Re: Do dark matter calculations make the stupid assumption that all mass is at the center of the galaxy?

On May 23, 10:35 pm, Jim Black <trams...@xxxxxxxxx> wrote:
frankli...@xxxxxxxxx wrote:
It would appear that the force of gravity within a ring of matter is
zero:

That is wrong.  It is true for a spherical shell, but not for a
circular ring.

http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/GravField.htm

And that link does not support the assertion.

--
Jim E. Black

under:

Gravitational Field from a Ring of Mass

The formula for force on P depends on x, the distance away from the
plane of the ring. If x is zero such that the particle is within the
plane of ring, the numerator becomes zero and therefore the force for
a particle within the plane of the ring (as would be the case for a
galaxy), the force is zero. Is this not correct? Therefore for the
case of calculating the force on a star within the galactic disk, the
force is zero for mass outside of the orbit.

Of course, I would still like to see something better than a paper
napkin calculation showing that dark matter actually exists. I have
seen references indicating that if you do take into account the real
mass distribution of galaxies, that the rotational differences can be
explained. I would find this far more likely than the existence of
"dark matter". Therefore, no dark matter is necessary.


.

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