2D Physic Engine collision response rotation of objects
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i'm writing my own basic physic engine and now I come to a problem I can't solve. Probably because I don't how to google this problem.

So here is my problem. I hope this image can explain it:

[Collision response][1]

I have two objects. The gray one is fixed and don't move and the green one which falls from the top.
The green object has three vectors: a force, the acceleration and the velocity. It collides with the fixed gray object.

The real question is how can I get the rotation of the green object when it falls down?

Hope someone understands me ;)


  [1]: http://i.stack.imgur.com/AFaDx.png