Re: displacement and graphs

[Larry Smith <Larry.Smith@SNOW.EDU>]

At 11:23 AM -0400 10/9/01, Spagna Jr., George wrote:
> Ken Fox asks:
>
> > I am puzzled by authors who ask for a graph of position as a function of
> > time but call it displacement. Am I alone? To me a displacement graph
> > should mirror the velocity graph.
>
> I'm confused. What do you mean by "mirror?"  As I understand it, in 1D,
> displacement and position differ only in the arbitrary choice of origin for
> position, and the establishment of an initial position for displacement.  In
> either case, velocity vs time should be the slope of the position or
> displacement vs time graph.

I have found the same confusion seems to exist in various authors' minds,
and consequently I have to make a lengthy explanation about the terms.  But
I'd still be interested in what the rest of the list does.

Does the term "displacement" refer to the vector r (from some arbitrary
origin) or does it refer to r_2 - r_1  (i.e. final - initial).  They are
clearly not the same thing but it seems texts use "displacement" for both
concepts.

Ken, it seems to me, is of the "displacement = r_2 - r_1" camp which means
that it would look exactly like a velocity graph sans the division by delta
t.  Others seem to think "displacement = position or location"--the _slope_
of whose graph would be the velocity.

In other words, is average velocity the slope of displacement or is
velocity just displacement divided by delta t?

Larry