[LEAPSECS] Look Before You Leap – The Coming Leap Second and AWS | Hacker News

[Jonathan E. Hardis]

> On May 19, 2015, at 3:02 PM, Richard Clark <rclark at noao.edu> wrote:
> 
> It was around the late 1600's that it started becomming possible (and necessary) to decouple weight and mass.

The sound you hear is the sound of chalk screeching on the blackboard.

“Weight” is an ambiguous term that can either mean “force” or “mass.”  If you believe physics textbooks since the dawn of the space age it means “force.”  If you believe the weights and measures community—including every box of breakfast cereal you’ve seen since the dawn of the space age—it means “mass.”  That box of Wheaties that is labelled “Net Weight 10 oz” would correctly weigh 10 oz everywhere on Earth, on the Moon, and on the ISS.  Both uses of the term are correct.

Indeed, it was not until the 3rd CGPM decided the matter in 1901 that the “kilogram” was officially recognized as a unit of mass, rather than force.  And since in the United States the pound (lb, as distinct from lbf) is, by definition, a specific multiple of the kilogram, it too is a unit of mass—regardless of what one might read in physics textbooks.

The best guidance on this subject is avoid the use of the term “weight” as much as possible.  Failing that, please see section 8.3 in http://physics.nist.gov/cuu/pdf/sp811.pdf <http://physics.nist.gov/cuu/pdf/sp811.pdf> .

     - Jonathan

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