Getting positional data from tracker

[AySz88]

I have an application where a VRPN tracker is linked to an object, and then that link needs to be severed to have the physics simulation take over. (In this case, I'm "tossing" a ball using a HiBall 3000 stylus.)

From what I can tell, the tracker doesn't calculate this (right?), so I need to estimate the object's velocity from position data myself. What's the best way to grab position updates from the tracker in a callback-ish sort of way? I tried redefining the ball's setPosition method, but that doesn't seem to work. Should I define some sort of setter function at all?