Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Optimizing spin time of a top

Learn more physics!

Most recent answer: 10/22/2007
Q:
How do you make a top that spins the longest? What variable do you change and how?
- Gideon (age 13)
Massachussets
A:
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).

Tom

(published on 10/22/2007)

Follow-up on this answer.