Neddy Bate
Valued Senior Member
If I'm a moving observer in a boosted frame (we really should use the boost lingo more often here), I can still legitimately argue that two scales read the same value while experiencing different forces as seen from my POV. I see two different forces, and two different scale inertias, leading to the same reading on both scales. All that matters is the forces are the same in the rest frame.
Yes, but Tach is saying there are two different forces applied to the spring scale in one frame. Either of the balls can be considered to be the rest frame, and he has two different forces in both frames.