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Re: [Phys-l] Weight?



James,

Quick question: You say that mg is the pull of gravity, I'm gathering from what you said below you mean the integrated contribution of all mass elements constituting the earth. Does that mean that g is not the free-fall acceleration of an object?

Joel

_________________________

Joel Rauber, Ph.D 
Professor and Head of Physics
Department of Physics
South Dakota State University
Brookings, SD 57007
Joel.Rauber@sdstate.edu
605.688.5428 (w)
605.688.5878 (fax)

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Espinosa, James
Sent: Wednesday, November 17, 2010 12:06 PM
To: Forum for Physics Educators
Subject: [Phys-l] Weight?

What is weight? Sometimes, when the origin of the lay word has not been
technically defined (as is done in mathematics), it helps to find its
etymology. The etymology of "weight" goes back to "lift." It,
therefore, appears that the weight of a body has been considered an
upward force. I tell my students that weight is a force of support,
pointing vertically upward. From Newton's laws it follows that weight
is equal and opposite to the force (pull) of gravity on the body. It
can then be shown that N = mg. What I emphasize to the students is that
mg (pull of gravity) is due to the whole Earth, including India, China,
the Pacific Ocean, etc.; but N (the force of support is due only to the
floor which is in contact with the soles of the shoes. The agent of the
force is completely different on the two sides of the equation. I do
not attempt to confuse the students with corrections that might exist
from the Earth's rotation, Special Relativity, General Relativity,
Quantum

Field Theory, String Theory, or any other such.

James Espinosa
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