Re: wrong physics explanations-misconceptions

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

Hi Cindy:
Here is something that I plan to try. Feel free to use the same
(or better) approach and share your observations. Let them
videotape the bob of a conical pendulum, looking from below.
Then ask them if the motion of the bob satisfies Kepler's
laws.

That "law" will be satisfied, the trajectory is likely to be
elliptical. By measuring a and b they will be able to locate
the two focal points. But the second Kepler law will not
be satisfied. I think that the process of demonstrating
this can be very instructive.

Another project (or an extension of the same project) is
to videotape two or three conical pendula swinging at
the same time. Is the Kepler's third law satisfied or not?
It would probably be wise to limit this part to nearly
circular trajectories and compare the values of D^3/T^2,
where D are "representative" (or average from six or
eigth)  diameters, if you prefer.

You asked for printed examples of bad physics. Who
would not be silly enough to write that the trajectory of a
conical pendulum is a good representation of a planetary
trajectory (except for circles)? But you can say "a friend
of mine claims that ... and I need experimental evidence
to show him wrong."  Then ask them to write about the
role of experiments in science, based on the activity.

Practical suggestions. (a) Do not exceed small angles.
(b) A plexiglass (or glass) plate with a rectangular grid
painted on it should be placed below the plane of
trajectories, as close as possible from the swinging bob.
(c) A special case of a simple pendulum (simple harmonic
motion) is worth taping and discussing, naturally. Then
ask them to "discover" that x and y projections of an
elliptical trajectory are trajectories of harmonic motions.
Ludwik Kowalski
P.S. It is possible that I read about this kind of activity
somewhere but do not remember. It is very hard to be
original in this field. Most often what I think is my idea
is really not mine. Let me know who should be credited.

schwarz wrote:

> I am teaching a new course called Physics in Motion - they learn
> physics through video taping things around them.  The class is all
> freshman and they must do alot of writing about physics.  I am trying
> to find some articles/stories that have some "wrong" physics
> explanations and or physics misconceptions in them.  Can anyone point
> me to some?  I will then have them rewrite the article with "correct
> physics" explanations.  Stuff on the internet or in
> magazines/newspapers that they could find would be really great.  If
> anyone has any suggestions please email me.
>
> Cindy Schwarz
>
> p.s.  I am currently only looking for things involving
> motion/kinematics/dynamics
>
> schwarz@vassar.edu