hi, just logged to say hello and congrats for this fantastic application, I find it pretty useful for fast app and game ideas.
on topic: You can retrieve force as Ashley said from Force=mass*acceleration but may be Im thinking you will need the algebraic equations of motion. Im refreshing my physics atm but from what I can remember may be you could use a=(vf-v0)/(tf-t0) for a given moment and given velocity. there is other equation for acceleration but its related with total displacement if im not mistaken.
cheers.