Christopher James Huff wrote:

> In article <3d9b20ac@news.povray.org>, Micha Riser <mri### [at] gmxnet>
> wrote:
> 
>> If the string remains taut the part of the force that is colinear with
>> the string will get compensated. In other words the particle will always
>> move in the plane perpendicular to the string! This means that all you
>> have to do is to project your force into this plane.
> 
> Not quite...if you move it along a straight line in the direction of
> force, the "string" will get longer and longer with time. You need to
> move it along a curved path:

Yes, I did not think at this. While the force and also the velocity is 
always perpendicular to the string, you need to correct the error made by 
the Euler method.

I have digged out my sliding macro and adapted it to run with POV-Ray 3.5. 
You can find the source and some examples in p.b.s-f. There I use trace to 
place the slider on the surface again after it has moved. You could use a 
similar thing in your case: trace against the imaginary sphere that limits 
the orbit of the particle.

I have made a pendulum animation (see p.b.animation) with exactly this way. 
The friction force is still as if it would move on the sphere... so it is 
probably not realistic in that respect.

- Micha

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