John Tauxe wrote in USMA 11327: >Well, my note about not being able to measure mass generated some >discussion, alsmost all of it identifying the balance as a way of measuring >mass. I still disagree. > >At 13:11 2001-02-27 -0600, Gene Mechtly wrote: >>On Mon, 26 Feb 2001, John Tauxe wrote: >I contend that mass can only be inferred by adjusting the weight by the >force of local gravity. > >On the balance, you are comparing weights in the same gravity field, so the >gravity field for both cancels out, and you can in effect compare the >masses directly. Of course, the comparison must be made to a standard >mass. And how was that standard mass arrived at without weighing it in a >gravity field? Ah - the 1000 cm3 volume of pure water. But again, there >you are measuring a volume, not a mass. > >Show me a device that can measure mass under zero gravity and I'll admit >that mass can be measured. > >- John > >_____________________________________________________________ >John Tauxe, PhD, PE voice: 505.662.0707 ext 15 >Neptune and Company fax: 505.662.0500 >1505 15th St. Suite B mailto:[EMAIL PROTECTED] >Los Alamos, NM 87544-3063 LANL mailstop M969 >___________ http://www.neptuneandco.com/~jtauxe _____________ Our standard of mass is the master Kilogram, which is preserved with all precautions. It was constructed to have the mass of one decimetre cubed atwater at 4°C. All other kilogram standards have been prepared by comparison with the master (or prototype) Kilogram. The comparison could theoretically be made anywhere: at sea level, on Mount Everest, on the moon. You are right that the weights of the two standards are being compared, but if the weights on the balance are equal, then the masses are equal, even though the ratio between mass and weight depends on the local acceleration due to gravity. The mass of an object is constant, the weight varies with location. Under zero gravity the only test of mass would be the measurement of the acceleration produced by a standard force, a most difficult procedure. It also raises the problem of standardizing the force. I think you suffer from the engineering tradition of considering the pound-force as a basic unit. An engineer wrote to the Globe and Mail that in going metric Canada was changing from using a system based on weight to one based on mass. He admitted that he would expect to get more butter in a pound in Denver than he would in Toronto, although the difference would be negible. Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
