Having a non-zero relative velocity is fine as long as the distance between the points isn't changing. This certainly holds for a rotating rigid body. As another example, take a ball on a string and rotate it in a horizontal circle. Is the ball moving relative to you? Yes. Is it moving towards or away from you? No. Therefore this part If they have a non-...


If the velocity of point B relative to point A is always at right angles to the line AB joining them, then the distance does not change.

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