Re: [Phys-l] Binary star question

[Ludwik Kowalski <kowalskil@mail.montclair.edu>]

On Dec 29, 2007, at 6:47 AM, Savinainen Antti wrote:
> I have a problem with one detail in quite a simple exercise. It goes 
> like this. Two stars are revolving in circular orbits around their 
> common center of mass. The masses are given and it is told that their 
> mutual distance remains constant (this distance also given). The 
> period is asked; with the above mentioned assumption they have the 
> same periods.
>
> Now this is not hard to do and I have no problems with solving the 
> exercise. I just can't see why their mutual distance remains constant. 
> I suspect it follows from a conservation law (angular momentum?). Help 
> would be appreciated as always :-).

1) Because the net force on each star (due to the other star on the 
same circle) is always directed along the diameter. The stars must be 
initially on the same diameter, and have identical speeds. Otherwise 
two stars will not share a circular orbit. That is how I see it.

2) It was a surprise that the above message came at the same time as I 
was composing a message with another question about a circular orbit 
problem. I will probably post it in an hour or two.  Perhaps i will 
find the answer by myself, after reading again what I wrote. In that 
case I will add the answer.
_______________________________________________________
Ludwik Kowalski, a retired physicist
5 Horizon Road, apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://csam.montclair.edu/~kowalski/cf/