The gravitational force acts only toward the center of mass of the
system. This cannot by itself produce two bulges.

When you say that, you are mixing two explanations. That doesn't work.

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth separately, and then ad the two, to
have a look at the combined forces. If you do not include part of the
rotation element, it works just fine.

If you only look at the gravitational forces, you can explain the two
bulges!

It is an abstraction. Not the "truth". Even if you include the
rotation it is still an incomplete abstraction. We are discussing
different incomplete models. We haven't yet reached anything near the
"truth".

When discussing different models it is important not to mix elements
casually.
I'm surprised that a mere sailor have to teach this to a professor.

Peter S/Y Anicula

"Nav" skrev i en meddelelse
...
Well Peter, I have to disagree there. The gravitational force acts
only
toward the center of mass of the system. This cannot by itself
produce
two bulges. To clarify this, try imagining the forces of gravity in
2D
on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the
far
side and more water as you move toward the moon... -two bulges would
not
be present.

Cheers

Peter S/Y Anicula wrote:
You make it sound as if the gravitational forces explains the
bulge
under the moon and the centrifugal forces explains the bulge on
the
side of the earth that turns away from the moon.
That is not right.

The gravitational difference alone can explain that there are
bulges
on both side of the earth. That's why it is sometimes the only
factor
mentioned when trying to keep the explanation simple.
The centrifugal element can only explain that there is a bulge on
the
part of the earth that turns away from the moon.
That is why it is one of the elements (and there are others), that
is
sometimes left out of the explanation.

While I think that in some cases it is a good idea to include the
centrifugal element in the explanation, I don't know exactly how
many
elements one should include to make it a good explanation - but I
haven't yet seen a complete explanation in a popular publication.

Peter S/Y Anicula
Sailor
The seven seas