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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 _______________________________________________ Forum for Physics Educators Phys-l@mail.phys-l.org https://www.phys-l.org/mailman/listinfo/phys-l
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