Realistic kinettic force?

Hi :) I'm working on a school project similar to Angry Birds. What i have now basically fullfills my schools criteria but whenever i play it i get annoyed by the physics.

Example: On top of a table i have an apple. Resting on that apple is a flat wooden board. I then take a hammer and hit the board to squeze the apple. How would i go about calculating the force the apple recieved?

I can see from the simulation in unity that the apple is affected by the hammer, but since it only reacts on OnCollisionEnter() it wont get force applied to it. switching it to OnCollisionStay doesn't solve the problem, but creates strange results :/

I Hope some of you can point me in the right direction, this has cost me 2 sleepless nights already :P