Re: wrong physics explanations-misconceptions

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

Regarding Ludwik's demonstration that fails to satisfy Kepler's laws.

> 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. ...

Actually *none* of Kepler's 3 laws are satisfied by the motion of
the bob, and this is even *after* we have taken the small angle
limit.  The reason that Kepler's 1st law is not satisfied is that
the ellipse generated by (2-dimensional) SHO motion has the center of
force at the center of the ellipse.  But Kepler's 1st law requires
that the Sun be at one *focus* of the orbital ellipse--not the
center.

David Bowman
dbowman@georgetowncollege.edu