NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/physics/23641

Internet Message Format | 1993-01-28 | 5.7 KB

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From: [e-mail] (Andrew Poutiatine)
Newsgroups: sci.physics
Subject: Re: satellite orbits
Message-ID: <[e-mail]>
Date: 26 Jan 93 23:41:54 GMT
References: <[e-mail]> <[e-mail]>
Sender: [e-mail]
Organization: Los Alamos National Laboratory
Lines: 95

In article <[e-mail]> [e-mail] (\) writes:
>
>Nothing wrong about it (except the spelling of "centripetal").
>The centrifugal force appears only if you use the satellite (or any rotating
>system) as a frame of reference.  You can avoid it by looking at the system
>from far away, using an inertial system as frame of reference.
>
>A non-astronomy example:
>On a merry-go-round, you'll experience centrifugal force, pulling you away from
>the centre, but that force cancels with the centripetal force, which acts on
>you through the chain tying you to the rotating part.  So the sum of all forces
>is zero, and you stay where you are - with reference to the rotating part.
>But the bystanders see it another way: You are held on a circular orbit by the
>chain pulling you to the centre and stopping you from flying away on a tangent
>and hurting yourself badly, which you'd do if there were no force (no chain).
>They observe only one force.
>
>Some people will tell you that centrifugal force is "not a real force" or
>"pseudo force" but I think that's dubious.  The Theory of General Relativity
>states that you can choose whatever system of reference you want, laws of
>physics will be the same.  Using the standpoint of an observer on the
>satellite is as reasonable as any other, only the forces you need are more
>complex.  That is, you need more assumptions to explain the same observations.
>Therefore, using Occam's razor ("Do not unnecessarily multiply entities" or,
>as Einstein put it, "As simple as possible - but not simpler")
>one normally chooses the frame of reference with less assumptions.
>
>BTW, that's the only reason why we say that the earth is rotating the sun and
>  not the other way around: for simplicity.
>


Some of what you say has merit, although the above is both wrong and
dangeroulsy misleading to those who wish to learn.

First, centrifugal means fleeing the center, and centripetal means seeking the
center.  Thus a satellite orbiting the Earth, with not other celestial
bodies acting on it, and assuming the simplest configuration, experiences
only gravity, and thus only a centripetal force.
In this case (again for the satellite) the centrifugal force _is_ in fact a
"psuedo force."  We see four forces in physics, gravitation, electro-mag.,
weak and strong nuclear forces (excuse me if I haven't given them their full
names).  Our satellite is not seeing substantial contributions from anything
but gravity, and then there is _no_ centrifugal force on the satellite.  Of
course, as mentioned in a previous post, the Earth feels a miniscule pull
from the salellite due to gravitational attraction, and this is away from the
center of the satellites orbit, and is thus a centrifugal force.  (No need to
nit pick about the actual location of the center of mass of the system, I am
trying to be ideal for the moment :^)  ).

Even when on a merry-go-round there is _no_ centrifugal force acting on you.
It obviously seems like it, but again, ask yourself if it is gravitational,
electro-magnetic, strong nuclear or electro-weak forces that cause it.  This
case is a little more complicated, since it is atomic interactions that
create the friction on your shoes, and prevent your hand from moving through the
bar that you hold on to.  But again, nothing pulls you away from the center
of the merry-go-round.

The problem is that in rotating reference frames we cannot apply our momentum
principles without taking into account the fact that we are being accelerated.
If we assume that on the Earth that we are in an inertial reference frame, we
end up with strange effects.  Usually it is ok for our purposes to assume
that our reference frame is inertial, but sometimes you see things like the
"Coriolis forces and accelerations."  Once again theree is no actual
physical force that is making storms rotate, it is only and effect of the
fact that we are in a non-inertial reference frame that makes it appear as
though there is a Coriolis force.  The storm wants to obey Newtons laws, and
gets itself set up in a sort of orbit, if you want to think of it that way.
Coriolis is a whole subject unto itself, and I'll leave that for specific
questions on the subject.

As far as what was said about Relativity, this in also not entirely true in
that Relativity _does_ distinguish between accelerating frames (i.e. non-
inertial) and non-accelerating frames.  This is in my understanding the
basis for the resolution of the twin paradox.

It is patently false to say that simplicity is the only reason why one
reference frame is chosen over another, and that that is the only reason
why we say that the Earth rotates about the sun, and not visa versa.  Conservat-
ion of momentum distinguises between the inertial and non-inertial frames,
which is why an inertial frame is called inertial.

If I am on a merry-go-round, standing on the outside edge, and throw a dart at
a dartboard that is at the center of rotation, I will see the dart curve
and miss the board, if I am rotating and throw as though I am in an inertial
frame.  This is the Coriolis effect again, and I cannot pretend to be in
an inertial frame and try to apply conservation of momentum.

Whew, I am glad that is off my chest.  Now you may flame, but just don't
pick nits please, I like my nits just the way they are ;^)

-AIP