Re: displacement and graphs

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

This confusion has prompted me to avoid the term displacement...do it
really serve a purpose that change in position would not.  We don't have
a separate word for change in velocity...why do we need one for
position.
Could it be that this preferential treatment of position, that is
creating a word for the change, is somehow connected with the naive
preference for position rather than velocity when thinking about how
objects move?  That just popped into me head so I'm not sure I would
defend it strongly...but it is curious.

joe

On Wed, 10 Oct 2001, Rick Tarara
wrote:

> A little sidetrack here:
>
> I've always had troubles at the beginning of intro courses trying to keep my
> nomenclature consistent.  The problem as I see it is as follows:
>
> 1)  We usually start out defining velocity as the (change in
> position)/(change in time).
>
> 2)  The change in position can be considered a vector--the displacement--so
> we can also say that the velocity is the displacement/(change in time).
>
> 3)  However, we can also express any position as a displacement vector
> (relative to some origin) and therefore the change in position is equivalent
> to a change in displacement.  Hence we can say that velocity is the (change
> in displacement)/(change in time).  In a calc course we will end up with v =
> ds/dt.
>
> 4)  Of course (3) is consistent with (2) since the change in displacement is
> indeed another displacement vector.  This is really no different then our
> definition of acceleratiion as (change in velocity)/(change in time) since
> the change in velocity is indeed another velocity--but we don't tend to talk
> that way.
>
> 5)  So--(change in position)/(change in time); displacement/(change in
> time), (change in diplacement)/(change in time) are all equivalent---but I
> suspect confusing to students.  It is difficult to stay with just one of
> these however when working through various examples.
>
> fwiw
>
> Rick
>
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> Saint Mary's College
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Joseph J. Bellina, Jr.               219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556