When one object accelerates away from another object, relativly they are both accelerating away from each other. The second object's speed is changing relative to the object truly changing it's speed. However the second object does not feel the "push" from the acceleration. I am having trouble understanding why only one object is truly accelerating and feels the "push" or force of acceleration. It is intuitive that only you would feel the force as you accelerated away from someone else, however I don't understand why. Another example is that the earth is rotating around the sun, however you could say that the sun and everything else is rotating around the earth, if you kept the earth centered. Though the sun does not feel the acceleration, the earth is truly the object changing its velocity direction and feels the acceleration force, but what determines this..?