Re: Displacement

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

Seems to me that there issue here is what we mean by
"change."  Generally I mean the later value minus the earlier value, but
of course that implies a definition of later and earlier...what a mess.

Anyway, I think you are correct on both counts...first the author ought
not alter a definition in mid-text....change is always final minus
initial.  Secondly the author should not avoid an important issue by the
slight of hand trick of reversing the definition.

joe

On Mon, 8 Oct 2001,
Ludwik Kowalski wrote:

> I would not know how to answer a student who might
> ask why the displacement is defined as Xfinal-Xinitial
> in Chapter 2 and as Yinitial-Yfinal in Chapter 5. I am
> referring to Section 5.3 of College Physics (5th edition)
> by Serway and Faughn (page 121 etc.).
>
> I suppose this was done to make a change of potential
> energy positive while the corresponding change of
> kinetic energy is negative, and vice versa. But should
> a displacement be always (final-initial)? Something
> is not right. What is it?
> Ludwik Kowalski

Joseph J. Bellina, Jr.               219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556