Adding physics to a melee animation?

I bet this has been asked before, but I looked around and didnt find anything. So here goes.

I want to have some realistic melee combat for a game Im working on, what I want is to attach a rigidbody to a limb (or a sword) and then apply a force to the collider (which also has a rigidbody)

So what I want to do is to get the velocity by calculating it from the position of the rigidbody between frames, has anyone tried to do this before? I kind of remember unity had an option to do this, but I cant find it in the new version 4.2 and the mechanim controller.