The unstoppable force has never been challenged. It has no purpose, no reason for existence until it encounters the immovable object. The same is true of the immovable object. It has never been challenged or had a reason for existence until it encounters the unstoppable force.
Both the unstoppable force and the immovable object have a desire to be true to their nature. They do not know challenge, so they will by nature seek to fulfill their empty purposes. Yet, they will remain in balance, because otherwise neither will be able to fulfill their purpose. To be unstoppable. To be immovable.
From this initial struggle develops mutual respect and an attraction. They will seek to understand each other, at first under the guise of overcoming the other's power. As that respect grows into understanding and eventually love, the natural desire will be to acquiesce to the other in order to demostrate the level of respect and admiration they have for each other and their purpose. Love, at its highest level, takes on the nature of sacrifice. Of giving everything one is capable of giving. The focus shifts from the lower, animal tendency to focus on one's desires and ambitions to helping the other fulfill theirs.
Unless they remain in balance, one would by definition be destroyed by the "surrender" of the other. So, they must acqueisce at the same time. Because they are perfectly matched, the yin to the other's yang, they would not allow the other to destroy itself, which would happen if one acquiesced before the other.
In order to give what is needed by the immovable object, the unstoppable force will come to rest. The immovable object would allow itself to be moved. In doing so simulataneously, the natures of the force and the object are changed, reflected by the needs of the other.
In allowing themselves to be changed by trusting and respecting each other, the unstoppable force and the immovable object will grow and become more than their original purpose granted. They come to realize that being unstoppable or immovable means nothing. They grow, evolve and become something greater than what they were by learning from and teaching each other.
Some people are unstoppable forces. They will keep going, unchecked and defying any and all who might try to stand in their way. Other people are immovable objects, refusing to change, refusing to grow, adapt or evolve. Sometimes they need to challenge each other, to learn respect and love for one another before they can become more than what they were. An unstoppable force has no real purpose outside of maintaining its perceived sense of power. An immovable object had the same problem.
If they do not allow the change, then they have only arrogance and ego to carry them. If that is all they have, then they are little more than the lifeless concepts you originally considered them to be.