The ancient rattleback spinner toy has excited human imagination since prehistoric times
Spin the Rattleback Spinner Toy clockwise and watch it stop, rattle up and down and then spin in the other direction! A scientific curiosity whose ancient and mysterious properties capture the attention of all ages. It also magnifies when positioned on its flat side.
If you’ve never seen a rattleback spinner before, prepare to have your mind blown. A rattleback is a sort of top that will only spin in one direction. Sound impossible? Spin the rattleback counter-clockwise and it spins effortless on the peak of its bent back. But try to spin the rattleback clockwise and it will steadfast refuse, spinning only a few times, stopping, rattling for a second or two, then begin spinning in the opposite direction.
The rattleback spinner toy has an unusual shape – flat on top and an asymmetrical ellipsoidal bottom. This is what is believed to cause its curious spinning action. Its motion has been studied by many scientists, but the reason behind its unusual behavior remains elusive. The simple explanation is that the long axis of the ellipsoid is not parallel to the long axis of the flat top – hence its predisposition to spinning in one direction. Some exceptional rattlebacks however will mysteriously reverse when spun in either direction. Maybe yours will meet this baffling criteria?
Rattleback spinners have been around for thousands of years. In that time, they’ve gone by many names including anagyre, celt, Celtic stone, rebellious celt, rattlerock, spin bar, wobble stone and wobblestone. The first rattleback spinners toys were made from wood. This modern Rattleback Spinner Toy is made from brightly colored, durable acrylic resin.
How the Rattleback Spinner Toy works – the physics behind the rattleback
When you spin a rattbleback one way, it turns a few times before the ends start to rattle up and down. The more it wobbles, the slower it rotates – until it stops spinning altogether. Finally, it starts to spin in the opposite direction. What could possibly cause this?
The first attempt to analyze rattlebacks was around a century ago. In the mid-1980s, two detailed mathematical analyses were done: one by Hermann Bondi (then Master of Churchill College, Cambridge) and the other by Mont Hubbard (Professor of Mechanical Engineering at the University of California, Davis). Bondi and Hubbard agreed that the rattleback’s amazing behavior needs three main ingredients.
- First, the curved base must have two different radii — one long radius for the lengthwise curve and one shorter radius for the tighter curve across its width.
- Next, the symmetry axes of the rattleback must be twisted slightly from its principal axes of inertia. Any rigid object has three principal axes of inertia. They sit at right angles to each other and if you spin the object about one of them, there is no tendency to rotate about the other two.
- Finally, there must be a different distribution of mass about each of the two horizontal axes of inertia — a long, thin shape, say.
Given these characteristics, the mathematics of mechanics predicts how the rattleback spinner toy should behave. However, to understand how the rattleback works in physical terms is nearly impossible. Even Hubbard says,
“It’s only clear through the equations. I don’t intuitively understand it.”
Still we can glean a little bit about its physics by the rattleback when it’s halfway through its run – the point where it begins rocking back and forth. Friction acts horizontally (at the point of contact between the rattleback and the tabletop) to prevent the rattleback from slipping. One part of this frictional force creates a torque that tends to rotate the rattleback about its vertical axis. Since friction always acts in opposition to the motion, this part of the frictional force acts in the direction opposite to the spin direction the rattleback had. That is, it rotates the rattleback in the opposite direction to the way it started.
However, to make it more complicated, the point of contact is moving all the time and the torque changes. If the inertial and symmetry axes of the rattleback spinner toy were the same, the average torque over a single oscillation would be zero. But for the rattleback, there is a net torque in one direction. And it is this that reverses the angular momentum.
Another way to understand how the rattleback works is in terms of energy. Each direction of spin is linked to a different mode of oscillation: if clockwise rotation feeds the pitching oscillation, then counter-clockwise spin would feed a side-to-side, or rolling, oscillation. So, when the rattleback spinner toy spins clockwise, any tiny pitching oscillation will grow exponentially. It feeds off the rotational energy which slows down the spin. But even when there is zero spin, the torque still acts. So the direction of spin changes. Now beginning to spin counter-clockwise, the energy stored in the pitching oscillation feeds into the counter-clockwise rotation (and the rolling oscillation).