Why does unbalanced forces cause change in the state of motion on an object?

Answer 1

To think of forces acting on an object we must first make one very important decision, and that is to define our point of observation. This is important because the frame of reference will tell us how an object is moving in relation to us and determine which one is to be viewed as an unbalancing force.

For example, one theory of the creation of our Moon is that about 4.6 billion years ago a protoplanet called Theia crashed into our ancient Earth. Both of these protoplanets were in orbit around the sun and in balanced motion. When the collision occurred it would depend on which one of the two, Earth or Theia, you happened to be standing on and observing what was happening to be able to form an opinion as to which one was the 'unbalanced force.' If you were on one, you would have most likely choosen the other.

For Newtonian physics, most things are a two body problem, that meaning that only two objects are considered at a time. If we consider that everything in the Universe is in constant motion, then we can look at all forces, if combined, are actually keeping things in balance.

Again, let me restate what I said earlier, an unbalanced force can only be determined if we have a point of reference and are talking about only two objects.