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Lately i have been watching the MIT Physics Lectures from Dr. Walter Lewin. I find his passion while teaching very fascinating and inspiring. Any way, in the end of the lecture about Torque he showed a weird phenomenon which ,he admitted, baffled himself for a while.

He place a piece of plastic onto the projector's glass surface and he spined it counter clockwise and the result was as expected. The body spined till the friction (however small) stopped it. However when he spinned it clockwise the friction brought it to a halt and then its direction of rotation is reversed.

For your convenience if you haven't seen the lecture. Click here (and go to 46th minute)

I asked my physics teacher and he assumed that there is either some kind of liquid in the plastic and the body when spinned changes it's moment of inertia and that's how the motion is affected or there is some monkey business. However that doesn't seem likely to me.

So my question is how is this motion possible? And could you share with me the mathematical approach of the motion?

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  • $\begingroup$ Are you talking about this type of stuff? In the video there is an object that 'only wants to spin in one direction' as well: It's due to the mass distribution inside. $\endgroup$
    – Danu
    Mar 14, 2015 at 12:10
  • $\begingroup$ As Danu pointed out it is about the mass distribution inside the toy, which called rattle-back. I also have the same toy and I can assure that as far as I can tell there is no water or no monkeys inside the toy! $\endgroup$
    – Gonenc
    Mar 14, 2015 at 13:23
  • $\begingroup$ I could have sword we had a rattleback question already, but I can't find it. $\endgroup$ Mar 14, 2015 at 15:25
  • $\begingroup$ Yes that's exactly what i am talking about. I'll try to work on the math, but if its easy for you please share with me the mathematical approach. $\endgroup$ Mar 14, 2015 at 17:01

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This quite special top is called a rattleback, or celt. See Wikipedia : http://en.wikipedia.org/wiki/Rattleback

I quote : "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). (...) 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."

You can find some for sale or even build your own : http://www.iop.org/EJ/abstract/0143-0807/11/1/112

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I suspect this is possible in the same way as when you strike a billiard ball below its center of mass: the ball moves forward, stops, and moves back because when it stops, it still has rotation energy and tries to keep rotating, but friction opposes this rotation and causes the ball's motion back. So the plastic piece probably still has some rotation energy when it stops. Why the results depend on the sense of rotation? Probably, because of some "chiral" mass distribution.

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