scott wrote:
>> OK, so I did all of that, and I *still* have the problem of the forces 
>> in RK4 appearing to be a different scale from the forces using the 
>> Newtonian method.
>>
>> Shouldn't I be able to just swap out the two, choosing Newtonian for 
>> speed, or RK4 for accuracy, and see relatively equal* results?
> 
> Yes, if your time-step is small enough for Newton to work roughly 
> correctly. Do you still get the same difference if you reduce the time 
> step by a factor of 100?  If you do, then it's a coding error, if you 
> don't then it's simply the fact the Newton is not doing very well at 
> your previous time step amount.
> 
> Did you try a simple system first, eg constant force or very soft spring 
> from one point?  With a constant force you should be able to calculate 
> the exact velocity and position by hand and compare with what your 
> algorithm gives out.

OK, I "fixed" the timestep at 1/200th of a second, and removed all 
particle collisions - now the only force is that of gravity.

Using the two integrators, the particles follow roughly the same path, 
but using RK4 they appear to have an increased velocity - they follow 
the same path much more quickly.

Which means I've got an error somewhere in my code...

-- 
...Ben Chambers
www.pacificwebguy.com

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