Re: "zero" gravity

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

I'm not sure there is a name for the case where the two massive
objects are "fixed in space," but there is for the very real
situation where two massive objects orbit each other.  This is
called the "restricted three body problem"--"restricted" because
we assume that the third body is small and, therefore, only
*reacts* to the forces of the other two; it does not itself
influence the two massive bodies.  In the case of perfectly
circular orbits there are five positions, known as the "Lagrange
Points," in the rotating frame at which the third body can be
placed and be in equilibrium under the combined influence of the
two primary bodies and te centrifugal force.

You will find more than you ever wanted to know by searching on
"Lagrange Points" or "Restricted three-body problem."

We regularly use "L1" as a convenient place to park solar
monitoring satellites.  "L4" and "L5" are quasi-stable (under the
additional influence of the coriolis force) and these points in
the Sun-Jupiter system have accumulated a number of asteroids
called the "Trojan Asteroids."  Just a day or two ago, I believe,
we launched the MAP satellite which is destined to be the first
satellite to be parked at "L2" which is permanently shaded from
the Sun by the Earth--the perfect spot to look for the weak
anisotropies in the cosmic background radiation, which is, not
surprisingly, MAP's mission.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm

On Mon, 2 Jul 2001, Dwight K. Souder wrote:

> Hello everyone.  I hope all of you are enjoying your summer.  I was wondering
> if someone could please help me out?  I have forgotten the name of a
> phenomenon and it has been driving me crazy!  I've looked all over my notes
> and the internet, and the name has escaped me, but I'm hoping that someone
> would be able to help me out.
>
> Here it is:  Technically, whenever you have a mass in space, no matter how far
> away you get from that object, there is always a little bit of a gravitational
> pull.  The same is true if you have 2 fixed objects in space, no matter how
> far away you move away from them, there is always a little bit of
> gravitational pull, except for one small point somewhere in between those 2
> objects.  Actually, the gravitational pull between the 2 objects are present,
> but they are also equal and in opposite directions to oneanother.
>
> In other words, if the earth and moon were 2 fixed objects (the only objects)
> in space and somewhere in between (a little closer to the moon side), a 3rd
> object at a certain point would be pulled with equal magnitude and in opposite
> directions.  Therefore, the 3rd object, if it wasn't moving to begin with,
> would not move to either of the other larger objects due to vector sums of the
> gravitational pull equaling zero.
>
> Does anyone know of the name of this?  I was discussing this with a fellow
> science teacher and we were both racking our brains trying to remember what
> this was called.
>
> Thank you,
> Dwight Souder