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Re: [Phys-l] definitions ... purely operational, or not



That's not a big problem for those of us who go with the idea that weight is the force due to a large, local gravitational attractor (I personally wouldn't use the term 'true weight') and apparent weight is what is measured by the scale. We accept that weight, unlike mass, varies from point to point (either because of the rotation of the earth, the local mass density of the earth beneath our feet, or the distance from the earth's center.) At the altitude of the space station, the earth's attraction (the 'weight' force) is about 90% of what it is on the earth's surface. As has been said by many and many times, the choice of approach is open as long as one remains consistent. For those of us dealing primarily with the Newtonian model, it is advantageous to stay in frames where the Third Law holds and the force pairs are easily identified. I like to use the idea that we often "experience" forces backwards--the sensation when taking off in a plane, of riding the Ferris Wheel or the Rotor Ride, simply of sitting in a seat (interpretting the upwards push of the seat as the experience of our weight).

Rick

Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
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----- Original Message ----- From: "Folkerts, Timothy J" <FolkertsT@bartonccc.edu>


>True weight is mg ...

The problem with this definition is that mg is the force measured on a ROTATING earth.

g ≠ GM/R^2

"Although the symbol g is sometimes incorrectly used for standard gravity, g (without a suffix) strictly means the local acceleration due to local gravity and centripetal acceleration, which varies depending on one's position on Earth (see Earth's gravity)."

http://en.wikipedia.org/wiki/Standard_gravity
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