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From: John Fox <j.fox@LATROBE.EDU.AU>
Date: October 22, 2006 10:00:42 PM EDT
To: HOPOS-L@listserv.nd.edu
Subject: Re: Weight vs mass
Reply-To: A Forum for Discussion of the History of the Philosophy of Science <HOPOS-L@listserv.nd.edu>
Dear Hopoi,
In Newton, I think, the theoretical distinction between mass and weight was
not a stating point but emerged down the line.
There was a conceptual distinction before Newton. Weight or heaviness (latin
'gravitas') had been for almost two millennia the favourite paradigm of an
intrinsic attribute of bodies; it was measured on balances etc. What Newton
called 'quantity of matter', and was soon called mass, was taken over from
Descartes; it was assumed to be something conserved. It was, I'm fairly
sure, widely thought that weight was a function simply of quantity of
matter, but the ideas were distinct.
Newton's revised notion of weight was derivative from a new relational
predicate he introduced, 'gravitare erga', literally having-weight towards
or being-heavy towards. Assuming his second law of motion, he calculated
that the force required to keep planets in elliptical orbit around a focus
was directed towards that focus and inversely proportional to the square of
the distance, and derived Kepler's laws; he noticed that both the moon's
motion round the earth and the behaviour of bodies in free fall could be
explained by the same inverse square law, and generalized it to the
universal law of gravity. With this law in place, the weight of a body
could be interpreted as the resultant gravitational force exerted on it;
i.e. in effect the vector sum of its weights-towards every body in the
universe! and indeed a function of its mass, but also of the mass of
everything else and of their relative distances!
Given gravitational theory, it could be predicted that gravity was not
invariant even if mass was, but would be much less on the moon, and a little
less even up high mountains. The former took a while to confirmed, but the
latter was confirmed quite soon.
That is my (as we used to say before decimal currency) two bob's worth.
John Fox
on 21/10/06 4:05 AM, jbellina at jbellina@SAINTMARYS.EDU wrote:
A physicist on the physics list wonders what the led to the
distinction between mass and weight. I suggested it probably was
with Newton, but I'm sure someone(s) on this list can provide a more
complete answer. I will forward whatever reply to the physics list.
thanks,
joe
Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556