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Legion
Daedalian Member

 Posted: Fri Mar 18, 2005 11:44 am    Post subject: 1 I was looking into the 'two tides' (why do we have two tides a day) question and came across the fact that we are a double planet, earth and moon rotating about a common centre, but how fast is this rotation?
Chuck
Daedalian Member

 Posted: Fri Mar 18, 2005 2:29 pm    Post subject: 2 One month, apparently.
Legion.
Guest

 Posted: Fri Mar 18, 2005 3:14 pm    Post subject: 3 Really, Why must this overall rotation be the same as the orbit of the moon?

 Posted: Fri Mar 18, 2005 3:56 pm    Post subject: 4 I am not sure about "must" but, I presume that this is why the same side of the moon always faces the earth. Instead of the reason that the moon has phases. Or, that could be backward.
Leptonn
Guest

 Posted: Fri Mar 18, 2005 4:21 pm    Post subject: 5 You are correct that the earth and moon are a two-body system, however, the centre-of-mass (about which both seen to rotate) is much closer to the earth than to the moon, so the moon pretty much follows a circular orbit around the earth. As to the two tides, here is a picture: It doesn't show exactly what I want, but never mind. Consider the moon being in the "full" position, and the earth rotating underneath it. Nearest the moon, the sea is "pulled" towards the moon with full intensity, while the middle of the earth is pulled with moderate intensity, and the opposite side is pulled with low intensity. Over the course of a day, we move from the side nearest the moon (high tides) to moonset (low tides) to the side opposite the moon (high tides) to the moonrise (low tides) and back to the start. However, the tides drag a little bit, so the tides aren't *quite* full at high moon. Seems weird, but this is how it works. Chuck is right... the moon goes around the earth in one month. I think it's actually 29.7 days or something like that. The same side of the moon always faces the earth for the same reason that the tides drag a bit behind the moon. It's called merely "Tidal Forces". Eventually, the same side of the earth will always face the moon, too. The moon has phases because it goes around the earth and gets shined on by the sun from different angles.
Legion
Daedalian Member

 Posted: Fri Mar 18, 2005 4:27 pm    Post subject: 6 I'm happy with the part about always seeing the same face of the moon, it's all synchronised after ages, but I'm not sure about the system rotation vs moon orbit vs earth rotation timings. I can't really picture it.
+1

 Posted: Fri Mar 18, 2005 4:34 pm    Post subject: 7 Yes the moon and the earth are orbitting around a common center. All orbitting bodies do this. In the case of huge differences in mass, like the Earth and the sun, the center of rotation is not significantly different from the center of the more massive body. When two bodies orbit each other gravity produces a tidal force on each. Basically the part of the object closest to the center of mass is rotating slower than the part of the object furthest from the center of mass. This produces a stretching along the axis. This causes bulges on the sides closest to and furthest away from the center of mass. All orbitting bodies tend to become tidally locked. But in the case of the larger of the two, there is generally some axial rotation (but tidal forces slow this down...our days are getting longer). In this case the "bulge" rotates around the surface of the object with the bulges on the inner and outer face. Since the Earth has fluid over most of it's surface this can be seen easily in the form of high and low tides.
Dr. Borodog

Dr. Borodog

 Posted: Fri Mar 18, 2005 9:09 pm    Post subject: 9 Hmm. I just realized that diagram doesn't make a damn bit of sense given my explanation. That elliptical shape is most definitely not perpendicular to the net force vectors in the upper and lower limbs. Must think . . .
Doc Borodog
Guest

 Posted: Fri Mar 18, 2005 10:22 pm    Post subject: 10 Ah. Of course the force vectors aren't perpendicular to the ellipse at their heads; they are perpendicular to the surface at their tails. Whew.
Coyote

Posted: Sat Mar 19, 2005 11:33 pm    Post subject: 11

Ah yes, of course. *nods knowingly* Force vectors at the tail, not the head...yes, yes, I see.

Ahahahaha!!! This is all way too heavy for me, but you said something earlier that got the clanking gears turning for a bit:
 Quote: The surface of the water will always be perpendicular to the lines of gravitational force. If it wasn't, it would "slide down the hill" until it was. This basically acts to minimize the gravitational potential energy of the water (it would take energy to maintain a "hill" of water, so all the water that the "hill" was made out of flows "off" the hill until there isn't one and the water is flat).

 Quote: ...the Earth is rotating, which creates an equatorial bulge all the way around, which we'll ignore...

We ignored it so far as discussing tides goes--fair enough, but now I'm wondering. If there were no tidal influences at all, would the oceans still serve to slow down the earths rotation? They've been spun up into a 'hill' due to rotation, but they're still trying to roll back down the hill. Would the energy they use in trying to roll down take away from the energy used in rotation, thereby acting as a brake even without tidal forces?
_________________
Hard work may pay off in the long run, but laziness pays off right now.
+1

 Posted: Sun Mar 20, 2005 12:43 am    Post subject: 12 Nope. That's a closed system.
Coyote

 Posted: Sun Mar 20, 2005 1:26 am    Post subject: 13 Hmmm...okay, but even in a closed system the energy can be distributed in different ways. The spin of the planet versus the tendency of the water to seek its own level could be converted into heat. [note:] I already know that you and Dr. Dawg have a way better grasp of physics than I'll ever have, and that I'm almost certainly wrong here. But what I want to know is why I'm wrong. Revealing the errors in our thinking is a time-honored method of learning, you know. _________________Hard work may pay off in the long run, but laziness pays off right now.
extropalopakettle
No offense, but....

Posted: Sun Mar 20, 2005 2:08 am    Post subject: 14

 Quote: The surface of the water will always be perpendicular to the lines of gravitational force.

I'm just geussing, but I think that taking everything into account, the surface of the water will always be perpendicular to the lines of the combined forces of gravitaional pull (which is always towards the earths center) and the centrifugal force (a virtual force?) due to spin, which is always directed away from the rotation of axis, perpendicular to it, but is strongest at the equator, and zero at the poles. Thus water is perpendicular to the lines of gravitational pull alone at the equator and the poles.

 Quote: If there were no tidal influences at all, would the oceans still serve to slow down the earths rotation? They've been spun up into a 'hill' due to rotation, but they're still trying to roll back down the hill.

I'm sorta geussing, but I don't think they are trying to roll back down any hill.
+1

 Posted: Sun Mar 20, 2005 2:17 am    Post subject: 15 Well, thanks for grouping me in the dr's class, but with a doctorate, I think he'd probably out physics me any day. For example, I will probably mix up things like force and work in any explanation I give. I'm trying to think of a good way to alleviate your trouble, and I'm not thinking of anything good. The only thing I can think of is to explain why the tidal force acts as a brake. Moving all that water around takes force and that force acts in an opposite direction to the force of the rotation. Just like a brake on a car (except that's friction, but anyway). You can picture the brake on a car as pushing the wheel in the opposite direction that it's moving. This slows it down. All that water on the Earth is part of the whole system here, or the wheel. The tidal action is pushing that part of the Earth in a direction opposite our rotation. This slows the Earth down because that water is part of the Earth. What you're talking about isn't rotational at all. It's all moving towards the center. To decrease the rotational energy you need to have a force that acts opposite to it. And I can't think of anything else. My brain is tired, sorry.
+1

 Posted: Sun Mar 20, 2005 2:22 am    Post subject: 16 And extro is right, I missed that bit. Rotation doesn't put anything up any hill. An object X meters from the center of mass has Y potential energy regardless of whether the body is spinning or not. A satelite in a geosynchronous orbit has the same potential energy as a satelite that's spinning around the Earth furiously, if it is at the same height.
Dr. Borodog

Posted: Mon Mar 21, 2005 12:47 am    Post subject: 17

 Quote: Hmmm...okay, but even in a closed system the energy can be distributed in different ways. The spin of the planet versus the tendency of the water to seek its own level could be converted into heat.

That's irrelevant to the rotation of the Earth. What makes the Earth rotate slower over time is not force (or energy) at all, but torque. The Earth cannot torque itself; it needs the Moon (and to a teeny, tiny extent, the Sun) to do that.

The reason the Moon is receding from the Earth is conservation of angular momentum, as the Earth's rotation slows due to tidal torque exerted by the Moon, the angular momentum she loses goes into the Moon's orbit, pushing her farther out.

 Quote: I'm just geussing, but I think that taking everything into account, the surface of the water will always be perpendicular to the lines of the combined forces of gravitaional pull (which is always towards the earths center) and the centrifugal force (a virtual force?) due to spin, which is always directed away from the rotation of axis, perpendicular to it, but is strongest at the equator, and zero at the poles. Thus water is perpendicular to the lines of gravitational pull alone at the equator and the poles.

More or less, if you're talking about the case without the Moon.

 Quote: The only thing I can think of is to explain why the tidal force acts as a brake . . .

I'm not sure I followed the explanation that followed, so I'm not saying you're wrong, but . . .

The tidal torque comes from the fact that the tides lag slightly, meaning that the net tidal forces are not axial.
+1

 Posted: Mon Mar 21, 2005 1:50 am    Post subject: 18 Torque was the force I was looking for.
Doc Borodog
Guest

 Posted: Mon Mar 21, 2005 2:14 am    Post subject: 19 I know what you mean, but torque is not a force.
+1

 Posted: Mon Mar 21, 2005 2:21 am    Post subject: 20 See, I told you I'd mix up terms.
Vinny
Promiscuous enough

 Posted: Thu Mar 24, 2005 5:22 pm    Post subject: 21 Here's a theoreotical question, what if the Earth has 2 moons, of the exact same mass, and they behave exactly the same except opposite of each other in location, e.g. on one side of the earth while the other one is on the other side. Would we not have tides at all then? And what if the Earth has 2 moons, of different size, and in some weird non-synchronize patterns. Would that mean the end of life of Earth?
Doc Borodog
Guest

Posted: Thu Mar 24, 2005 6:15 pm    Post subject: 22

 Quote: what if the Earth has 2 moons, of the exact same mass, and they behave exactly the same except opposite of each other in location, e.g. on one side of the earth while the other one is on the other side.

No, we would have tides twice as strong as we do now.

The other situation you describe would be unstable as long as the two moons were of sufficient mass to cause tides in the first place; in other words at least one of the moons would be ejected from the system eventually.
extropalopakettle
No offense, but....

 Posted: Thu Mar 24, 2005 6:29 pm    Post subject: 23 What if the Earth had two moons of the same size, in the same orbits, but going in opposite directions, and they were perfectly hard solid moons, like those steel balls on those click clack thingies: Would that be weird or what? Would we still call it Earth, or would it be "the Click Clack Planet"?
Mackay
Saviour of Spiders

 Posted: Thu Mar 24, 2005 7:01 pm    Post subject: 24 I think extro's post is one of my favourite ever.
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