As an electrical engineer, I have to suggest: *Rotation is certainly periodic motion. Periodically, the same point on a shaft passes a fixed point of observation. *Revolutions per second could certainly be expressed in hertz. In the electrical domain, a signal is sometimes viewed as a frequency in hertz, sometimes as a linearly increasing phase. The position of a shaft is entirely analogous. The use of hertz, which describes the rate of complete cycles, would avoid any confusion with angular rate in radians per second. Revolutions per minute, by whatever symbols, should be avoided entirely; all it does is throw an annoying factor of 60 in further calculations (Such as explaining why a generator must turn at 3600 rpm, or 1800 rpm, or a few other numbers, to generate 60 Hz electricity. Without the annoying factor of 60, it is the shaft frequency times the number of pole pairs.) This is obviously a departure from long-standing practice. So was "Hey, we don't need those pesky calories. We can express thermal energy in joules." On the subject of radians, the name is a placeholder. The radian is dimensionless as an arc length divided by a radius. But naked numbers beg the question "what the hell are you talking about" and using the radian as a dimensionless placeholder saves a lot of explanatory words. Implied usage of radian can be confusing; in the interest of clarity, I believe its use should be explicit (stated as a unit).
--- On Mon, 3/23/09, Bill Hooper <[email protected]> wrote: From: Bill Hooper <[email protected]> Subject: [USMA:44082] radians To: "U.S. Metric Association" <[email protected]> Date: Monday, March 23, 2009, 5:41 PM On Mar 22 , at 2:47 PM, James R. Frysinger wrote: So if I say, "The motor is running at 8600/s" what do I mean? Better to say, "The motor is running at a shaft rotation rate of 8600/s" or "The motor is running at an angular velocity of 8600 rad/s", whichever is the case. Of course those differ by a factor of 2 pi. I agree with this (above) and would further argue that, if we indeed do insist on naming the measured rate by proper names like "rotation rate", "angular velocity" etc. then it should be possible to see that rotations are not units and the, correspondingly, neither are radians. (Since degrees per second are also used for angular velocity, one would need different names for these two things. I'd suggest "degree velocity" and "radian velocity".) Abandoning the practice (built firmly into SI) of treating the radian as a unit would, in my opinion, be progress. Regards, Bill Hooper Fernandina Beach, Florida, USA PS And I remembered to change the subject on this as I go off on a tangent. ========================== Make It Simple; Make It Metric! ==========================
