Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



On 12/11/2013 10:33 PM, Paul Lulai wrote:

Net force isnt *A* force it is a sum of forces.
An account balance is neither a debt nor a deposit.

If we adopt that approach, it means force is not a vector.

I say that because according to the definition of vector,
the sum of two vectors must be *A* vector. This is called
/closure/. This requirement is so fundamental that it
sometimes goes without saying when people are listing the
vector-space axioms:
http://en.wikipedia.org/wiki/Vector_space#Definition
but it is a requirement nevertheless:
http://en.wikipedia.org/wiki/Vector_space#Alternative_formulations_and_elementary_consequences

Going back to dealer/feeler notation...what is the dealer for the net force?

It looks like we have conflict between
a) the idea that every force should have a dealer and a feeler, and
b) the idea that force must be a vector.

We have to choose.
a) We have a physical intuition about forces.
b) We have a mathematical intuition about vectors.

That sounds like a tough choice, but not really, because
a) The notion of dealers and feelers is only /sometimes/ important.
b) The notion of closure is /always/ important.

It is common to start with a less-than-general example, and
then extrapolate to the general case. The world will not end
if we say that dealer-feeler forces are not the general case,
just a starting point, just a foothold, just a springboard,
just a subset of all forces.

As so often happens in pedagogy, and life in general, where
you start out is not where you want to end up.

There is a built-in conflict between the dealer-feeler notion
of force and the vector notion of force. I did not create
this conflict; I just called attention to it.

Such conflicts are not particularly rare:
-- We have two very different notions of "energy" (the physics
energy and the DoE energy aka "available" energy)
-- There are at least two different notions of "conservation".
-- There are two different notions of "acceleration".
-- Within physics we have two different notions of "gravity".
-- There are two incompatible notions of "adiabatic".
-- There are at least four or five incompatible notions of "heat".
-- Even something as crucial and fundamental as the word "and"
has two strongly-conflicting meanings.
++ etc. etc. etc.
http://www.av8n.com/physics/weird-terminology.htm

Such conflicts are a serious impediment to making sense of
the subject ... and not just for students. I've seen experts
get seriously fooled.