Re: [Phys-L] Units, gratuitous complication - and memories

[Leigh Palmer <palmer@sfu.ca>]

On 2012-09-16, at 9:00 AM, Bernard Cleyet wrote:

> On 2012, Sep 15, , at 11:04, Leigh Palmer wrote:
>
> > Nothing really bad has happened to confuse students since I moved to Canada. I was jolted somewhat when the definition of mass changed
>
> Please elaborate, and JD pointed out (phys-l)that the fed. (US) govt. has decreed the pound is a unit of mass.
>
> http://www.phys-l.org/archives/2008/11_2008/msg00054.html

The modern definition of mass is the same as what I was raised to think of as "rest mass". Rest mass is the inertial mass of a particle at rest. I don't know when the amendment was made to the old definition which ignored the possible relativistic modification of inertial mass. I guess it happened about 1965.

Canada is not yet entirely free of the British Imperial System. Body weight and height are still most commonly expressed in the old units. Elevations of geographic places have pretty much been converted to meters in common use, but I still think of them in feet. Temperature and volume are all changed to what is called "metric" in the USA. The mass-slug-pound-poundal-force problem seems much less troublesome here now that everyone is familiar with kilograms, if not newtons. My bathroom scale still reads in pounds, though I could choose to have it read in kilograms - or even stones and pounds - if I wished to do so. Weight in kilograms still offends me slightly, so I won't switch the scale.

I looked at the document JD cited, and I didn't see the word "mass" anywhere. I guess that it is implied.

I think it is important to tell students that the scientifically conventional units are to be learned and understood. It is also important to let them know that the old fashioned MKS system has been replaced by the SI which should be thought of as a KS system now that the standard of length is defined in terms of the second. 

Leigh