Linear motion requires a reference frame to be defined; time, velocity, occurance is all relative to your deffinition of the reference. Eienstein in his book of "Relativity" states: "Suppose two lightning bolts strike a train track simultaneously; but then, what does that mean? If I am stadning CLOSER to one than the OTHER, the speed of light makes it appear that the CLOSE bolt strikes FIRST..."
Rotational motion, OTOH, is absolute --- no reference need be defined, no relative matter considered; as a body rotates it experiences centrifugal force; thus there IS a concept as "zero rotational velocity", but no concept as "zero linear velocity"...
If mass is modeled as a QUANTIY existing at a POINT, then the rotational equation is dimensionless...