Re: Circular orbit

In article <1134591782.611087.285060@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "PD" <TheDraperFamily@xxxxxxxxx> writes:
>
>gmarkowsky@xxxxxxxxx wrote:
>> Hi all,
>>
>> I was looking at an old Physics exam, and am confused by the following
>> question:
>>
>> A golf ball moves under the influence of a central force given by F(r)
>> = -k/r^n. If the ball's orbit is circular and passes through the force
>> center, show that n=5.
>>
>> OK, so here's my problem. First of all, it means that the force is
>> always directed towards the force center, no? So what I don't
>> understand is that, at the time when it's passing through the force
>> center it will have 0 acceleration, so wouldn't it just go in a
>> straight line path after that? It would be like starting it out at the
>> force center in a particular direction. It would just go straight out,
>> then straight back. How could it go in a circle?
>>
>> Greg
>
>I would think so too.
>This looks like a poor problem.
>n=5 is not a case that permits a closed orbit solution.


"Permits" is a wrong word here. r^n for any n permits closed orbit
solutions (as special case), it just doesn't mandate closed orbit
solutions as r^2 and 1/r potentials do).

Mati Meron | "When you argue with a fool,
meron@xxxxxxxxxxxxxxxxx | chances are he is doing just the same"
.