It's generally taken that Galileo established the concept of mass; and usually this is illustrated by the apocrophyl story of two cannon-balls of differing size thrown simultaneously from the top of a tower.

But in fact he had a theoretical argument, which I suppose is in his Dialogue on Two World Systems (the Ptolmeic and Aristotelian).

Is this supposition correct?

Lucretious also had an argument for mass, which he reported in De Rerum Natura as:

... lastly, why do we see two

Objects of the same size differ in their weight? Instead,

If a ball of yarn contains the same amount of mass as lead,

Then they should weigh the same, since mass's property is to press

All downwards, while the property of void is weightlessness

This, I suppose is the same argument in Galileo.

Is this supposition too also correct? Or is there some substantive difference?


I'm not, if it needs saying, disputing the efficacy of modern physics; but simply establishing consonance a and differences between different modes of physical thinking at different times.

It's worth adding, given the comments below, and to avoid confusion that the above notion of mass, is not to be understood anachronistically as either inertial or gravitational mass - as it appears in Newtonian Mechanics - but closer to the notion of mole in chemistry.

  • You seem to totally misunderstand the concept of mass. To begin with, which mass are you speaking about? The inertial one or the gravitational one? The primer is a property of resistance to change, while the second one is the charge of the gravitational interaction. Concept of mass and resistance to change already existed in Aristotle physics, but this wasn't at all the same mass as the inertial mass involved in second Newton's law. – sure Aug 26 '15 at 8:20
  • @sure: this is the word used in the translation I'm looking at; by AE Stallings; and no doubt anachronistic. It's more about the notion of 'amount of stuff'. – Mozibur Ullah Aug 26 '15 at 8:25
  • It's enough to establish that, as in Galileo and Lucretious, that things fall at the same speed despite differing size. – Mozibur Ullah Aug 26 '15 at 8:30
  • On the contrary, weak equivalence principle shows that you cannot discriminate "amount of stuff" with respect to their resistance to change (that is, it is harder to push a car than a pen): they all fall similarly and are indistinguishable. Therefore, if you were a greek guy who didn't know all this terminology, you would be right to conclude that mass is about pushing objects, not them falling... – sure Aug 26 '15 at 8:33
  • @sure: how is that different to what I just said? To put it in concrete terms, take two cannon-balls that are exactly the same, drop them from the apocryphal tower, because they are the same, they fall in exactly the same way (and this is a principle of symmetry); repeat this experiment but with the cannon-balls touching; this makes no difference in how they fall; both these two experiments come from experience; now imagine these are protons; then you've just demonstrated an ion of Helium falls like an ion of Hydrogen. – Mozibur Ullah Aug 26 '15 at 8:43

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