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Arguably Aristotle made a worse error in his definition of mechanics, particularly of motion. For him friction ruined acceleration. "Continuation of motion depe

by rurban 3y ago

Arguably Aristotle made a worse error in his definition of mechanics, particularly of motion. For him friction ruined acceleration. "Continuation of motion depends on continued action of a force".

canjobear 3y ago
Aristotle’s physics is correct for objects immersed in a fluid. https://arxiv.org/abs/1312.4057 https://arxiv.org/abs/1312.4057
- pdonis 3y ago
  More precisely, it is a good approximation for objects immersed in a fluid that is sufficiently dense relative to the density of the objects, on time scales long enough for the effects of the fluid to be significant given the relative densities. Rovelli fails to point out this critical factor, although it is implicit in the equations he gives, and fails to recognize that this factor invalidates his criticisms of high school textbooks and teachers who, he claims, mislead students by telling them that heavy and light objects fall with the same acceleration. In fact, what high school textbooks and teachers tell students is that if we neglect the effects of air, heavy and light objects fall with the same acceleration. That's what I was taught in high school, and it's perfectly correct. (Note that this phrasing already eliminates cases like motion in water, for which the Aristotelian model is a much better approximation.) Some high school textbooks and teachers (like the one I was fortunate enough to have) will even go on to point out that, for a wide range of objects falling in air, the effects of air are negligible for a significant enough amount of time to make feasible, for example, the famous (but probably apocryphal) experiment of Galileo dropping two cannon balls of different weights from the top of the Leaning Tower of Pisa and seeing them hit the ground at the same time. And Rovelli's equations, when you plug in the relevant numbers for such a case, say the same thing--but Rovelli himself doesn't. While this in itself does not invalidate the actual math in the paper, which is fine, I found it very disappointing that Rovelli went beyond what his math actually justifies in making the criticisms of high school textbooks and teachers in the paper. He should have known better.
  - pdonis 3y ago
    > While this in itself does not invalidate the actual math in the paper, which is fine Actually, on going back and re-reading, I realized that even the math in the paper has an error. The formula for buoyancy force is wrong! Rovelli's formula for buoyancy force is V rho (in the upwards vertical direction), where rho is the density of the fluid in which the object is immersed. But this is wrong; it doesn't even have the right units. This formula says that the buoyancy force is equal to the mass of the displaced fluid; but mass is not force. The correct formula is V rho g, where g is the acceleration due to gravity; in other words, the buoyancy force is equal to the weight of the displaced fluid. (To check against your intuition, you can plug in numbers and realize that, according to Rovelli's formula, an ordinary helium balloon of the kind you get for birthday parties would not rise in air!) This just makes the paper even more disappointing.
lo_zamoyski 3y ago
I will add as a footnote, and I know you likely did not mean this, but it happens often enough, that one should distinguish between Aristotle's scientific works and philosophic works. That is, while his physics may be obsolete (though we should show gratitude, as this was an early attempt from which our later traditions drew), his metaphysics is not, for example.
- pdonis 3y ago
  > his metaphysics is not It isn't?

by rurban 3y ago

Arguably Aristotle made a worse error in his definition of mechanics, particularly of motion. For him friction ruined acceleration. "Continuation of motion depends on continued action of a force".

canjobear 3y ago

Aristotle’s physics is correct for objects immersed in a fluid. https://arxiv.org/abs/1312.4057 https://arxiv.org/abs/1312.4057

pdonis 3y ago
More precisely, it is a good approximation for objects immersed in a fluid that is sufficiently dense relative to the density of the objects, on time scales long enough for the effects of the fluid to be significant given the relative densities. Rovelli fails to point out this critical factor, although it is implicit in the equations he gives, and fails to recognize that this factor invalidates his criticisms of high school textbooks and teachers who, he claims, mislead students by telling them that heavy and light objects fall with the same acceleration. In fact, what high school textbooks and teachers tell students is that if we neglect the effects of air, heavy and light objects fall with the same acceleration. That's what I was taught in high school, and it's perfectly correct. (Note that this phrasing already eliminates cases like motion in water, for which the Aristotelian model is a much better approximation.) Some high school textbooks and teachers (like the one I was fortunate enough to have) will even go on to point out that, for a wide range of objects falling in air, the effects of air are negligible for a significant enough amount of time to make feasible, for example, the famous (but probably apocryphal) experiment of Galileo dropping two cannon balls of different weights from the top of the Leaning Tower of Pisa and seeing them hit the ground at the same time. And Rovelli's equations, when you plug in the relevant numbers for such a case, say the same thing--but Rovelli himself doesn't. While this in itself does not invalidate the actual math in the paper, which is fine, I found it very disappointing that Rovelli went beyond what his math actually justifies in making the criticisms of high school textbooks and teachers in the paper. He should have known better.
- pdonis 3y ago
  > While this in itself does not invalidate the actual math in the paper, which is fine Actually, on going back and re-reading, I realized that even the math in the paper has an error. The formula for buoyancy force is wrong! Rovelli's formula for buoyancy force is V rho (in the upwards vertical direction), where rho is the density of the fluid in which the object is immersed. But this is wrong; it doesn't even have the right units. This formula says that the buoyancy force is equal to the mass of the displaced fluid; but mass is not force. The correct formula is V rho g, where g is the acceleration due to gravity; in other words, the buoyancy force is equal to the weight of the displaced fluid. (To check against your intuition, you can plug in numbers and realize that, according to Rovelli's formula, an ordinary helium balloon of the kind you get for birthday parties would not rise in air!) This just makes the paper even more disappointing.

lo_zamoyski 3y ago

I will add as a footnote, and I know you likely did not mean this, but it happens often enough, that one should distinguish between Aristotle's scientific works and philosophic works. That is, while his physics may be obsolete (though we should show gratitude, as this was an early attempt from which our later traditions drew), his metaphysics is not, for example.

pdonis 3y ago
> his metaphysics is not It isn't?