"Nav" wrote in message
...


Peter S/Y Anicula wrote:

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!


Well you keep saying that but it is not so. Unless you unclude the fact
that the system is rotating you cannot make two bulges on opposite
sides. Jeff posted a URL, have a read and then you will see the problem
-I hope.

There is no problem. The "differential" explanation starts by subtracting out
the total, net gravitational force and looking at just the differences at
various places on the Earth. What is the effect of this net component? It
accelerates the Earth towards the center of the Earth-Moon system. Thus, when
looking at the left over differences, you're already accounting for the rotation
of the Earth in this way.

As I mentioned in my other post, the net gravitational force subtracted out is
simply the opposite of the centrifugal force you've mentioned. To my mind,
neither of these causes the bulges, its when you subtract (or add its negative)
and looking at the differences around the Earth that you get the answer.