spring effect in Python

[pochis40]

I'm trying to write in Python something similar to this:
(Java)
http://java.sun.com/applets/jdk/1.4/demo/applets/GraphLayout/example1.html
or these:
(Proce55ing)
http://www.cricketschirping.com/processing/ExportAtlas/
or
http://www.cricketschirping.com/weblog/2005/12/11/force-directed-graph-layout-with-proce55ing/
or
http://www.cricketschirping.com/weblog/2007/02/26/force-directed-graph-layout-with-processing-take-2/

I've problem to find something that give that physical effect (like a
spring).

Any idea?