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# Re: [Phys-L] Ex: Weight of equal masses

• From: John Mallinckrodt <ajm@cpp.edu>
• Date: Sat, 9 Jan 2021 22:37:13 +0000

I won’t weigh in on the central question here, but just a quick note on the “inadvisability” of "mentioning centrifugal force":

Doing so IS inadvisable for the simple reason that centrifugal force IS gravitational force, not 1) because "it is a pseudoforce," or 2) because "it confuses things,” and CERTAINLY not 3) because "one must only calculate gravitational forces in inertial frames.”

I’m not such a purist that I don’t understand and even make use of other, more Newtonian, ways of looking at it, but we should all be clear that this has been accepted physics for over a hundred years.

John Mallinckrodt
Cal Poly Pomona

On Jan 9, 2021, at 2:05 PM, Brian Whatcott <betwys1@sbcglobal.net> wrote:

You may be familiar with the question: which is heavier, one tonne of feathers, or one tonne of steel?If you take the view that weight is due solely to gravitational force, then there is no doubt how you would answer.
Suppose you take the slightly more sophisticated view that weight is the net force due to gravitational force less centrifugal force in a rotating frame, and though the value of g varies from place to place, on this account, you may still hold that their weights are the same.
But now, suppose you take the slightly more sophisticated view yet, that weight is what a scales indicate, where gravitational force is opposed by both centrifugal force in this rotating frame and a buoyancy force due to the air displaced.
I will readily concede the inadvisability of mentioning centrifugal force, but you soon ascertain that the density of this steel is 8000 kg/m^3 , the air density is 1.2 kg/m^3and for convenience we agree a density value for the feathers equal to that for a closed hollow tube of keratin, which is 2.4 kg/ m^2 and which forms 15% of the feather volume.Let us next agree to calculate a cumulative volume for the steel: 1000/8000 m^3or 0.125 m^3 which displaces 0.15 m^3 of air, which weighs 1.2 *0.15= 0.18 kg.Hence in mid latitudes for which the g constant incorporating a centripetal component is 9.81 N/kg, the steel weighs (1000kg - 0.18)*9.81 newtons = 9808 N .
Finally we calculate the volume of the feathers using a density figure of 85% of 1.2 kg/m^3 and 15% of 2.4 kg /m^3 = (1.02 kg + 0.36 kg)/m^3 = 1.38 kg/m^3We quickly find the volume of 1000 kg feathers is 1000/1.38 m^3 = 725 m^3discounting the external volume of air trapped by the feathers. This gives a buoyancy of 725 * 1.2 * 9.81 newtons.This leaves us with the weight of feathers as measured by a scales:1000 *9.81N - 725*1.2*9.81 N = 9810N - 8535N = 1275 N
This leaves us with the uncomfortable weight comparison of 1 tonne of steel weighing more than seven times as heavy as 1 tonne of feathers.(An argument due to Prof. Cristobal Cortes, U.Zaragoza)
If I object that the feathers, which are hollow and filled with air are leaky, then I can discount the internal air volume, and count only the volume of the keratin structure which is 15% of the feather's volume, then the effective buoyancy is greatly reduced to 15/100 of 8535N = 1280 N giving a total feather weight of 8530N In THIS case, the steel weighs only 9808/8530 of the feather weight - a mere 15% more. I welcome critiques of Cristobal's and my estimates....
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