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Reconsidering Weight Versus Mass

May 23, 2011 | A version of this story appeared in Volume 89, Issue 21

The following statement regarding weight and mass appears in a letter by Edward Boudreaux (C&EN, March 21, page 4): “So within Earth’s environment of gravity, the concepts of weight and mass are interchangeable, as is also true of their units.” In the same letter, Boudreaux states, “The conventional units of mass—the kilogram or gram—are the same units for weight.”

These statements are inaccurate and misleading, and they should not appear in any scientific publication. The mass of an object is intuitively defined in many first-year textbooks as the amount of matter in the object. (More rigorous definitions can be found in many physics textbooks.) The unit of mass in the SI system is the kilogram. Weight is the gravitational force exerted on an object and, as such, has the unit of force. This unit, in the SI system, is the newton (N) where 1 N = 1 kg m/sec2. It is clear that weight and mass are different concepts and have different units.

To bring up a well-worn example, I believe most beginning science students know the weight of an astronaut on the moon will be less than on Earth, even though the astronaut’s mass will be the same in both locations.

The Boudreaux letter was written to rebut an earlier letter by Harvey Carroll (C&EN, Jan. 24, page 4). In my opinion, Carroll’s letter was well written, accurate, and to the point. Given his inaccurate statements, it would appear that Boudreaux is guilty of the very charges he levels against Carroll.

Stanley R. Radel
Yonkers, N.Y.



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