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

[Jeffrey Schnick <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of Philip
> Keller
> Sent: Saturday, October 24, 2015 3:47 PM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] fundamental notion of force --> using an arrow to
> represent something more than a vector
>
>
> 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.

Check out 
<https://en.wikipedia.org/wiki/Free_body_diagram>.
Under Modelling the Body it has:
"rigid extended. Stresses and strains are of no interest but turning effects are. A force arrow should lie along the line of force, but where along the line is irrelevant. A force on an extended rigid body is a sliding vector."
and
"non-rigid extended. The point of application of a force becomes crucial and has to be indicated on the diagram. A force on a non-rigid body is a bound vector. "
John is not making this stuff up.

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