Bill Hooper wrote in USMA 19398: >I wrote: >>> Any given number of pounds always has the same number of kilograms of >>>mass IF >>> we remain in the same gravitational conditions (e.g. on surface of planet >>> Earth) > >and you replied: >> Why IF? The pound is legally defined as a unit of mass. > >I wish it were universally agreed that the pound is ALWAYS to be considered >a mass unit, but I fear that is not so (even if the official definition of >the size of a pound is in terms of the kilogram). > >So long as some people consider the pound to be a measure of weight (pull of >gravity, roughly) there will be those who insist that conversion of weight >(in pounds) to metric means convert it to newtons (the SI unit of force).
This discussion reveals a fundamental, but not generally appreciated, split between physicists and engineers. Anglo-Saxon engineers use the pound as their unit of force, and the slug as their unit of mass. 19th century Anglo-Saxon physicists used the pound as a unit of mass and the poundal as their unit of force. The law recognizes the pound as a unit of mass and does not have a unit of force. A spring balance measures force of gravity, and hence the engineer's pounds. A true balance (honest weight, no springs) compares masses, and hence measures the physicist's pounds. Bill explained the engineer's viewpoint. I replied as a physicist. Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
