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

[Philip Keller <pkeller@holmdelschools.org>]

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