🚀 go-pugleaf

On Sat, 22 Apr 1995, Barry S. Riehle wrote:

> Jim Stagliano mentioned that the reason for the inverting of the tippy top is
> that the cg of the tippy top is nearer the stem "so when the top rests on the
> spherical surface, it has more potential energy than when it is sitting on the
> stem."
>
> If that is an accurate explanation then the football whose cg is in the middle
> would not rise for the same reason nor would the class ring whose cg is most
> certainly nearer the stone end that rises to the top.
>
> Donald E. Simanek suggests the opposite occurs: the cg rises.  The cause is
> friction but why would it do this?  I am still puzzled by this.
>
> Barry S. Riehle
>

If you have a tippy top handy, you can easily establish that the cg is
not 'nearer the stem'. Sit the top on the table (not spinning). It rests
happily with round (heavy) end down. Try to balance it on its stem. It
won't, it will fall.

It's hard to communicate gyroscopic motion without diagrams, and I know
no satisfactory way to do it without calculus. See the references
previously cited, which have both. (Barger and Ollson's mechanics book
was mentioned.)

However, the concept that an external friction force can *increase* the
energy of a body is not hard to understand at all. Earlier I offered the
example of the automobile, where the friction of the roadway on the tires
is in the forward direction of the car's motion. Ultimately this is
because the engine is rotating the tires in a backward direction (opposite
to the car's motion) against the ground. The friction of the table or
floor on the tippy top can supply a torque to the top in the correct
direction to change the top's spin axis as well as to raise its cg. Again,
the friction arises because of the top's motion and the fact that the
top's axis tips a bit initially, away from the vertical, so the friction
has a lever arm.

Another example. Place a sheet of paper under a block. Pull on the paper.
The friction of paper on block increases the kinetic energy of the block.
There are lots of examples to illustrate that friction from external
agents can increase the energy of a body, to counter the false notion that
external friction always reduces the energy of the body. (How *do*
students acquire these false concepts, anyway?)

Nip false concepts in the bud soon, folks. False concepts are far harder
to eradicate later, after they are well implanted.

The neat thing about a tippy-top is that it reorients its axis 180 degrees,
while the football and the egg only reorients 90 degrees.

        -- Donald