I spotted this sign today. The letters R.M.S. are important in Physics.Think about the molecules in the air. The distribution of velocities is random. That means that there will be as many going left at 6 m/s as there are going right. We could say left is 6 m/s and right is -6 m/s. So the average velocity of all air molecules would be ZERO. That's nonsense. So what we do is SQUARE all of the velocities. That removes the minus signs so we get a sensible non-zero MEAN. To get the number back to normal size we have to take the ROOT. Put backwards, that's ROOT MEAN SQUARE - RMS.
