I am a beginner in Physics and my teacher taught us "Relative Motion" yesterday. He said that the "Observer is assumed at rest." Is the observer assumed to be at rest only to simplify calculations, or is there some physical basis for this assumption?
For example, observer A is moving with an acceleration of 2 $\frac{m}{s^2}$, observer B is at rest, and A sees B accerating at $-2 \frac{m}{s^2}$. But we know that acceleration comes from Force (Newton's Second Law). So for B to be accelerating, they should have some net Force. But in reality, we know that B does not have any net force acting on it?