XPost: alt.usage.english   
   From: wollman@hergotha.csail.mit.edu   
      
   In article <874j6agbg8.fsf@parhasard.net>,   
   Aidan Kehoe   wrote:   
   >   
   > Ar an naoú lá déag de mí Méan Fómhair, scríobh Christian Weisgerber:   
      
   > > Yes, the natural logrithm of 5 approximates the conversion factor   
   > > between miles and kilometers; specifically one mile is about ln(5)   
   > > kilometers.  It's accurate to four digits.   
      
   >pounds to kilograms. What I learned from my father (born 1945, went to school   
   >before it was neglected) was that a kilometre is 5/8 of a mile, which helps in   
   >converting the speed limit signs in Northern Ireland to what my speedometer   
   >shows.   
      
   That's the approximation that's much easier for mental arithmetic, and   
   similarly, 8/5 when converting the other way.  Otherwise, 1.609344 is   
   the exact conversion, and 1.609000 is good enough for almost every use   
   (unless you're doing weird stuff like survey miles, which are based on   
   the old foot of 1200/3937 meter,[1] rather than the "international"   
   foot of 0.3048 m).   
      
   However, an approximation that has actually proved useful to me is   
   that 1 m/s ≅ sqrt(5) mi/h.  As everyone knows, sqrt(5) ≅ 2.236068, or   
   2.2 if you only need two significant figures, which is much easier   
   to remember than 3600/1609.344 ≅ 2.236936.   
      
   -GAWollman   
      
   [1] One will note that this conversion defines a meter to be exactly   
   39.37 inches, although that's never how it's actually phrased.   
   Sometimes that's a more convenient conversion than the modern one even   
   if it's no longer exact.   
   --   
   Garrett A. Wollman    | "Act to avoid constraining the future; if you can,   
   wollman@bimajority.org| act to remove constraint from the future.  This is   
   Opinions not shared by| a thing you can do, are able to do, to do together."   
   my employers.         | - Graydon Saunders, _A Succession of Bad Days_ (2015)   
      
   --- SoupGate-Win32 v1.05   
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