link to Home Page

Re: ZetaTalk and Spaceguard UK (D8)

Magnus Nyborg wrote:
> Orbital speed for ideal circular motion of a low-mass object
> circling a high-mass object M (which refers to it's mass) is
> determined by the formula
>   v = sqrt( G*M / r )
> which gives the following...
> Ground orbit (if possible) -
>    v = sqrt( 6.67E-11 * 5.976E24 / 6.378E6 ) = 7905 m/s
> Satellite orbit -
>    v = sqrt( 6.67E-11 * 5.976E24 / 6.478E6 ) = 7844 m/s
> Geostationary orbit -
>    v = sqrt( 6.67E-11 * 5.976E24 / ? ) = ? m/s
>       (didn't find an accurate number for distance in the hurry)
> Moon orbit -
>    v = sqrt( 6.67E-11 * 5.976E24 / 3.844E8 ) = 1018 m/s
> reservation made for constants (G only with three significant
> digits effecting the accuracy of the calculations negatively).

You used 5.976E-11 for Earth Mass where Eric George computed this to be
5.9763e+24 kg.  I assume this to be essentially the same. I'm assuming r
is the distance between the objects, per MC Harrison post (below).
You've got this divisor progressing from a ground orbit to the Moon's
orbit.  I don't know how to read these numbers, but assume you've got
the distance, from center of Earth or whatever, per points made in your

M.C. Harrison writes.
> The force of gravity is an inverse square law, which means
> a mass will experience a force due to another mass according to
> the equation F=M1*M2/r^2 where M1 is one mass, M2 is the
> other mass, and r is the separation of the masses. ...

But Magnus, how can the gravity pull be THE SAME for a satellite and the
Moon!  Gravity is a factor of BOTH objects, the pull between them.  Not
sure what a satellite weighs, but Eric George computed the Mass of the
Moon to be 7.3508e+22 kg, or 73,696,438,000,000,000,000 Metric Tons!
The formula you used, for a "low-mass object circling a high-mass object
M" may work for a satellite, but the Moon is NOT a low-mass object.
Give me a velosity equation that has a spot for the Mass of the MOON, as
well as the Earth being orbited!  Lets plug in an equivalent Mass of the
Moon, using the same basis as we do for the Mass of the Earth in your
computations above!

Paul Campbell writes:
> Mass is not based on speed but mass can be solved by
> F=GMm/r^2 and solving for m. The weight of the moon is
> zero, the mass can be calculated by the above formula. If you
> change the mass of the moon then the Moon's orbit as
> presently observed would not happen. Therefore the moon's
> mass must be what it is regardless of it's composition.