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Re: [Phys-L] feeler-dealer, third law, et cetera



"By way of analogy, note that in introductory physics it is absolutely routine
to use one weight to lift another weight with the help of massless strings
and frictionless pulleys. We speak of one weight pulling on the other
weight, and we abstract away the string that transmits the force."

It should NOT be "routine". It undercuts the important concept that objects
in the surroundings exert forces on objects in the chosen system, either
through distance forces or through "contact" forces. I judge it to be
highly important NOT to say that one weight pulls on the other weight,
because their distant gravitational interaction is negligible, and there is
no contact.

(A not entirely unrelated issue is that massless strings and frictionless
pulleys and zero-resistance wires and infinite-plate capacitors can
introduce pathologies and/or misconceptions. It's much better to say
low-mass string and low-friction pulley and resistance small compared to
other resistances in the circuit and plates large compared to the gap. As
one insightful student commented, "You could never discharge an
infinite-plate capacitor, because it would have zero fringe field to drive
current in the neighboring wires.")

Bruce