Is how we choose to describe relative velocity arbitrary? For example, say I am floating in space with only Ball A and Ball B for reference. Ball A is moving away from me at 10mph, and Ball B at 20mph. I could say that I am stationary, and Ball A and Ball B are traveling at 10 and 20mph, respectively, or I could say that Ball A is stationary, and I am moving away from it at 10mph, and Ball B is moving away from me at 10mph. Assuming acceleration is not involved, is how I describe this situation totally arbitrary? Thanks!