Re: Explain the dynamics of the 'Dynabee'

Jack:

Your question is:

"A Dynabee is a sphere with a gyroscope inside. It spins freely on its axis like an ordinary gyro. However the points of the axis are inside another circular component free to rotate inside the sphere (with somewhat more friction than the inner gyro). Once spinning, this device accelerates when oscillated. There is great resistance to this motion, but the gyro accelerates faster and faster. I can get over 10,000 rpm with no connection to the inner rotating Gyroscope. These are sold in sporting goods stores."

I know about the Dynabee thoroughly because I have 3 of them and have had 2 of them apart for various reasons (Most often when I drop them while trying to make them go faster and faster, and they sometimes break apart at the seam when they hit the floor!! As you probably know, they can be glued easily.). Being the curious physicist that I am, I have myself often pondered how they work, and I think I have figured them out.

The circular component that you describe in your question ("free to rotate inside the sphere") appears to be unnecessary for the operation of the Dynabee. In fact, the 2 units that I have had apart do not have the "circular component" anymore because I removed them, and the Dynabees still work as well as ever! I suppose they have a purpose, but I have not been able to figure out what they are for. I have wondered whether the "circular component" is supposed to sweep dust and debris out of the groove, or keep debris out.

But the important part of the Dynabee is the ends of the shaft. The ends of the shaft that goes through the center of the gyroscope part of the Dynabee are constrained by a groove in the outer "sphere". As you may recall from some science or physics course you may have had, a spinning object such as a gyroscope "resists" changes in direction. Physicists describe the spinning object as having angular momentum, and just like linear momemtum, angular momentum is a conserved quantity. The angular momentum vector of a spinning object (see angular momentum) points along its spin axis, and due to complicated reasons having to do with the vector nature of angular momentum, if you try to tip the end of the spin axis it will move at right angles to the direction you are trying to tip it.

In the case of the Dynabee the ends of the spin axis are directed around the circumference of the groove as you move your wrist, and so the gyroscopic action described in the above paragraph forces the ends of the shaft to be pressed against the upper and lower sides of the groove. With this friction and the fact that the gyroscope is spinning, the shaft ends roll inside the groove. Your wrist action accelerates the turning and rolling of the shaft, so the spin rate of the gyroscope also increases. As it is rolling the shaft ends continue to change direction, and so the gyroscopic action continues to force the shaft ends against the sides of the groove. It is a situation in which the one action increases the other, and so on. I'm sure you have noticed that it is easier to keep the Dynabee going once it reaches a sufficient spin rate. This is because the gyroscopic action increases as the spin rate (and angular momentum) increases, which increases the frictional force on the sides of the groove, which makes it easier to make the gyroscope increase speed as the shaft rolls on the groove, etc.

There is also at least one previously answered question from this site which talks about gyroscopes in general. You will find it at previous. Also try searching on our search engine for other answers dealing with gyroscopes.

I hope this helps!

John Link, MadSci Physicist