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Re: [Phys-l] g...



Anthony Lapinski wrote:

I mean that a scale does not always show your "true weight (w = mg). Scales show apparent weight -- how heavy you THINK you feel.

What is the difference between how heavy you feel and how heavy you THINK you feel? And how can a scale (used properly) do anything but show how heavy you feel?

So when an elevator accelerates upward, you feel heavier. You feel more pressed to the ground, and a scale would register a higher reading. It seem like you are heavier, but you are not as your true weight has not changed (since g is still 9.8), but your apparent weight has.

My personal position is that a scale (properly used) shows how heavy you ARE, i.e., your "weight." This is the result of my preferred definitions for

1. Gravitational force, (vector): Whatever force remains when all other forces have been identified and accounted for. That is,

gravitational force = m*a - sum of all other forces

2. Weight, (scalar): the magnitude of the gravitational force on an object in the rest frame of the object. That is,

weight = magnitude of (sum of all nongravitational forces)

With these definitions we don't need to talk about "true weight" or "apparent weight." There is only "weight." The only other similar quantity of interest is the gravitational force on the object and we can simply call it "the gravitational force."

With these definitions astronauts, like all things in freefall, ARE weightless even though the gravitational force on them is not zero in an earth-based reference frame.

With these definitions the weight of a person in an elevator accelerating upward IS larger than that of the same person standing on the ground.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>