From: EMILEBRU@aol.com
Date: Sat, 31 Jan 1998 00:53:50 EST
To: pinhole@exploratorium.edu
Subject: Re: Pinhole Daily Digest
Suzy,
I don't know if you've received a reply yet, but I thought that I'd throw one
back.
The "gravitational friction" that they're talking about is also known as the
"tidal force". On the Earth the tides that we experience are due to a number
of factors, but the primary two are the gravitational attraction of the Sun
and the gravitational attraction of the Moon. Since gravitational attraction
is dependent upon distance, the side of the Earth closest to the Moon will be
attracted more than the side of the Earth farthest from the Moon at any time.
This causes the Earth to pull out into an oblong type shape. The Earth,
however, is also spinning, so each time it turns a bit with respect to the
Moon, a different part of the Earth is pulled out towards the Moon, and on the
opposite side of the Earth a different part is bowed out the other way - as if
the Earth were rotating under the bulge in its surface. This double bulge in
the surface of the Earth obviously causes some strain and friction as
different parts of the crust lift in a wave every day. This is the
gravitational friction that will slow down the Earth's rotation over time.
Interestingly, this supposedly actually happened to our Moon. It used to
rotate, but the Earth caused huge tidal bulges in its surface that resisted
its rotational motion and eventually stopped it in its current position.
Incidentally, the Sun also causes these tidal bulges on the Earth, but since
distance is more important to gravitational attraction than mass, its effect
isn't quite as great as the Moon's. The Moon and Sun can actually work in
concert to create the spring tide (when they are aligned with each other) or
the neap tide (if they are at right angles to each other).
The point about the Earth's atmosphere slowing down its rotation must be
false, because the Earth's atmosphere rotates at the same average rate as the
Earth itself (otherwise you could fly in a helicopter from New York to San
Francisco simply by going straight up into the atmosphere and allowing the
Earth to rotate underneath you).
So, in sum, the gravitational attraction of the Sun and the Moon cause tides
in the crust of the Earth (the water rises the easiest, but the crust raises
up about a foot, too!). The tides are due to the fact that one side of the
Earth feels much gravitational attraction while the other side feels less
(since gravitational force depends upon distance). If you were to drop a water
balloon off of a tall building and the bottom of the water balloon were to
fall at 9.8 m/s2 and the top of the water balloon were to fall at 9.6 m/s2 you
would get a similar effect - a double bulge that resists the spinning motion
of the Earth. Or you can imagine the shape resulting from spinning a water
balloon on a string around your head.
anyways, that's it
hope I didn't babble too much
good luck
emile