Olin Lathrop wrote:

> Gerhard Fiedler wrote:
>> Not necessarily. It could be that the energy would get transformed from
>> kinetic energy into other forms (heat?) anyway -- float or no float.
>> Maybe the float just redirects a small part of the energy flow that
>> goes from kinetic to thermal into electric instead; in that case, the
>> float wouldn't introduce an additional slowing, but would cause a
>> certain ever so slight cooling.
> 
> My point was where the energy was coming from in the first place.  I say
> it comes from the kinetic energy stored in the rotation of the planet. 
> You seemed to disagree so I wanted to hear where you think it does come
> from. 

No, I didn't really disagree with that... Maybe Howard?

> But to address your comment above, you are saying that the energy in
> moving the water up and down as tides gets dissipated anyway, therefore
> directing it to make electricity doesn't remove any additional energy
> from the source. I disagree with this also.  My main argument is that
> much of the energy is not dissipated, but returns to the system.  It's
> sortof like bouncing a mass at the end of a spring.  Once you get it
> going, it take relatively little power to sustain a large oscillation.
> That's because little is being dissipated.  The energy just gets sloshed
> around between the spring and the mass.  Only the little bit lost to
> friction and air resistance must be replaced by the driving mechanism
> each cycle. 

If this is true, then I agree with you.

> Certainly there is energy dissipated by the oceans as they rise and fall
> due to tides, but most of it is stored and released as potential energy
> in the height of the water column.  Think of a planet that has a
> uniformly thick ocean layer over a perfectly smooth solid core.  The
> tides would be traveling waves.  They would loose a little energy in the
> internal friction of the water due to viscocity, and some more in the
> friction with the ground, but most of the energy would be transferred
> around the globe in a big wave.  The driving force is essentially in
> phase with the motion. 

Not sure how much the internal friction contributes. Any numbers?

> Now think of a large float used to extract this energy into up/down
> motion of a fixed mechanical system.  If there is no force on the
> mechanical system (the float just bobs in the water), then no energy is
> extraced.  To extract energy, there has to be a force in the direction
> of motion.  This means the float must be lagging the wave, causing a
> phase shift in the wave itself. Once the wave is lagging its driving
> force, the driving force must be imparting more energy to the wave.  The
> extra energy has to come from somewhere, which is the kinetic energy
> stored in the earth's rotation long ago. 

Yes, I agreed already to the source of this energy being the kinetic energy
of the rotation. The question is really whether the energy would be
conserved as kinetic energy if the float wasn't there, or if it would get
lost (as kinetic energy) anyway -- maybe at the coast line. It seems to me
that there's quite a bit of kinetic energy that gets lost (as kinetic
energy) at the coast lines...

Gerhard
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