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Re: [Phys-l] Weight?



I also have considered that 'weight' might be used to denote a psychobiophysical sensation, much like loudness and pitch are considered psychoacoustic interpretations of amplitude and frequency. Weight could be considered to be the brains interpretation of the sensations of how tightly the muscles in the legs and feet and gut and butt are held to support everything. When they don't have to support as much (like a downward accelerating elevator or orbit around a planet or the Tower of Terror at Disneyworld), you have the sensation of less weight; when you have to support more, you have the sensation of more weight.

So, maybe as physicists working problems on massive bodies we would do well to not even use the word weight as a physics quantity. What I say when drawing a 3-body diagram is "we have downward force due to the gravitational attraction of the planet. Let's call the magnitude mg." Yes, I'm one of those that defines g as the gravitational field of the planet. I try never to talk about weight, except as a sensation. I do talk about mg. For elevator problems, we generally either calculate the normal force that the floor of the elevator exerts, or the acceleration of the elevator relative to the planet's surface. I try to be careful with definitions, so I also warn students that if we consider the rotation of the Earth, either g will have to vary slightly or we'll have to consider an extra term.

Bill

-----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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