Re: "Simple pendulum"

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Referring to:

> > Are you opposing the idea of collecting misconceptions,
> > Jack? I mean misconceptions some of us had, and also
> > those we identify in minds of students. No, I am not
> > referring to misconceptions such as "cows must drink
> > a white fluid to produce white milk."

Inge H. A. Pettersen" wrote:

> I think that your idea of collecting misconceptions is a great idea.
> I think also it would be wise to distinguish between misconceptions
> of central concepts (work, energy, heat, ..) and principles compared
> to misconceptions in an analysis of specific physical systems.
> Misconceptions in the first group are the most dangerous ones.
> Let me also point out the recent article H. T. Williams "Semantics
> in teaching introductory physics" AJP 67 (8) Aug 1999 pp 670-680
> which has in my opinion many valid points. One of the principal
> ones is that many terms in physics, maybe most of them, are from a
> pedagogical point of view best defined ostensively (i.e. by examples).

This begs for a definition. What are misconceptions? Let me
follow the advice and express myself by the way of an example.

As a student (and also when I started teaching) I accepted the
convention that acceleration is positive while deceleration is
negative. It prevented me from solving some kinematics
problems in one step. The vertical throw problem, for example,
would be solved with the y axis up till v becomes zero and
with y axis down for the free fall step. I do not remember the
origin of this. Then I learned a better way (see below) and I
solve the same problem in one step, with y pointing up.

Was a less effective way of practicing kinematics a
misconception? Is it worth of being described as an item
on our list? What is a better name for the "list of
misconceptions"?

As stated, for example,  in vol I of "Physics, a Contemporary
Perspective" by R.D. Knight, "The sign of acceleration has
nothing to do with weather or not the object is speeding up
or slowing down". The correct interpretation is that a positive
acceleration vector points away from a chosen origin (for
example, along the positive x axis) while a negative
acceleration vector points toward the origin (for example,
against the positive x axis).

In other words, a component of an acceleration vector is
positive when its direction coincides with the direction of
the velocity component. And it is negative otherwise. The
convention that "the direction of v is positive when an object
moves away from the origin (and negative otherwise)", does
not confuse students as much, when they learn kinematics,
as what we say about a.

Acceleration and deceleration terms are used in plain English
as references to opposite things (see also contaminate versus
decontaminate, activate versus deactivate, etc.), That is the
source of confusion. Fortunately the sign rules become
less confusing after the F=m*a formula is digested by
students.

Ludwik Kowalski