Re: [Phys-L] fundamental notion of force --> using an arrow to represent something more than a vector

[Philip Keller <pkeller@holmdelschools.org>]

Well I know you are probably just teasing.  But I will say that I find 
JD's "rants" to be helpful and thought provoking.  And even if they 
venture well beyond what I would teach in my class, I still want to get 
the physics right.

In this case, either force is a vector or it is not.  I'm going to tell 
my classes that it is.  I'd feel better doing that if I knew I could 
address this particular issue if it were in fact to come up.


On 10/24/2015 5:24 PM, Richard Tarara wrote:
> From the safety of full retirement, let me repeat what I often say 
> here....take your audience and the context of your courses into 
> consideration.  IMHO, unless you are teaching graduate level physics 
> students or preparing for a round-table discussion with the past 25 
> Nobel Prize winners in physics (and even then....) you can probably 
> ignore an awful lot of want JD rants on about.  Case of 'no harm, no 
> foul'. :-)
>
> rwt
>
>
>
> On 10/24/2015 3:46 PM, Philip Keller wrote:
> > I admit that I draw diagrams like that all the time!
> >
> > I'd be impressed if any student asked me: if forces are vectors, why 
> > can't I put the force wherever I want?
> >
> > I would say that you CAN, but that doesn't mean you have to!  I can 
> > strongly recommend where you should draw the force arrow, but it is a 
> > heuristic, not a law of physics.
> >
> > It's somewhat analogous to the question of where to put the pivot 
> > when solving these kind of problems.  There is a theorem that says 
> > you can put it anywhere, but you will find your path to the solution 
> > easier if you put the pivot where the greatest number of unknown 
> > forces act.  Not a requirement, just a helpful suggestion.
> >
> > Similarly,  you can put the force arrows anywhere you want.  But you 
> > are likely to avoid confusion if you draw them in a way that reminds 
> > you of what the lever arm vector is.  It's not a law of physics. But 
> > you are asking for trouble if you do it any other way.
> >
> > I am not sure I would discuss this with students if they didn't ask.  
> > But if anyone argues that force is a "bound vector", I am not seeing it.
> >
> > >   Maybe you assiduously avoid drawing such diagrams, but
> > >   nevertheless they exist in the world.  There are textbooks
> > >   that feature such things.  I kid thee not.
> > > https://www.google.com/search?tbm=isch&q=%22bound+vector%22+%22free+vector%22+force+point 
> > >
> > > https://www.google.com/search?tbm=isch&q=%22extended+free-body+diagram%22 
> >
> > _______________________________________________
> > Forum for Physics Educators
> > Phys-l@www.phys-l.org
> > http://www.phys-l.org/mailman/listinfo/phys-l