🚀 go-pugleaf

On Tue, 11 Apr 1995, Jennifer Wright wrote:

> Matt,
> We were actually playing with one of these tops in my physics classes
> right before our break.  My info comes from the book entitled "Toys in
> Space" by Dr. Carolyn Sumners.  According to her, there is no point on
> the top for it to spin on.  As it spins then, gravity pulls it over and
> therefore a it has a "wobbling" axis.  Gravity pulls enough that the top
> flips entirely over onto its handle, which is a stable axis and
> continues to spin.

Did they observe the *direction* of the spin before and after the flip?
If Carolyn Sumners doesn't say more than you quote above, it isn't very
helpful in *understanding* what's going on. One can easily make a top
with no 'point' which does *not* flip. In fact some of those you buy are
so poorly shaped that they are reluctant to flip. Spin a round wooden
ball. Does it flip? No. Does it have a point? No.

The description above, with the stuff about the wobbling axis, seems to
be confusing the 'tippy top' with that other toy marketed as the 'space
pet' the latter being an elongated plastic object about 3 inches long,
which does wobble before changing its direction of rotation.

The tippy top flips because of frectional torque where it touches the table.
The analysis is clearly beyond the high school level, or even the
freshman college level. Mason's book on Mechanics does the analysis. You
need to understand moments of inertia and torques in 3-D.

Anyway, note that the direction of spin is the same after flip. Therefore
the direction of the top's spin with respect to its own axis changes to
the opposite direction! It's amazing how many people observe the top
without noting that *very important* detail. (At what exact point is its
rotation about its own axis momentarily zero?)

Paint a little arrow on the top to confirm this. Now ask, does Sumners
*really* understand it? Do the students? Should they? If you *really*
understand something, you can make correct predictions about it,
confirmable by observation.

        -- Donald