Rattleback
A rattleback is a type of spinning top that prefers to spin in one direction due to its unique semi-ellipsoidal shape. When spun in the wrong direction, it will automatically shift the momentum until it spins the opposite direction. This is caused by an asymmetrical mass distribution and the instability between the rotational axes. As the mass shifts, the rolling axis compensates in the other direction, which in turn causes the mass to shift more. This continues until the top is spinning in the preferred direction. But when spun in the “right” direction, the mass needs no compensation.
Challenge a know-it-all to explain the physics behind these simple shapes.
Dimensions: 3 ⅞" x ¾" x ½"
RATTLEBACK
A rattleback is a semi-ellipsoidal top which will rotate on its axis in a preferred direction. If spun in the opposite direction, it becomes unstable, "rattles" to a stop and reverses its spin to the preferred direction. This spin-reversal appears to violate the law of the conservation of angular momentum. Moreover, for most rattlebacks the motion will happen when the rattleback is spun in one direction, but not when spun in the other. Some exceptional rattlebacks will reverse when spun in either direction. This makes the rattleback a physical curiosity that has excited human imagination since prehistoric times.
The spin-reversal motion follows from the growth of instabilities on the other rotation axes, that are rolling (on the main axis) and pitching (on the crosswise axis). File:Spoon Celt.webm Rattleback made with spoon exhibiting multiple spin reversals. Original video and idea by Prof. Christian Ucke. When there is an asymmetry in the mass distribution with respect to the plane formed by the pitching and the vertical axes, a coupling of these two instabilities arises; one can imagine how the asymmetry in mass will deviate the rattleback when pitching, which will create some rolling. The amplified mode will differ depending on the spin direction, which explains the rattleback's asymmetrical behavior. Depending on whether it is rather a pitching or rolling instability that dominates, the growth rate will be very high or quite low.
This explains why, due to friction, most rattlebacks appear to exhibit spin-reversal motion only when spun in the pitching-unstable direction, also known as the strong reversal direction. When the rattleback is spun in the "stable direction", also known as the weak reversal direction, friction and damping often slow the rattleback to a stop before the rolling instability has time to fully build. Some rattlebacks, however, exhibit "unstable behavior" when spun in either direction, and incur several successive spin reversals per spin.