Physics question

[Reidy]

Do items of different density or specific gravity (are these the same?) fall at different speeds in a fluid medium such as air or water, holding other factors equal?

We all heard in high school or college that Aristotle said objects of different weight fall at different speeds and wasn't he dumb. I came across the statement, in Apostle's edition of the De Anima (Peripatetic Press 1981, page I), that Aristotle's word is ambiguous among "weight" and the two above and that he always assumed some medium (since he thought a vacuum was impossible); thus, says Apostle, he was not so wrong after all.

[BaalChatzaf]

Do items of different density or specific gravity (are these the same?) fall at different speeds in a fluid medium such as air or water, holding other factors equal?

We all heard in high school or college that Aristotle said objects of different weight fall at different speeds and wasn't he dumb. I came across the statement, in Apostle's edition of the De Anima (Peripatetic Press 1981, page I), that Aristotle's word is ambiguous among "weight" and the two above and that he always assumed some medium (since he thought a vacuum was impossible); thus, says Apostle, he was not so wrong after all.

Density = mass/volume

Specific gravity of a test substance = density of the test substance / density of a fiduciary substance.

Water is often used as the fiduciary substance.

In a viscous medium terminal velocity is a function of the viscosity of the medium and the cross section area of the object falling through the medium. Denser the medium, terminal velocity is reached sooner. All other things being equal the object of lesser mass will reach terminal velocity sooner than an object of greater mass, so Aristotle was partially correct. However in a near vacuum all objects fall with the same acceleration. This generalized to a perfect vacuum is Einstein's equivalence principle. All objects have the same acceleration due to the force of gravity, once drag and viscosity are eliminated.

Space is filled with fields so it really isn't empty in the sense of having nothing in it. However portions of space can be devoid of matter or very nearly so. In outer space hydrogen molecules are there but rather far apart relative to their size. There is the occasional speck of dust in a given volume in outer space. Light exerts some pressure on objects and space is just filled with whizzing photons. But the drag is nearly zero and the viscosity is zero.

See http://en.wikipedia.org/wiki/Terminal_velocity to know what the terminal velocity of an object with a given drag coefficient is in a medium of a given density.

Ba'al Chatzaf

[syrakusos]

Do items of different density or specific gravity ... he was not so wrong after all.

Right. When I was performing public demonstrations at the Ann Arbor Hands-On Museum (2005-2006), I would have a kid from the audience push a desk across the floor. When he stopped pushing, the desk stopped moving. I explained that Aristotle was not wrong, only that Newton took a wider viewpoint, a broader perspective, including more specific cases in his generalization. We call it a paradigm shift, a different context in which to view the phenomenon. Or, easily, what Ba'al said.