impact

The questions been boggling me for quite some time and I'd like to know how do I get the exact position at which a collision happens from one object to another. For example: (a 2D game) ObjectA meets with ObjectB and blood appears. Much like you shoot something in real life and the blood or dust...

I belive I've made a thread about this before, although I did not get any help there really and it seems like it was a tough nut to really comprehend. I'm talking in terms of recoil and impact force which get applied by shot projectiles at a target or basically an enemy or yourself. Obviously...