NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/physics/23219

Internet Message Format | 1993-01-21 | 3.6 KB

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From: [e-mail] (Richard M. Mohring)
Newsgroups: sci.physics
Subject: Re: satellite orbits
Message-ID: <[e-mail]>
Date: 21 Jan 93 17:53:43 GMT
References: <[e-mail]>
Sender: [e-mail] (Richard M. Mohring)
Organization: New Jersey Institute of Technology, Newark, N.J.
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A combination of gravity and the satellite's tangential velocity is what
holds a satellite in orbit.

Take for instance a mass tied to the end of a string. If you twirl the
mass around by holding the end of the string, you see that the mass
travels around in a circle. I think you would agree that a tension in
the string is what holds the mass in this circle, rather than letting
it fly off into your neighbor's window. This force on the mass directed
toward the center of the circle it is traveling in is called a "centripetal"
force (and it is quite a *real* force, no additives or preservatives.)

Likewise, when a satellite is traveling with a velocity around the earth
in a circle, a similar centripetal force is provided by the satellite's
gravitational attraction to the earth. In actuality, the orbit isn't
necessarily circular (it most likely isn't, in fact) but will be elliptical
(a circle is just a special ellipse.) But, the principle is the same. If the
velocity of the satellite is within a certain critical range, then gravity
is always just enough to keep the satellite turning in orbit, but neither
too much as to suck it into the atmosphere nor too little as to let it fly
off into space.

As for the "centrifugal forces", they are not *real* "forces" in themselves,
although it does feel quite "real" when you're whipping around in a circle
on the "Rotor" at your local amusement park. :)

When you consider a force of motion, you usually consider it as something
that gives rise to an acceleration of some sort. This is all fine and good
as long as you are either standing still or moving with a constant velocity.
Now, if you're in a system that itself is accelerating, it becomes hard to
differentiate between which accelerations are being caused by forces, and
which are just there because your whole world is accelerating also.

The "centifugal force" is one of those phenomena that get lost in the shuffle.
When you feel yourself trying to fly off the ride radially, it's really because
the *ride* itself is accelerating toward the center of the circle (exactly
like the mass-on-a-string or the satellite is) and relative to you, it seems
like *you* are accelerating outward.

Another way of looking at it is in terms of inertia. A body at rest tends to
stay at rest. This feeling of a "centrifugal force" is analogous to the feeling
you get when you accelerate in your car: You feel pushed up against the back of
your seat. This is because your body was quite content with being at rest while
you were stopped at the drive-thru window at McDonald's. ("content" might be
the wrong word at a McDonalds, but that's another story. :) ) When you
accelerated to leave, your body tried to stay at the drive-thru window, but the
stubborn car was strong enough to force your body to accelerate and leave.
So you *felt* like you were being pushed backward, but you really weren't
being pushed at all.

I hope that helps...Sorry 'bout the length. I'm in a good mood. :)

Regards,

Rick Mohring
NJIT Physics Dept.

"After all, the conservation of energy looks good, and it is nice,
 and I do not want to throw it away."    -- Richard P. Feynman