Tops slow down because of the friction forces at work. The biggest
effect usually comes from the point of the top and the floor, where
they touch. There's also some friction with the air. If the top
wobbles, it can increase the friction forces on the tip as the wobbling
may drag the tip across the floor.
Reducing air friction usually just means making the top smooth and
symmetrical about its axis. Actually, putting golf-ball style dimples
might help -- experiment!
The most important part is the point. Reducing the size of the
point is probably the best thing you can do. If the point has some
large size to it, then as it turns, parts of it will rub on the floor.
The wider the point, the farther a bit of it will travel as it rubs on
the floor. A pinpoint or nail point may do nicely for an optimum top.
Choose a material for the point and the floor that will not bend out of
shape or dent. Using the pinpoint top on an unvarnished wooden floor
will probably dig the pinpoint into the floor and slow the top down
fast. A steel or concrete surface would work better. Pins are not very
sturdy, and it is easy to bend them out of shape. If the tip of the top
is not exactly on the axis of rotation, it will make the top wobble,
and make the point slip on the floor as the top spins, losing energy
due to friction. So don't bend the tip! This is why many good tops have
metal tips which are stubby and sturdy, rather than long and thin.
Given all of the ways to lose energy, you may want to make a top
that can store the most energy. A heavy top with mass concentrated far
away from the center will have more energy when spinning than a light
top (it will also push down harder on the tip).
(republished on 07/13/06)