If, hypothetically, one force is \"unstoppable\" and the other \"unmovable\", then in the event of a collision both would be tested to see if both qualities are true. If one is weaker than the other in their un* ability, eventually the one with the stronger ability will win out and the weaker one will be overcome (the unstoppable object will stop or the unmovable object will shift). If they are equal, both would stop doing their thing: the unstoppable force would cease to move, and the unmovable object would become movable.