Re: [Phys-L] Violation of Newton's third law in motile active agents?
The issue appears to affect the anomalous behavior of microscopic entities in
a viscous environment. Geoffrey Taylor touched on the topic as early as 1951:
see https://royalsocietypublishing.org/doi/10.1098/rspa.1951.0218
He notes: " The dynamics of a body as small as a spermatozoon—say 5x 10-3 cm.
long with a tail 10~5cm. diameter—swimming in watermust, however, be completely
different from that of a fish..."
On Monday, October 30, 2023 at 03:29:44 PM CDT, Prof. Keith S. Taber via
Phys-l <phys-l@mail.phys-l.org> wrote:
If I was a betting man, I would put good money on it not being anything
that contradicts N3.
The snippet quoted claims that energy is converted into force (rather
than being conserved?) - which rather undermines confidence in their
physics (or possibly their English).
Why would Newton’s third law be violated due to 'inner activity' (cf. a
hand grenade that flies apart due to inner activity without such an
issue), and why would you need to consider it an open system if the
activity is internal?
I tried to find where in the paper they discuss Newton's third law, but
as far as I can see they refer to it in the abstract and
introduction…and then do not come back to it later? Or have I missed that?
Hm, published by the American Physical Society and several months
getting through review. I await with interest the comments of far better
qualified colleagues than me.
Keith
On 30/10/2023 19:48, Antti Savinainen via Phys-l wrote:
> Hi,
>
> I came across an article which claims that Newton's third law doesn't apply
> to "active motile agents." It also claims that they would exhibit
> "nonconservativity of energy." The article is freely available:
>
> https://journals.aps.org/prxlife/abstract/10.1103/PRXLife.1.023002
>
> Here is an excerpt from the introduction:
>
> "Motility is one of the main features of living matter, from a single cell
> to a swarm of birds or a human crowd. In the
> last few decades, the dynamics of motile active agents, both individual and
> collective behavior, have been intensively studied, giving rise to a
> rapidly expanding research field in physics bridging nonequilibrium
> statistical physics, biophysics, and continuum mechanics, now known as
> active matter and living
> matter physics. A crucial feature of these systems is that inner activity
> units convert energy into mechanical forces. In turn,
> Newton’s third law may be violated when we regard it as an open system,
> with its mechanical energy being injected from
> microscopic active units. Therefore, the mechanical interactions between
> the units can be non-reciprocal."
>
> As far as I can tell, Newton's third law, expressed in terms of momentum
> flow, is pretty fundamental in physics.
>
> Any thoughts?
>
> Regards,
>
> Anti Savinainen, PhD
> Finland
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--
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the choice of making metaphysical commitments or avoiding them, but of
adopting a good or bad metaphysics.”
<https://science-education-research.com/commonplace/>
Dr. Keith S. Taber
https://science-education-research.com/
<https://science-education-research.com/>
Emeritus Professor of Science Education
University of Cambridge
http://www.educ.cam.ac.uk/staff/taber.html
Senior Member
Homerton College, Cambridge
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/RSC Advances in Chemistry Education/
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