I think the problem is that you call it in a while - loop.
But I don´t know it for sure.

Here´s a good example for interaction and adding forces to newton entities:

Thread:

http://www.coniserver.net/ubbthreads/sho...true#Post355859

Archive:

http://www.zingerhead.com/NGD_CustomScripts_Vol1.zip

no science involved