Nav wrote:
The earth-moon body rotates around a common point and water tries to
move away from the centre (water in a bucket swung on a rope
analogy).
The moon exerts gravity which is stronger on the side of the moon.
Thus
water forms two bulges on opposite sides and makes two tides as the
earth rotates...


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





"Nav" skrev i en meddelelse
...
Jeff,

I think the term centrifuigal is appropriate in this context. It is
a
term in the system that can be appreciated without needing to
consider
Newtonian forces. To understand centripetal forces is a lot harder
than
just demonstrating the effect. similarly gravity can be demonstrated
without maths. Thus the explanation becomes really simple e.g.:

The earth-moon body rotates around a common point and water tries to
move away from the center (water in a bucket swung on a rope
analogy).
The moon exerts gravity which is stronger on the side of the moon.
Thus
water forms two bulges on opposite sides and makes two tides as the
earth rotates...


Cheers

Peter S/Y Anicula wrote:

Nav 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 trouble is that the gravity difference does not "explain" the two
tides -it may seem to but that is not the case. The correct explanation
resides in the difference between the inertial force and gravity.
Interestingly, it predicts larger tides than are observed (and predicted
by the differerntial model). However, that is because the land masses
and friction reduce the tide height (to what is actaully observed). That
differential gravity appears to produce the "right" answer shows how
shallow (pardon the pun) that "explanation" really is!

Does this make sense?

Cheers