# The Naked Scientists Forum

### Author Topic: What causes theTidal forces to be symetrical?  (Read 2618 times)

#### tommya300

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##### What causes theTidal forces to be symetrical?
« on: 18/09/2010 07:36:05 »
I understand that the moon gravity affects the tide.
Low tide looks logical.
On the rough scretch below, discribes how it would look like.
With respect to N and S shows low tide.
E and W shows high tide.

The moon is positioned on the W side of the globe, and it looks obvious that gravity is pulling the water to high tide.
What force is making the tide high on the E side, when the moon is on the W side.
Is gravity pushing it away? How does this exactly happen?

.
« Last Edit: 18/09/2010 11:20:55 by tommya300 »

#### Soul Surfer

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##### What causes theTidal forces to be symetrical?
« Reply #1 on: 18/09/2010 17:56:52 »
Tides are not just a function of the gravitation of the moon they are a function of the differential gravitational attraction over the surface of the earth caused by the moons gravitational field.  One simple way of expressing it is that the moon attracts the water away from the earth where the moon is overhead and on the other side it attracts the earth away from the water.

A more correct and scientific approach is as follows.  The earth and the moon are in orbit round each other. (let us forget the sun for the moment this has a significant effect too but just complicates the picture)   This means that they are both moving around their common centre of gravity in a state of free fall where the orbital forces are exactly balanced but the earth is of a finite size and the moon is at a finite distance  the moons gravity is weaker on the side away from the moon and stronger where the moon is overhead  the orbital motion also gives a centrefugal (centripetal) force away from the moon  which is stronger on the side away from the moon and weaker where the moon is overhead so there are therefore two extra forces acting on the water that produces the double bulge

#### MartinTheK

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##### What causes theTidal forces to be symetrical?
« Reply #2 on: 21/09/2010 04:06:28 »
Doesn't the gravitational field actually warp space-time so that gravity doesn't "pull" anything. Space-time distortion is actually making the earth skinnier along the axis of the earth-moon pair so that water piles up on the antipodal surface as well.

#### Soul Surfer

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##### What causes theTidal forces to be symetrical?
« Reply #3 on: 22/09/2010 09:33:08 »
No. All the distortions are explainable in terms of classical mechanics and gravitational forces.  There are similar tides in the shape of the solid earth but they are much smaller of course because the earth is far more rigid than water.

#### imatfaal

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##### What causes theTidal forces to be symetrical?
« Reply #4 on: 22/09/2010 16:04:39 »
Soulsurfer - I seem to remember that these tidal movements of the actual body of the planet can generate significant heat; to the extent that the moons of jupiter and saturn (which experience much greater forces) could have liquids (incl poss water) under the surfaces despite their great distance from the sun.

#### Soul Surfer

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##### What causes theTidal forces to be symetrical?
« Reply #5 on: 22/09/2010 23:43:21 »
That is very true.  The heating effect of tidal forces was not seriously considered until it was found that Io was so hot when it was observed by the voyager probes.  Tidal forces do heat the earth  but nuclear radiation is thought to contribute more energy and this has helped to keep the earth's core molten

#### granpa

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##### What causes theTidal forces to be symetrical?
« Reply #6 on: 23/09/2010 03:46:16 »

centrifugal force can and should be ignored completely.
the tides are due to the difference in the force of gravity at different points on the earths surface.

« Last Edit: 23/09/2010 03:49:52 by granpa »

#### tommya300

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##### What causes theTidal forces to be symetrical?
« Reply #7 on: 26/09/2010 06:19:00 »

#### granpa

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##### What causes theTidal forces to be symetrical?
« Reply #8 on: 26/09/2010 06:31:39 »
No. All the distortions are explainable in terms of classical mechanics and gravitational forces.  There are similar tides in the shape of the solid earth but they are much smaller of course because the earth is far more rigid than water.
Quote
Google solid body tides, or earth tides.

They aren't tiny. They are about 30% of the height ocean tides (Hint: The Earth's K2 Love number is 0.3).

http://volkov.oce.orst.edu/tides/

#### The Naked Scientists Forum

##### What causes theTidal forces to be symetrical?
« Reply #8 on: 26/09/2010 06:31:39 »