R almost exclusively stands for radius, like in math. However, it depends on the situation. For example, in the equation PV=nRT, R is actually the gas constant, which is 8.31 J/(mol*K). I recommend looking at the units, as that will almost always show what 'R' should represent; anything in a single dimension (ie meters, parsecs, kilometers) R is most likely radius.
time
Mega joules.
perpetual motion
Velocity/speed.
The R in the RSPAC stand for Royal.
W. R Hindmarsh has written: 'Approach to physics' -- subject(s): Study and teaching, Physics
Velocity/speed.
THe R's stand for Reduce, Reuse, Recycle.
R. Zallen has written: 'The physics of amorphous solids' -- subject(s): Solid state physics, Amorphous substances
A. R. Meetham has written: 'Basic physics'
C. R. Emigh has written: 'Radiation and particle physics' -- subject(s): Particles (Nuclear physics), Radiation
Probably: The Antiproton Decelerator A machine to cool down antiprotons at the CERN physics laboratory.