Re: [Phys-L] Violation of Newton's third law in motile active agents?

[John Denker <jsd@av8n.com>]

On 10/30/23 12:48 PM, Antti Savinainen via Phys-l wrote:

> Newton's third law, expressed in terms of momentum
> flow, is pretty fundamental in physics.

Yes.

> 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."

Generally it's not a good idea to rubbish an article without
reading it, but I'll make an exception in this case.

Newton's third laws is violated if and only if momentum conservation
is violated. That's won't happen. That won't even appear to happen,
not even for a complex system.

In particular, consider the proverbial boxcar containing birds
and monkeys that bang off the walls at random times. The center
of mass of the system as a whole continues to move at a steady
rate. The impulse/momentum theorem applies just fine to the system
as a whole. In other words: You can't hide momentum.

Conservation of energy is separate from conservation of momentum
i.e. the third law. Energy arguments about the third law are kooky.

Keeping track of the energy in a complex system is very much more
complicated than keeping track of the momentum. Keeping track of
the KE is even harder. For example, consider a black box containing
two counter-rotating flywheels. It contains a lot of KE, but we
cannot know that without unsealing the black box. The KE could be
reduced by mutual friction between the flywheels, and we would
never know without unsealing the black box. As a consequence, the
pseudowork/KE theorem must be stated very carefully and used with
caution when applied to a complex system.

In other words, momentum cannot be hidden, but energy can be hidden
after a fashion, if you don't look too closely.

Note that there were fairly extensive discussions of the pseudowork/KE
theorem in this forum a few years ago, from which I learned a lot.
https://www.phys-l.org/cgi-bin/namazu.cgi?query=pseudowork+ke

My thinking on this topic, as of a few years ago, is summarized here:
  http://av8n.com/physics/kinetic-energy.htm