Certain stones, or toys, can change the direction of their spin without being touched. They spin one way, slow down, stop, and then spin the other way. How is that physically possible? Find out.
The toys have a lot of different names. Rebellious celts, celts, rattlebacks, wobblestone, spin bar, space pet, bizzaro swirls; the list goes on. One thing they all have in common is the fact that, when spun on a table, they start rocking. Eventually, they stop spinning entirely, only rocking back and forth, and then they spin the other direction. Some will spin one way, but not another. Others will just switch off. How on earth do they do that?
It turns out, that's really hard to describe. Really. However, everyone makes clear is that the rattleback has to have two curves at the bottom, a long smooth lengthwise one, and a sharper one across the width. Mass has to unevenly distributed along these two curves. The interplay of these curves and the mass results in an uneven application of frictional force. One direction has to fight against more friction than the other. Probably the clearest way to see a rebellious celt at work is this video below:
You'll notice the video makes a point of showing that the handle of the spoon is bent so that the spoon is unevenly balanced when it is spun. Most people have experience with this. Anyone who has spun around carrying a bag or a heavy object in one hand (or on thier back), knows they feel a little extra kick of force in one direction. Since that kick is diagonal to the axis of spin, it makes the spoon move a little too fast that way, rocking up, until gravity causes it to rock back. This motion slows the spoon down.
But what causes it to move in the opposite direction? the bottom of a spoon has an uneven curve. At the 'front' it is long and smooth, but at the back, towards the handle, it's sharp and steep. In the video above, the spoon rocks diagonally, first one way and then another. If, looking from the side of the handle, the back curve to the left side of the handle is rocking, it turns to the left. If it rocks along a diagonal that includes the curve to the right side of the handle, it spins to the right. That's uneven application of force at work.
Most people have had an injured leg at one point or another. Whether its pins and needles, a sprained ankle or a full on break, one leg pushes more than another. Those people know what happens if they're not careful. They start walking crooked. The left side pushes powerfully off the ground, pushing forward and slightly to the right. The right side is weaker can't counter with a push to the left. This means the person keeps moving to the right without knowing it.
The same thing happens with the spoon. One side (say the front left) pushes back and to the right very hard. The right side relies more on its sharp curve (meaning the front of the spoon has to lift itself up higher to keep the rocking motion going) and doesn't push back as much. The force against the ground is uneven, and the spoon experiences an overall 'push' to the right.
It spins, it's uneven mass causes it to pitch and rock to the other side, and the toy 'rebels,' changing direction again.