tv Earth Focus LINKTV March 1, 2012 6:00pm-6:30pm PST
we're going to talk today about ocean tides. ocean tides. i've got a question for you. the tides are influenced by the moon. the tides are influenced by the sun. which plays the greater role in raising tides, the moon or the sun? what is it? the moon. i got a different question for you. there's a force of gravitation between the oceans of the earth, the oceans of the earth and the moon. there is also a force of gravitation between the oceans of the earth and the mass of the sun, so both are pulling. which do you suppose pulls harder on the oceans of the earth, the moon or the sun? how many say the moon? stand up. how many say the sun? how did you guys know it's the sun? how do you know? where did you find it?
how could you not read the chapter and be astonished by that? i always used to think that the moon pulled harder on the oceans because it's closer, and the sun's so far away that the pull is a lot less. that's what i used to think, and then i get into my physics and boom, sometimes in physics you find things that are kind of like counterintuitive, don't they kind of say, hey, wait a minute, what's this, huh? and what's going on? it turns out that sun is far away, but honey, it is big. let me put it this way. you know the sun's hot and when you step from the shade into the sunlight, you can feel the heat of that sun, right? and you might say, wow, it feels-- i can feel the hotness because the sun has got such a high temperature, but you know what gang? i can bring you to welding shops wherein the welding, the torches of some of those flames are hotter than the surface of the sun, hotter, and you walk by that welding shop and you don't, whoa, go like that, but you step from the shade into the sunshine
and whoa, you feel it. it's not that the sun has a high temperature. you know why that sun is so hot? because it's big. i mean, big, big, very, very big, but it's far, but it's bigger than it is far and it pulls on the earth's oceans more than the moon. so how come we get then the tides by the moon? and the person to figure that out was named isaac, guess what the last name is gang? you know, how many people would not be knowing? isaac newton is the one that figured that out. let's suppose we have a ball of putty or a ball of taffy or a ball of jell-o, something that's really pliable, something that you can whang around a lot, okay? and let's suppose i pull it across the room, and i pull it everywhere with just the same amount of force. what it would do is it'll race across the room
and when it got here if you took a snapshot of it, it would still be in a ball shape, okay? let's suppose instead that i pulled on that jell-o a lot stronger here on this part and so-so in the middle and over here a little bit weak-- quite a bit weaker. ah, now i have a difference in pulls across the jell-o. that jell-o, gang, is gonna race across the room. when it gets over here and you take a snapshot of it as it goes by, would it have that shape? no it wouldn't have that shape because this side here is accelerating more than this side. it's a pliable stuff now, so this side here would, maybe, get out to here. this side here is not being pulled so far, that would lag behind over here and so what would happen to the shape of the jell-o, gang? it'll be kinda like this. it'd be kinda stretched out, wouldn't it?
and guess what behaves the same way? it begins with o and ends with cean, try it. the ocean of the earth. because it turns out, this is our earth, here's the moon over here. the moon's pulling this side harder than it's pulling this side. why? the answer is easy. check your neighbor to be sure we're all there. why is this side being pulled harder than this side? how many say because this side's closer. it's closer. when you're closer the gravity is stronger, hey, no big deal, huh? and that's right. so this side is being pulled out into a shape and this being left behind. now is the earth really moving towards the moon? we on earth think, no, no, no, no. the moon is moving toward the earth. there's a force on the earth, i mean, there's a gravitational force between the earth and the moon that pulls the moon this way, right?
why don't the moon crash? because it's moving like that. so as it's being pulled instead of coming over here, it's moving like this. it doesn't get to here, it gets to here. it's being pulled like that, instead of getting here, it gets to here. and so it gets pulled around and around and around in a circle, but guess what the direction of the acceleration is as it's going around and around and around the circle? it's toward the center of the earth, see? so we think of the moon as going around and around and around us like i take a rock in the end of a string and i go whirl it around, around, around. we talked about the idea that the force along the string is acting toward me, right? the rock is really accelerating toward me. if it weren't it would go somewhere else, but it keeps pointing toward me all that time, huh? i mean, it's just huh, huh. well, that's what's happening here too. the moon is being-- but if you lived on the moon, how about the moon-types? what would they see? hey, no, no, nothing, the moon is center of the universe, the earth going around us. who's going around who? we're both going around each other.
hey, you guys think the earth is just like this and the whole universe is going around you. come on, okay. it turns out the earth and the moon going around each other, a common center of gravity. that common center of gravity is still inside the earth. nevertheless, you can think of the earth as accelerating around the moon and this part closer accelerates more than this part and gets stretched right out and then the solid earth underneath like this rotating every day, huh, every 24 hours. so what do you do, you rotate right into a high bulge of water. then we rotate into a low bulge of water and then to a high bulge again and then low and high and low and as this sort of turns, see, as it turns-- let's suppose the moon's over here, huh, pulling, and as all the time would bulge out like this. hey, i'm simplifying, gang. i'm pretending that this earth mass isn't here. that makes variations that are very important. we're really simplifying, and consider the whole world to have no land masses that are going to get in the way, just the overall idea of tides, a bulge on this side, a bulge on this side and kind of shallow in here.
so a person right here says, oh, wow, moving back here, the tide is high. maybe it's about a meter higher than usual over here. well, the tide just went out. the tide got low, okay? and back here, the tide got high again, low, high, low, high, low, high. so how many high tides do have every 24 hours? beginning with a "t," two, i'm sorry. how many low tides do we have every 24 hours? it turns out every 23 hours, you know why? because the moon just don't stay like that. the moon is kind of turning as it's happening to-- so we could say, roughly speaking two tides a day, two high tides and two low tides. that's kind of easy to see, isn't it? stretch right on. if you're walking along the street and someone grabs your shirt and pulls it and they grab this part and pull it and this part and pull it, pull every part of your shirt the same, you'll move over but your shirt won't get ripped. but you walk along the street and someone only pulls this part and they don't pull the other part, it might rip your shirt
because one part's being pulled harder than the other and when you get a difference in pulls, then you're going to get a stretching effect and that's why we get the tides. we get a difference in pulls of the moon. do we get a difference in pulls of the sun? answer begin with a "y," end with a "p." try it. guess then, all right, the middle is a "u," okay, yup. you do. you get tides from the sun as well, okay? but the sun's so far away that it pulls just about the same on this side as on this side. very, very far away that difference is not so much. so the sun is pulling like mad, 180 times harder than the moon but on nearing this far side, not so much different. up close, yeah. you know what happen to that moon if that moon gets closer and closer to us. by the way, we--guess what has two bulges as well, it begins with an "m" and ends with oon, try it. the moon. the moon gets two bulges too, okay?
but the moon, of course, as--the bulges stay fixed because it rotates just as often as it revolves. we talked about that last time. so on the moon, you don't see it going up and down, up and down every six hours. it just stays up. it's like a football shape, okay? you know what happens when the moon gets closer and closer? the tidal forces get bigger, smaller or stay the same? bigger. and, you know, what happens when the moon gets too close? they get ripped apart, ripped to shreds and all those shreds will just orbit around and spread out like a great big ring. do you know there are planets where that's already happened? one begins with s, ends with turn. do you know what it is gang? saturn, that's right and the other one is uranus, neptune, okay. rings, moons that have gotten too close and just simply got ripped apart, they are whole lot of boulders and they're just all spinning around. that's what they are. tidal forces can be enormous when you are very, very close to something because the difference in pull between near and far might be greater than the force
with which everything is held together with. makes sense? so tidal forces don't occur too much for long, long distances but for short distances where the difference in pulls is a lot, the tidal forces can be enormous. we don't get the same depth of tide every day. some days, the tide is higher and lower than others and the reason for that is because both the moon and the sun are pulling at the same time. you see, if i have the moon pulling here and i have the sun out here pulling in the same direction, that's going to make these bulges even more. the sun contributes about 1/4 as much influence as the moon. i mean, it's still as big, okay? so when they're lined up, you get extra high, high tides and extra low, low tides. and it turns out that if the sun is on this side or this side, you get the same effect. it's just if they're lined up. now when the sun is on this side,
what do we call that condition in a month gang when both the moon and the sun are on the same side in between the earth or when the moon is between the earth and the sun? do you know what we call that? we call it a new moon because at nighttime over here, these people here don't see any moon in the sky. you know, the moon is not out of every night, gang. sometimes the moon is out in the daytime, okay? and if it's exactly in front of the sun what do we call that? not an eclair, what do we call that gang? an eclipse, okay, and notice a shadow coming right down here, it's basking that shadow, okay? but usually it's not lined up exactly, and so we call it a new moon. you look up, you can't see it because you're looking at the dark side, see, and then with a night sky. but how about the other case when the sun is on the other side and here's the earth in here and then we get the moon over here. what kind of moon do we see gang? full moon, that's right.
because what's happening now, here's the dark side of the moon, here's the dark side of the earth. people up here look up and they see the whole moon full, you know. it's like the globe right here. if i play flashlight tag with you guys and i put the globe up here and when i put all the lights in the room out, black and everything, okay, and i get in different parts of the room and shine my flash light on the globe. can you look at the globe and tell where the flashlight is? how about of the whole bottom of the globe will lit up in the top are dark, dark room, where will the flashlight be? how many say, "oh, probably up there somewhere." who? stand up. where will the flashlight be gang? down underneath, yeah, yeah. and let's suppose you look at the globe and it's all lit up, where is the flashlight? maybe in back of you or in front of you but along a line where you are, yeah. and if you see like a quarter of it lit up over here, just a quarter lit up, wouldn't that be back here, maybe? this part be lit up and you guys could see this part lit up in here.
so how the moon looks depends, of course, where the sun is, you know that. it's all the same. but when you have a full moon, how about the tides, extra high? the answer begin with a "y." you got a friend and your friend wants to go clam digging with you. you guys know what clam digging is? you ever eat clams before? you know where the clams come from? the tidal flats, the mud, you gotta go up there and you gotta dig 'em. you gotta dig with a pitchfork. you dig 'em up and you do that at low tide, and when the tide's really low, honey, you can go way up and get more clams. i used to do that a lot, okay, clam digging. let's suppose you're going to go clam digging and you wanna go at the right time in a month and some guys says, "hey, i know a good time to dig some clams. "saturday night is supposed to be a full moon. "let's go saturday. we'll go dig clams at the time of the full moon." you are not too sure about that, so you consult with your friends
who are knowledgeable about the wonders of the universe and those friends are sitting right beside you now, and you say to your friend, "gee whiz, would that be a good time to go digging or not a good time to go digging?" check. what would be the answer, gang? how many say, yeah, "i think "that's the time of the month to dig your clams, honey. that's the time i'm gonna go." show hands. 1, 2, 3, okay. how many say, "no, i think i'd go to the time honey "when it's got the least fullness. i'll go at the time of the new moon." hey, even steven seem saying, "no, i'm gonna go when it's a half moon." or, "i don't know "i'm not too good thinking about such things. i'm here to take notes, gang." you tell me, i'll write it down, teach and i'll memorize it and i'm just looking for a "c," anyway. at the time of the full moon, how's the high tide going to be? extra high?
are you gonna go digging clams at the high tide? are you gonna wait till low tide comes? you wait six hours later and you go, right? now should you go at the time of a full moon? how many say, yeah, that's the best time because you can get more clams because the water go away, way out and you get a real, real, low, low tide. show hands, only two people on this side of the room. i got something to invite you all to do saturday night when you take your bath. when you take your bath, i want you to get in the tub and fill it up and i want you to start sloshing the water back and forth. and when you slosh the water out in front of you and when it gets extra, extra high, that's the time of the full moon. turn around quick and look behind you and guess what you're going to find? guess what you're going to find, extra what? no, not high, extra what? extra low and that's time of the full moon, the extra high tide is going to be matched by an extra what? conservation of water, there's only so much water to slosh around, see that? so the people back here are wonderful.
they can kind of see that. if it's extra high one place, honey, it got to be extra low somewhere where the water come from. so yes, you go dig in your clams at the time of a full moon right on because you have extra low, low tides. how many say, "oh, maybe the tides are only confined to the water." it certainly couldn't be confined to the solid earth itself because it's solid and rigid, right? is the earth solid and rigid? about like an orange, what's underneath the surface, what's underneath the skin, huh? you've been over the big island lately? okay. it turns out as all molten and that molten is that molten part of the earth. are parts of that molten part closer to the sun and the moon than other parts? okay. so what's that gonna do to it? stretch it out and you know what, gang? we have earth tides, earth tides about a quarter of a meter puckered back and forth, back and forth about every six hours just like the ocean tides, tides in the earth.
when you suppose the highest probability is of getting a earthquake or a volcanic eruption, full moon or some other time? beginning with f, full moon, why? it's extra stretched. san francisco earthquake, 1906, very close to the new moon and they're all lined up, extra stretched. you also get tides in the atmosphere, the atmosphere of the world, okay. it's very, very low mass, they're not big, but there they are, and at the top of the atmosphere we have all these ions. ions are charged particles made by the cosmic rays coming in and splattering the air atoms and what we get is we get a big flux of a change, a change in the distribution of those ions with the position of the moon and the sun. and when you get a full moon or a new moon, you get the deepest, you get the deepest atmosphere and that gives rise to what is called these changes magnetic tides and these magnetic tides at the top of the earth
regulate the amount of cosmic rays that come down and hit us. did you guys know cosmic rays are coming through the ceiling right now and going through you? and if they go through without making a hit, who cares? but sometimes they make a hit, good or bad? it depends who was hit, and if it's hitting you, it's not good, okay. but hitting maybe some you don't like or-- these things going through us all the time, and there's a little blanket up there that regulates how much come through and guess when the greatest change from deep to shallow is? guess what time of the month are the magnetic tides up there, when the moon is what? begins with a "f." full moon. have you noticed that your friends are sometimes a little more weird at the time of a full moon? it might have to do with physics. lee. would it be the same thing, they're just about as strong as the new moon? yeah, oh, new moon or full moon. when i say full moon, you're right. new moon would have the same effect, that's right.
so the time of a new or a full moon, twice a month. yeah. do you guys be knowing what a blue moon is? this is not physics, a blue moon. i found out just two years ago what a blue moon is, once in a blue moon. how many people know what a blue moon is? one, but i tell you-- if i told you, i would rob you of years later finding out what a blue moon is. you say, "hey, son of a gun, that's what a blue moon is? "how about that? "hey, man that's pretty-- i'll never forget that." but if i tell you in class, you forget it, right? see if you can find out from lee or someone else. "hey, what do you mean by a blue moon honey?" what's a blue moon? is it really blue? the answer begins with a "n." all right, ends with an "n" and with the o. try it. yeah, okay guys. but see if you can find out what a blue moon is, not physics. kind of interesting. question? yeah, what's a harvest moon?
check that out too. here's your earth gang. we stand at the surface. we're pulled down with a force. we're pulled down with a force of gravity between our mass, the mass of the world, the distance between our belly button and the belly button of the world squared, quick boom, we get that force called our weight. isn't that true? so, we weigh, we have a weight because of the force of gravity. let's suppose we went down inside the earth and get right at the center in a hollow part. make believe it's hollow and right down here, how much would you weigh, a lot or a little? a little. the answer begins with-- check your neighbor. see if your neighbor yonder this. answer begins with a what? not x, not y but z, zip.
honey, you get no weight down there? you know why? see up here, you're pulled like this but you're pulled over here too. you're pulled over here, pulled in all these directions, you know, and all these directions pull you as if they were all acting straight down, see? all of those have a result and it's straight down but when you're down here, you're pulled to all this, you're pulled to all this, you're pulled here, you're pulled here. what's the net force? all these things combine to be what? not as straight, huh, combined to be a zero. so in the center, you got no weight at all. did you get me? at the center of the world, you just bathroom scale you wouldn't, you wouldn't squash the springs. as every part is being pulled in all directions, all the forces cancel out. how much you get halfway down? how much you dig a hole? and down here you put a bathroom scale on it. would you weigh more or less? how many say, "i'm not sure. i'm not sure of anything in this world but i think less."
show hands. yeah, everyone except the front row, except one. okay, all right. it's not right, you guys go in the back next time, okay? see the other guy's got hands to look at, see? see this back row types, and see the hands go up and they join the chorus, see? and you guys don't have that privilege. i'm just kidding. it turns out you would weigh less and why you weigh less? well, you could say with the zero here and the weight up here, it's got to be in between. or you could say, "no, wait a minute, it's not the way to look at it." when you're right here, you're being pulled down by all these ground over here but you're being pulled up by all this over here. this is sort of like an unpulling being done, isn't that right? because we're attracted to every particle that makes up the earth. so these parts here up, here up, here up, just off the scale a little bit and it turns out you would weigh less but rather than draw a line like this and say we can have this cancel that. let me just give you the results of something really, really neat.
it turns out there's another way to analyze it and that's this. it turns out that you're halfway down right here. if you take this particular radius which is less than the radius of the world now and consider that ground and take the mass of that ground and the mass of you and this distance square, that will be your new weight and it turns out if your half way down, it would be half, half weight. and, you know, what about all this ground here, you can throw it all away because all of this stuff here doesn't pull on you at all, not a net pull. whenever you're inside a shell of uniform composition, that shell won't pull on you. you can just throw it out and as you get the smaller and smaller amounts, you can see that weight keeps going down, down, down, down and finally when d gets zero, you're down at the zero
because there's no mass left but it's the mass outside in that -- shell and you can throw away. lemme show you a little argument for that. here's a shell of warm composition. i think you can see that in the center, any object in the center would have no gravity because it's being pulled in all directions equally. do we see that? but what i want to do is i want to convince you that it would cancel out no matter where you are and so what we'll do is we'll take a point that's about here where this distance here is twice this distance. do you think it would still cancel out this side and this side? what we'll do is we'll consider a solid cone--of material. we'll call this m1. we'll call that d1.
we'll call this m2 and 2d. well, just d and 2d. let's look at our equation, let's have the equation guide you are thinking again. a force over here would be our mass, say the mass of a point, okay, times m1 divided by d square. that's this force. the force that's pulling over here, you might think it's more because it's more mass or you might think it's weaker because it's further away and guess what one does to the other, okay? when you're out here, it'll be twice as much this way but twice as much this way, so how big would m2 be compared to m1? do you see it's four? it's four, so m2 would be four times m1. that would be like four m1s times your mass "mp" divided by this distance here, 2d square,
but when i take the twice the distance and i square it, what do i get, gang? isn't that like 4d square? and what will the force do? and what's this? the same done thing i got here. so, you know what, the pull here, the pull here, equal. i just took a place that was easy because i get twice the distance here as here. but you put any distance you want, and that'll happen. anywhere inside there, there'll be no gravity. there'll be no field region, inside a shell like that, there'll be gravity-free region. there'll be no gravitational field due to this mass. it would cancel out everywhere. so if you ever get to a big, big hollow planet, the people on the outside of that planet would walk around as if all the mass would concentrate at the center, but down inside there, you know what they do? they just float around, honey. the gravity would cancel out everywhere, very, very neat. so all we're doing over here
is when we take a shell, all this cancels out. no matter where you are, get right here all the-- all the--to here will cancel, all the--to here for that shell really, really kind of nice. let's take a case that sort of has a different result all together where we get closer and closer to the center. let's take the case of a collapsing star. let's suppose you're standing on this planet, star or whatever and here you are and you've got a certain weight. let's just call it 1, that's your weight. you know what that weight is, that weight's the result of your mass, the mass of the whole star, divided by this distance square. well, that distance is here to here, okay? if this is a very, very massive star, you're going to have more weight.
if you're more massive, you have more weight. if the distance is greater, you have less. what we're going to do is we're going to pretend that the star collapses and we're going to see what will happen to our weight at the surface. let's suppose the star collapses to become half size. well, you're still at the surface. it still has the same mass. if you take a loaf of bread and squeeze it up to its half size, it still has the same mass, it's just more compact, yeah? okay, so we still have the same mass, you have the same mass but their distance now is half as much as before. well, that make you weigh more, less or stay the same? well, we can kind of put the numbers in. here's your mass, the mass of the star but now the distance is half the distance that we have over here but it's squared, isn't it? and cannot write that like this?