# Re: Lesson 1: Friar Broccoli apparently ignorant of Newton's Laws

In article <apagano-62uvu5tql3e65j30abfen1m8ljl72r50aq@xxxxxxx>,
T Pagano <not.valid@xxxxxxxxxxx> wrote:

Apparently Broccoli doesn't exactly remember Physics 101 too well.
Newton's laws reveal only that of two or more bodies in a rotating
system all bodies will revolve around the center of mass.

They say no such thing. Here are Newton's three laws:

1) An object at rest will remain at rest unless acted on by an
outside force; an object in motion will continue to move in
a straight line at constant speed unless acted on by an
outside force.

2) A body acted on by an outside force will accelerate with an
acceleration propotional to the force and inversely proportional
to the mass.

3) Every action has a reaction equal in magnitude and opposite in
direction.

Then there is his law of gravitaion, which really ought to be
included too, but whatever. Anyway, while it is true that two
bodies will orbit their mutual center of mass, it is not so simple
when you introduce more. If you have two large bodies similar enough
in mass that their center of mass lies somewhere in space between
them, they will appear to orbit that point, yes. But you cannot
then make another smaller object go around that empty common center
in a smaller orbit. Some science fiction authors have planets or
asteroids orbiting the center of mass of a binary star system, inside
the orbits of the two stars, but they are getting it wrong. In any
case, this hardly applies to this argument anyway.

From far outside a system of bodies, you can sort of approximate
gravity as acting towards the center of mass on average, but the
closer you get the less well this works.

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