water in the bucket, what am i going to do? i am gonna go right up my head. and if get to the head top, i'm going to stop, right? no, i ain't gonna stop, okay? nor that i could help it. i'll just go, okay maybe towards you guys, huh, okay? you have confidence in the laws of physics, huh? [laughter] but this is water gang, okay. how come the water didn't fall out?

how many say, "well, there is probably no reason for that, it's just that when it's spinning, the laws of physics break down. okay. how come the water didn't fall out, why? i take the ball on the string and i swing the ball around and around and around. all right, swing it around like this, okay? or like that. how come a ball goes in a circular path? maybe, because something is pulling the ball out. that's what the little kids say because you guys say, "wait a minute, there is nothing pulling the ball out." which way is that ball being pulled gang? in or out? begin with i, now try it. [laughter] okay, that's being pulled in, do you know a lot of people don't know that? they think that something is pulling the ball out.

they think that when i whirl the water around and around and around, that something is pulling the water out and you have a name for that. you know what they call that? they call it centrifugal force, center-fleeing force, a force going away from the center, but guess what, gang? there is no centrifugal force pulling outward on this ball and there was no centrifugal force pulling outward on that water. the forces that acted on both these things was pulled which way? inward. i have to pull on that string. if i stop pulling and let it go, what's going to happen? the ball is gonna go out or just continue going, and this is law of inertia. you know, the slingshot bit, take this thing like this. let's suppose the string breaks right at the bottom, right at the very bottom, right, like that. when it is at the bottom, which way does the ball go? everybody here i think know, everybody knows. how many say all of a sudden, go and hits the floor? how many say, no, it keeps going in a circle?

because a body going in a circle tends to keep going in a circle, no force needed. come on, come on! how many say, you know, when that string breaks, honey, there is a ping that's gonna keep going in a straight line path. see, so long as the force acts, it will pull it in a circle. if the force doesn't act anymore, then pew, the law of inertia takes place, huh? the idea that the object moves in a straight-line path, constant speed, we don't say straight-line path, constant speed, we got two words to say the whole thing... constant velocity, man, constant velocity. constant velocity means straight-line path constant speed. and when no force acts on this thing... tends to go at constant velocity. so there is an inward force: a force pulling toward the center. you're riding in your car and all of a sudden you're in a car and your friend takes a left turn, and phoom. you hit the door. so somebody asks, "how come your hands--" "oh, i don't know. that's centrifugal force pulled me out against door," true or false? no, what happens as you are driving along in the car, you tend to keep going straight, but the car goes like this

and you didn't put your seatbelts on that day, so what do you do? you tend to keep going straight, and the car comes in and hits you. so you didn't hit the car, the car hit you. or if the car hits you, you hit the car. there is an outward force on the car, but not on you. there is an outward force, there is a going away force on the string, but not on the ball, see that? see, the string pulls the ball in, in, in, in, in. if the string pulls the ball in: action, reaction is the ball pulls the string out, but no outward force on the ball. ain't that kind of neat? tomorrow's space colonies might look something like this.

within a great big huge tire, a couple of kilometers wide, and this thing would be spinning at a constant rpms, and people out here will walk around. if you took a regular balloon tire from a car or a bicycle and you put some ants in there, and you just took the tire and you just dropped it off a cliff, what would the ants do? the ants would just free-fall, it wouldn't push against them anymore. all the supports gone and they would just all fall. and they would say, hey wow, someone cut out the gravity, honey. but if you take that wheel and you spin it, and then you drop it, the ants will all get thrown on the outside, isn't that true? and the ants will think, there is a force pulling them out, but no, it's a force holding them in the circle. if all of a sudden there is a little hole there, the ants keep going, but the ants pulled here. so the ants are pulled, pulled, pulled. people inside a space colony, these people right here, head, feet, okay. this thing is rotating like this.

and these people are thrown against the outside rim. and these people think that there is a force pulling them out like this. they think that and if you've gotten one of those amusement-type devices, those great big centrifuges, did you ever get one of those, gang? you stand up against the wall like this, okay? and then i start revving around and when it is going fast, fast, fast, they drop the floor out and you stay there-- and you stay there, you're like, you can hardly put your head away boom, and you turn around like this, push-ups are hard to do man, okay, because you see yourself being pushed this way, but it's not happening. you're being thrown out that way, but instead of ending up down there, you're over there, because something pulls you in so. so there is an inward force pushing in on you, and you can go it is hard to put your neck out like, it goes right back, okay. centripetal force, a center seeking force, and that's what we will enjoy tomorrow when we are in space vehicles. now we can get out in regions where there is not earth's normal gravity and spin that son of a gun to a point

where we can feel ourselves as if there were gravity acting on us. there is probably people in this room that will do such a thing. and i know there is one person in this room that's already designing such type things. and this is where the human race is going, outer space. we can make our own gravity, simulate gravity, just by rotating at the proper speed. let's suppose you wanted to climb up a little tube in here and at the center, this is the hub. in the center, would you have any speed? this kind of speed? in the center, "r" would be zero and so you would be in a state of free-fall there, the little ant that was thrown to the edge inside the tire in the hub would just be floating around as that wheel fell off the cliff. and people way out beyond the moon, in one of these rotating things right in here,

would not feel any gravity effect, but people out here would. the human race, just talks about that today. the human race doesn't do that because the human race hasn't got to the point where we have these things operational, yet. do you know why? because the human race is back in the 1900s. and when the human race gets up to where there is the two instead of a one, then the human race will be doing things like this, at first a few people and then later, more. all you people have calculators. you get them in your rice krispies box, when you open your cereal. i can remember when calculators first came out, they were so rare and so expensive that at my school in san francisco, we were told, "you can't use a calculator for exams." why? because only the rich ones can have them

and that's discriminating against the poor folk, so nobody can use a calculator, they are too expensive, too hard to get. and how about today? like i say in your rice krispies in the morning, there is a calculator, okay? everyone has them. and so today, how many people orbit around the world, a lot or a little? how about tomorrow? i talked to an astronaut type couple of summers ago. and he belongs to a club where to be in that club, you have had to orbit the earth at least once. i thought to myself, must be awfully small club, it's a big club. i forget what it is, it's more than a hundred. you know what it is, tinnie? something like about 300 people that have orbited the world. yeah. there is probably someone in this room who will orbit the world.

which one? stand up please. tinnie, you're going? gang, i've got a nice homework problem for you. oh, you see that dandy, you're gonna be loving it. and here it is, it's not in your book. here it is. i got a can of chili beans. the chili beans roll down the hill. i've got a can of pineapple juice. the pineapple juice will roll down the hill. i wonder if they roll down the same.

well, one is juice, one is a liquid, the other is beans, hard packed beans. which one will get to the bottom first? something with liquid in it? or something with solid in it? this is a homework problem, you hand it in two more days. how are you gonna get the answer? you call up your friends, all right? you look in some other physics books. you talk to a physics type or how many say, "do it myself, do an experiment." do it, gang. and then see if you're cleaver enough to explain why you got the results you got. hint: do all the beans roll? does the liquid inside, roll or slide?

[laughter] think about such things, write it out for homework next time, so try it. take a can of liquid, take a can of soda pop and take a can of like hard packed beans and try it at home and see if you can explain, why one seem to roll faster than the other, okay? catch you next time, physics.

travel advisories to small business loans. retirement savings to medicare coverage. id theft protection to contacting elected officials. student loans to taxes on-line. whether you have information to get or ideas to give, usa.gov is the official place to connect with your government. from surplus car auctions to finding a new job, our new mobile apps will keep you updated on the go. so from marriage records to passport applications, veteran's benefits to birth certificates, patent applications to energy saving ideas, product recalls to home buying tips, check out usa.gov. because the country runs better when we stay connected.