[USMA:36395] Re: Another Lord Kelvin quote

[Martin Vlietstra]

For the record, the value of "g" varies by about 0.25% between sea level and the top of Mount Everest (at the same latitude) and by about 0.5% between sea level at the equator and and the poles.   At the equator, "g" has a value of 9.780 m/s² while at the North Pole it has a value of 9.832 m/s².   The relationship between sea level and the top of Everest can be derived as follows:   F=GmM/r² (Equation of gravitational attraction).   Differentiation and substitution gives dF/F = -2dr/r.  Setting r=6400km and dr = 8km gives dF/F = 0.25%. ----- Original Message ----- From: Bill Hooper To: U.S. Metric Association Sent: Tuesday, March 28, 2006 7:22 PM Subject: [USMA:36394] Another Lord Kelvin quote Here's another of my favorite quotes from William Thomson*, Lord Kelvin. It points out the fact that mass and weight are not the same thing in spite of the absolutely ridiculous definitions of a mass unit and a weight (force) unit that are both named "pound". *Lord Kelvins family name, Thomson, does not have a "p" in it. I spelled it incorrectly in my earlier email. Thanks to those who pointed out the error. =========================================== "By taking the gravity of a constant mass for the unit of force** it makes the unit of force greater in high than in low altitudes. In reality, the standards of weight are masses, not forces. They are employed primarily in commerce for the purpose of measuring out a definite quantity of matter; not an amount of matter which shall be attracted to the earth with a given force."  =========================================== ** This statement refers to the practice of  defining a force of "one pound" as the weight (force of gravity) of "one pound" of mass (or vice versa). Thus, there are defined two DIFFERENT units, one a mass unit and the other a force unit, both of which are named "pound". Regards, Bill Hooper Fernandina Beach, Florida, USA ==========================    SImplification Begins With SI. ==========================