and he knew this very, very accurately and he knew this very, very accurately. and when he did that, guess what happened, gang? this side pulled down a slight, slight bit. it got out of equilibrium. so what von jolly did was he put some other little masses on here, now he didn't have this equal like this, i mean, you can see the darn thing, okay? but he puts some other little masses there and restored the balance. when he put the other little masses here to restore the balance that's to say he has the force that pulled this down. he knows the force? he knows the mass? 1, he knows m 2 and he could measure the distance very accurately. he has everything except g. all you got to do is take that force, numerical value, divide it by the product of the masses,

divided by the distance square and when he did that he, got a tiny, tiny number 6.67 times 10 to the -11, that's not-- 6.67 times 10 to the -11, that is what he got for the ratio. and you know what? change the masses, put a bigger one or a smaller one, make no difference, change of distance, you'll always get the same ratio of force to mass over distance squared just as you'll always get the same ratio for big circles, circumference to diameter, circumference to diameter. so this is the constant of proportionality. we don't call it 'pi' gang, guess what we call it? 'g' see, that's not called 'pi', that's 'g', big 'g', right, the big 'g'. and so we can say now we have the exact equation f = g... when this experiment was done the science writers correctly interpreted it. they said cavendish who did it first

has just measured the mass of the world. because see this piece of chalk? can you measure the mass of the chalk? yes, you know distance between it and the center of the world? yes. do you know that the g constant? yes. can you get the force? yeah, put it in the bathroom scale, you got the-- what don't you have? the mass of the world and at that point, then one could calculate what the mass of the world is and that's wild because when i was a kid and i was told that the science types knew what the mass of the world was, i say, including the mud puddles they don't know about in ceylon? including the sticks and stones and the elephants they don't know about up in india? including all the lava that they don't have a good handle on way down underneath and what's at the center of the world, they know what that is too? how do they know such thing? they'd be knowing such thing, honey, because...one left and that's the mass of the world. ain't that neat?

next time, we're gonna be talking about ocean tides. and we'll be talking about ocean tides, we're gonna be talking about the role that the moon plays when it pulls on the oceans of the earth, okay? but let's talk a couple of things about the moon now. you probably all know that there's only one side of the moon that can be seen from the planet earth, yeah? you look up and there's one face looking at you all the time, alright? now why is it that one face is looking at us all the time? some people will say, well, the fact that one face is always looking at us and it wasn't until the astronaut types went out and looked at the other side that they knew, you know what? maybe it was a great big-- you know, it could have been anything back there like a great big plastic mold, you know, to entertain earth types. you know, maybe like a hollywood set back there and they went back and they found the other side was pretty much the same as this side. the russians were the first to do that and the u.s. types right after that, yeah?

so we know we do have a world with two sides but nevertheless, one side always faces us. now the popular thinking is, ah, the fact that one side always faces us is evidence, is evidence that the moon does not spin about its axis like the earth. we know the earth spins about its axis. we know that. it turns around, around and around, okay? and it makes one turn every 24 hours. the sun's up here, we got what day over here and we get night and day and night, right? ain't that true? we know the earth is spinning. is the moon spinning similarly about its axis? take a stab at it right now with your neighbor. yes? no? i don't know but i'll take a guesstimate? that guesstimate will be called the hypothesis, go. how many say, you know, i think that moon is spinning like a top as well? show hands. how many say no, i think the earth might be spinning like a top, but i think the moon is frozen

so that it doesn't spin at all? show hands. how many say, well, i saw a lot a hands up before and i might, i kinda think that, but i ain't gonna put my hand up 'cause someone might know i'm wrong, and it'll be terrible, right? and that reminds me of the time i was a roller skating, man. i used to be a roller skate freak. oh, i loved to roller skate. i wish you guys had a roller skating rink around here but i'm skating at the rink and one time i'm skating with a lady, i'm skating with her and you know, go a bit where're you from? what's your name, all these bit like that you know? and she volunteers some information, she says to me, i said, you know, how long have you been skating? she says, "well, i've been skating nine months." and she says, "and i haven't fallen down yet." she was very proud of the fact that she had never fallen down. i said to myself, honey you can't skate either, she couldn't skate worth a darn, okay? she clopped, clopped, clopped, being very, very careful and she went nine months without falling.

she thought that was an achievement. and you know what, all the honchos in the rink? the honchos in the rink, gang, they're all rosin all over their bodies, all smeared with, every time you fall down, you slide in the rosin. and the honchos are falling down all the time. you try this spin, you try this jump, you fall down, you get up, you keep going. and this lady thought it was an achievement that she never fell down. how many of you guys will go through this course and at the end of the course say, you know what? i never volunteered a wrong answer to my neighbor, not once. of course, i didn't learn anything, but i never fell down. do you guys have a hang-up about saying something that might be wrong, huh? come on, it's okay, it's okay to be wrong. you gotta keep falling down, if you don't fall down, hey, you can't skate, huh? so learn how to skate, it's okay to fall down, it's okay to be wrong at certain times. but anyway, let's get back to this idea here. here's an eraser and we are on the moon

and let the eraser be the moon and we can read the side, can't we? this is the side that faces the earth, okay? this side faces the earth and doesn't the moon go around the earth? well, watch how the moon goes around the earth. what am i doing to the eraser, gang, as it goes around the earth? i'm twisting it because it turns out the eraser's going like this as it turns. and when it makes one turn like this, it'll also make one rotation. so it'll go... and--stopped it, it would just keep... stop it for a minute... ain't that neat? so the moon is turning around and around and around. and you know what? there's a reason why one side always faces us and it has to do with center of gravity and torque.

did you pick that up on the chapter on rotational motion? did you pick that up? and when you picked that up, did you say to your mother and father, hey, you know what i just i found out? there's a reason why one side of the moon faces us all the time. how many people did share that with their parents? how many say, well, my parents don't give me anything, i'm not gonna give them anything? do you people share these ideas with the people at home? how many of you people do share these ideas, some of these ideas with your friends, your family or someone you care about? how many do? how many say, not me, it's mine, i paid my tuition, no one's gonna get it. no, you gotta share these ideas. hey, here's a neat one gang, get this one. here's how it goes. let me get this out of the way a little bit. this one i get kind of excited about. here's the earth down here, and here's the moon. it turns out the moon up here, gets pulled into kind of an oblong shape.

we'll get into this when we get into tides. it turns out that this side is closer to the earth than this side and this side gets stretched out, so it kind of stretches into that shape, okay? that's a whole tide bit, we'll talk about next lecture. let's suppose the moon is like this. well, the center of mass of the moon about which rotation takes place is right there, but the center of gravity of the moon isn't there, because this part here is being pulled with more gravity than this part. why? 'cause it's closer, inverse square law, over here, it's being pulled harder than here, right? so what happens is the center of gravity is about there and so the earth is pulled as if all its mass is there, but it's rotating about this point. there's a line joining the two centers. what's this little distance right here called, gang? begin with 'la'. that's your lever arm

so you've got a little lever arm in this instance and you've got a force. when you've got a force through a lever arm that gives you a torque. and so a torque produces a what? a rotation. and so what it does is it rotates down like this. now if it overshoots to the other side, which way does the torque tend to rotate it now? back. like a compass needle in a magnetic field. it'll line right up with the field. you know, a compass needle, one needle's being pulled like that and one's being pulled like that and they kinda pulling this straight. this straightens right out and a compass needle will line right up with the magnetic field like this, huh? the moon lines up with not a magnetic field, what kinda field? the gravitational field, because surrounding that world is a gravity field, honey. and that moon is out in it and what happens is this lines up, so both those centers. here's the condition of equilibrium,

one that acts right through there. what's the torque when that force passes right through the center of mass? begin with the 'z', end with a 'p'? - zip. - zip. no torque, no torque, no what? so what it does is it hangs right in there? so it's not a coincidence that the moon has one side frozen to us. it is not a coincidence. in fact, when we go out and look in the solar system, you'll find out things that have been orbiting around long enough will eventually face each other and be frozen. that's the normal state in the universe. and even the earth is approaching that. one day the earth and the moon will be locked on each other and that day is coming, not for a while so you can kinda live it up, okay? i wanna ask you a question that next time when you come to class, i want you to have it. it's a question you can ask your friends at home and you'll get a variety of answers, but you will know the answer and here's the question. we all know that the moon is tugging on the oceans of the earth, we all know that.

we call that tug of what, the force of what? gravity, right? and we know that force of gravity have to do with tides, right, okay? we all know that the moon is pulling on the oceans of the earth, right? because the moon has a mass, the oceans of the earth have a mass, there's a distance between, there is a force there as well. i've got a question for you. which do you suppose pulls harder on the oceans of this earth, the moon or the sun? i want everyone to come to class next time knowing that and guess where you can find the answer if you're not sure now. guess where you can find it, gang? begin with 'b' end with book? in your book, okay? so next time, let's come in knowing that, hey, physics, huh?

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