On Wed, Aug 16, 2017 at 10:18:54PM -0400, Christopher Sean Hilton wrote:
> On Wed, Aug 16, 2017 at 01:35:20PM -0700, C. Berry wrote:
> >  An object motionless in a g field has constant potential energy,
> >  hence no energy input is required to keep it there.
>
> I'm not 100% certain that this is the case.

It is the case.

> In your example g is very small being a function of a large
> r^(-2). But in fact an energy transfer is happening between objects
> on Terra (the oceans) and Luna.

That's because the oceans are not motionless in Luna's g-field.
Earth's rotation drags them around.  The power lost per unit mass is
microscopic, but the Earth has a lot of mass and it has been rotating
for a very long time.

> Here's a relevant thought experiment. Can you hold a 20kg mass, 1m off
> of the sea level surface of Terra without other support forever?

Human muscles are terribly inefficient, and require power input just
to hold things steady.  That's not a fundamental physics limitation,
it's just a biological trade-off.  Other animals don't necessarily
have the same limitations, nor does it apply to most non-biological
methods.

> Or do you get tired from paying the ~ 200J energy cost and have to
> put it down at some point in time.

No, my muscles get tired because they pay a biological power cost
entirely unrelated to the 200 J potential energy difference of the
mass.  The 200 J energy cost is paid *once*: while lifting the mass
from the surface to its new position.  Anything after that is just
biological wastefulness.

> I don't know the names for these two balanced forms of potential
> energy. Someone else would have to tell me whether or not ~ 720kW
> are spent keeping the mass off of the ground for an hour.

No, there is no power expenditure involved (and the units are wrong
anyway).

- Tim