Re: [Phys-L] treating force as a vector ... consistently

[Moses Fayngold <moshfarlan@yahoo.com>]

 On Aug 28, 2016, at 1:24 PM, Scott Orshan <sdorshan@aol.com
> <javascript:;>> wrote:


> > As a preface, I'm not big on using momentum as a beginner concept. I
> can't see or feel momentum, and it changes with the frame of reference.

"Feeling" or "seeing" the momentum may be another interesting topic for discussion, but I won't go into it now. As to momentum dependence on reference frame (RF), it is not an argument against its use. Lots of observed properties of objects (more accurately, their measured numerical values) are frame-dependent, which does not preclude them from being useful and meaningful description of reality. If you consider collision of two bodies, the momentum of each will be different in different RF, but their rates of change determining acting forces during their collision, will remain equal and opposite to each other in any frame.     
> > This next one is not one I've seen mentioned, but I think it's an axiom
> as well:
> > They start and end together. Same start time, same duration, or the
> slightly more complicated parallel for variable forces. (In the real world,
> reactions tend to follow actions.)

  I do not see the value of this axiom. As I said in one of my previous messages, "action-reaction" concept is most simple and fruitful only in classical and static situation, in which case the notion of their starting and ending together is meaningless. In more realistic situations involving dynamics this notion does not work or is, at best, not straightforward. Consider, for instance, an electron-positron pair production from collision of 2 neutral particles in the field of a distant proton. Each member of the newly-born pair immediately feels the field of the proton and experiences the corresponding action, but the proton will start feeling their field (and respective reaction) much later. So the axiom does not work for this system. It may work if we include the particles' interaction with each other's field as an intermediate agent, and the changes of the respective field momentum. But that would be no less complicated than the rocket-fuel-outgoing jet stream interactions in Scott's own example.      
> > That last axiom means that the weight of a book can not be a force pair
> to the table's normal force. The book was interacting with the Earth long
> before it was placed on the table.

This does not preclude the emergence of another "action-interaction" pair (book-table) when you put the book on the table.  
> > Real world forces exist as pressures (not applied at a point), and real
> world objects are deformable and/or elastic. 
The gravity force on the Earth is not applied at a point either since the Earth is not a point mass and is also deformable. This does not preclude usefulness of a point-mass model even in describing lots of real situations, including those with planetary motions etc.
 Moses Fayngold, NJIT
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