What is the difference between a regular spin top and a forever spin top (which spins for far longer than a regular spin top).? The forever spin top is made out of stainless steel.
I would like an answer from a physics point of view.
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3$\begingroup$ Article about The ‘Forever Spin’ Top. $\endgroup$– FarcherAug 31, 2019 at 6:31
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2$\begingroup$ @Farcher I can’t get past the paywall. What is the gist of it? At first glance it seems like angular momentum should decrease with time due to friction regardless of what the motor/counterweight does. $\endgroup$– Ben51Sep 1, 2019 at 1:03
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1 Answer
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[LATER EDIT]
PM 2Ring has pointed out that the foreverspin top and the LIMBO top (by Fearless Toys) are not the same top; different manufacturers.
If I remember correctly: in the category non-driven top the world record is somewhere in the region of 20 minutes. On the foreverspin top website the manufacturer writes that the foreverspin top will spin several minutes. That is, as I understand it the foreverspin top spin time is not longer than that of other tops in the same price segment.
[END OF LATER EDIT]
The instagram page of the fearless toy company shows a picture with an exploded-view image of the top
The forever spin top is hollow inside and contains:
- an accelerometer and a processor
- a battery
- a motor
Clearly, energy lost to friction is replenished by the motor.
I assume that in order to sustain a particular spin rate of the top the rotation of the motor must be tuned precisely and that is why there is electronics to measure the current acceleration and a processor to adjust the motor speed accordingly.
Attached to the motor is a weight that is all on one side. That is, the shape of the weight is a half-circle, putting all of the weight on one side. So: by design the center of mass of the weight is away from the axis of rotation of the top as a whole. Presumably that off-center position of the weight allows the motor to replenish the energy lost to spinning friction.
About the energy replenishment mechanism:
I assume that the internal moving weight is positioned at the same height as the height of the center-of-mass of the top as a whole. That way the internal motion of the weight will not cause a wobble. At the same time internal motion of the weight will make the external of the top trace out a (very small) circle. This small circle may well be unnoticable with the naked eye.
If the surface that the top is resting on would be perfectly frictionless that small circular motion would still not be of any help. But if you get the amount of friction just right it would. You want low enough friction so that spinning friction is low, but sufficient so that the contact patch isn't completely sliding.
That is the only mechanism that I can think of that would allow the motor to pump the top as a whole.
On the kickstarter page the manufacturer recommends that the buyer also buy the base that they manufacture:
Fearless toys writes:
"The base was designed specifically for LIMBO, with a surface that has just the right texture and right amount of friction"
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$\begingroup$ That's a Limbo top. The ForeverSpin top has no motor, it's a traditional top. $\endgroup$– PM 2RingSep 1, 2019 at 11:36