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Re: [Phys-l] ? passive force of constraint



I agree with Paul here--have for the last 30 years. I've never used the action/reaction nomenclature other than to identify the 3rd law as students have already heard it stated. I too work with the idea of force pairs exclusively. The action/reaction words bring up an image that one force is primary and also prior to the second (secondary) and later force. That's just not the case. It is this idea that forces come in pairs that also keeps me from trying to analyze forces from inside an accelerating frame of reference--that is a non-Newtonian viewpoint. We stick with the Newtonian world for quite a ways into the course so we look at accelerating systems from that viewpoint--where to be a legitimate force there must be a second paired force. That's as far as I try to take it for my Gen-Ed class, but for the Science majors we eventually work our way towards more modern viewpoints.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu
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----- Original Message ----- From: "Paul Lulai" <plulai@stanthony.k12.mn.us>


I've recently switched from action/reaction to interaction pairs. A switch in reference frame might change which is perceived as the action & which is the reaction. Action/reaction also tends to imply one of the objects 'caused' the phenomena to occur while the other was an innocent bystander. This is obviously not true.

If we discuss every such occurrence as an interaction, the 'fault' of the collision is gone. It is easier to see which objects are interacting (while easier, I'll admit it doesn't magically & completely remove the conceptual issue). It is also a bit smoother when discussing impulses. Stating that impulse-momentum methods work when we are interested in information just b4 or after an interaction is smoother than discussing action/reaction at that point. It also allows an easy transition & reinforcement with N3L discussions than if I subbed in action/reaction or collision etc...



To wonder is to begin to understand.

Paul Lulai
Physics Teacher
St. Anthony Village Senior High
Saint Anthony Village, MN
55418
(w) 612-706-1144
(fax) 612-706-1020
plulai@stanthony.k12.mn.us



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