Re: [Phys-l] bound vectors ... or not

["Rauber, Joel" <Joel.Rauber@SDSTATE.EDU>]

When I present FBD (free-body diagrams) is to use them in the context of situations where one can approximate the object to which the diagram is being applied as a point-like object.  The object is represented by a dot in the diagram and the forces external to the object, but acting on the object are represented as arrows, the tail or the arrow-head can be in contact with the dot in the diagram.

For this version of a FBD, the point of application of the force is immaterial and the concept of force as magnitude and direction only is emphasized.

Obviously this version cannot be used for analyzing situations where rotational dynamics is pertinent, i.e. the structure of the object in question is important.

For those situations I draw diagrams where the force vectors are correctly indicated with the correct line of action (for rigid bodies).  For non-rigid bodies . . .
I also, don't call those diagrams FPD's; though I suppose some people do.

Joel

_________________________

Joel Rauber, Ph.D 
Professor and Head of Physics
Department of Physics
South Dakota State University
Brookings, SD 57007
Joel.Rauber@sdstate.edu 
605.688.5428 (w)
605.688.5878 (fax) 

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Josh Gates
> Sent: Tuesday, September 07, 2010 1:37 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] bound vectors ... or not
>
> Denker:
>   As a related pedagogical issue:  We need to talk about
>   _Free Body Diagrams_.
>
>   FWIW when I was in school, I never saw or even heard of
>   a Free Body Diagram ... even though I got a very good
>   education, beyond what most people can even imagine.
>
>   One salient thing about Free Body Diagrams is that you are
>   supposed to draw the "force vector" in the "right place"
>   i.e. at a place that corresponds (to the extent possible)
>   to its point of application (or, failing that, at least
>   somewhere along the correct line of action, assuming a
>   rigid body).
>   This one, I haven't seen.  Until students get to rotation, I was
> taught (and teach) students to draw forces as vectors with tails
> attached to a dot (representing the object).  In this way, our different
> pushes in the same direction but applied at different points do indeed
> look exactly the same; the only things that are important are the
> direction and magnitude.
>
> When they get to rotation much later, we draw diagrams as you say, and
> at this point, the students (should) have the intuitive understanding of
> N's 2nd to recognize that the point of application is irrelevant when
> considering the motion of the CM.
>
> jg
>
>
>
>   So, it would seem, students who are taught to draw Free
>   Body Diagrams are at risk of developing a notion of "force"
>   (and of "vector" in general) that does /not/ uphold the
>   principle that such things have a direction and magnitude
>   but not a location.
>
>   I'm not sure how to proceed on this.  I reckon Free Body
>   Diagrams are still a good thing, but we need some way
>   to deal with the idea that the arrows on such a diagram
>   are not, strictly speaking, force vectors.  They're
>   something else.  This seems like it "should" be an
>   easy-to-solve problem, but I need to think about it
>   some more.
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