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## It’s just multiplication, have no fear

(A true story, with some added sarcasm, to illustrate that using equations is safer in the long run than trying to avoid using equations)

SERIOUSLY?  The equation is:   RCF = RPM2 [min-2] x Radius [mm] x 10-6

It is multiplication!  Granted, getting out a ruler and measuring from the center of the rotor to the tip of the holder can be physically exhausting and is best left to the young athletic types in your lab.  No argument there.  But risk your experiments rather than do multiplication?   And he learned this at Yale?  This is a very smart person, a very good scientist, yet the thought of doing multiplication is so distasteful, that he relies on a number he once heard from someone he considered trustworthy.

What is this ‘culture of equation avoidance’ doing to our scientists? He may someday find himself with a new centrifuge, of a different size, and his 1.0 RPM could lead him to bad data.  Troubleshooting will be close to impossible, and he will abandon his beautiful experiment and not get a grant.

* I object to the use of “g” as unit for this purpose, although I appreciate that thinking in terms of our constant companion gravity is a comfort. The correct name for the parameter in question is Relative Centrifugal Field or RCF.  Without regard for reality, however, RCF is traditionally reported in units of g.  RCF has dimensions of length over time squared (L T-2), which is mm/minutes squared in the above equation (rotation is dimensionless). RCF is determined entirely by the rotations per minute and the radius of the rotor. On the other hand, gravitational force has units of, surprise, force, i.e. Newtons, meaning its dimensions are mass x length / time squared (M L T-2).   What happens to the mass when you convert to RCF?  Traditionally, they’re not telling.  So, writing “an RCF of 125 g” is an abomination.  However, I have gotten over this and moved on.  Really.