Howard Winter wrote:
> No!!!  While tidal effects caused by the Sun's gravity can be said to
> be because of the Earth's rotation (and thus the apparent movement of
> the Sun), the major cause of tides is the Moon,

Yes.  That was probably unfortunate wording on my part.  I wasn't trying to
distinguish between sun/moon causing tides, just that tides are caused
because the earth rotates thru an externally caused gravitational field.

> It comes from the potential energy of the water being pulled towards
> the various celestial bodies, so forming a higher column as water flows
> in from surrounding areas, and then lowering again as the external
> influence "moves away".  In one way it's similar to a "storm surge",
> where an area of low air pressure inside (say) a hurricane causes a
> hump in the water below it, but in this case the extra upwards force
> comes from added gravity above, rather than reduced air pressure.

Right, but you haven't said exactly where the energy to move the water comes
from other than "potential energy" of various celestial bodies.  So are you
saying that if we remove power from tides, the potential energy stored in
the earth-moon system will decrease?  In other words, the moon will move
farther closer?

>> By extracting the power from the float, you are making the planet slow
>> down ever so slightly.
>
> Well you are extracting the power from the water, which depending on
> the direction of flow towards your collection point may in fact cause
> pro-spin friction between water and Earth, so having the opposite
> effect!

But I'm only extracting power from vertical movement, not horizontal.

Here's a thought experiment:  Pretend the planet was rigid with no oceans.
There is nothing to deform, so there are no tides and no power lost in tidal
friction.  Now suppose you had two very large masses at opposite sides of
the equator.  If you hold them fixed, you can measure a slightly lower force
to hold them up (weight) when in line with the moon and a higher weight when
at right angles to the moon.  Now suppose you raised the masses when their
weight was lowest (moon overhead or 12 hours away), and lowered them when
their weight was highest (moon at horizon).  On the whole you would be
extracting energy from somewhere since you get back a little more energy
from lowering the mass than it takes to raise it.

Where does that energy come from?  Think of the gravitational forces on the
two masses.  The rest of the planet is rigid and the center of gravity
doesn't move since both masses are always moved in equal and opposite
directions together, so it cancels out.  What we are left with is the
gravitational forces between the masses and the moon.  Since gravity
decreases with distance, the mass closer to the moon will have a larger
effect, whatever that effect might be.  So let's consider the force on a
mass as the moon appears to rise and fall viewed from the mass over 1/2
rotation.  For the first 1/4 rotation the mass is low.  When the moon is
directly overhead, the mass is raised and then stays high for the next 1/4
rotation.  The mass will therefore be a little closer to the moon as it is
rotating away than it was when rotating towards it.  This means it will be
pulled forward a little less than pulled backward.  The net effect is a
torque on the planet opposite of its rotation, thereby slowing its rotation.
The difference in rotation speed represents a drop in the kinetic energy of
the planet, which is where the energy you got from raising/lowering the
masses came from.  If you raised/lowered the masses the other way, you would
be supplying energy, which would end up increasing the planet's rotation
speed.


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