Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Displacement and position (was: displacement and graphs)



I'm confused.

Displacement is change in position, no? Has there been any disagreement
about that?

<Change in velocity> is change in velocity. We just don't have a special
name for <change in velocity>.

So, what would a plot of <change in velocity> be? Well, from my point of
view, it depends on the reference...I need to know "change from what?" You
can't arbitrarily plot <change in velocity> and expect people to know that
it is the change over the time increment chosen for the graph. Similarly,
you can't arbitrarily plot displacement and expect people to know that it is
the displacement over the time increment chosen for the graph.

It seems to me that people will naturally assume you mean the displacement
relative to the initial (fixed) position, wherever that happens to be.

I think that is what JSD is saying. If so, what is the problem? If not,
please help me.

----------------------------------------------------------
| Robert Cohen Department of Physics |
| East Stroudsburg University |
| rcohen@po-box.esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
----------------------------------------------------------

-----Original Message-----
From: RAUBER, JOEL [mailto:JOEL_RAUBER@SDSTATE.EDU]
Sent: Wednesday, October 10, 2001 10:42 AM
To: PHYS-L@lists.nau.edu
Subject: Displacement and position (was: displacement and graphs)


I'm simply going to have to disagree.

John D's "displacement", "displacement generally means
position relative to
some arbitrary reference." is the less-general case as it
requires reference
to an origin, i.e. it is displacement from the origin, a
particular point,
my and the quoted reference's displacement requires no such particular
point, although it is covenient to use such a fiduciary point for
calculational purposes.

Or to say it slightly differently: What John D calls
displacement is the
special case of displacement from the origin. (and hence
less-general).
John D's displacement is what I call "position", which I define to be
displacement from the origin.

When I look at Feynman page 47-4, Feynman's use of the word
"displacement"
appearing in figure 47-3 as well as in the verbiage and math
of the text
appears to me to match my definition and use of the word displacement.

And to further name-drop, (not really a valid method in scientific
discussions other than to point out popular usage and
definitions of terms)
the 9th edition of Sears and Zemansky "University Physics"
page 10 follows
my usage. (Note: the 9th edition no longer has the names
Sears and Zemansky
on it, which I consider a bit of sacriledge on the part of
the publishers,
but that is another discussion.)

And to quote John D:

"Citing examples of less-general usages is not going to prove
anything."