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Comment by kayson | Hacker News Reader

I'm hoping someone with better mechanics knowledge than I can help me understand something that's been bothering me about this: if you have a simple circuit of the "spin battery" driving a "spin resistor", at equilibrium (when the chain is moving with constant speed) isn't the torque applied by the battery necessarily the same as the frictional torque in the resistor? Otherwise you'd still have some kind of acceleration.

For a twice larger "spin resistor", then, the equilibrium torque would be twice as large, and intuitively, the chain speed would be half as fast. So the same amount of work is being done by the "spin battery".

Doesn't this mean, then, that the "spin battery" is constant power, rather than constant voltage? Their FAQ says the unit of "spin voltage" is Newtons, and that the battery is 3 spin volts, but this doesn't agree with my understanding of the mechanics.

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I think Spintronics resistors will resist motion with a force proportional to rotational speed, which I think follows from the resistance coming from oil in the bearings. So the torque will be the same for both resistors (since the battery applies constant torque), but the speed at which the resistors apply this torque will be different by a factor of two.

So you're saying the bearing friction is twice as large in the bigger resistor but the speed is half, resulting the same torque? This seems like the simplest explanation, and what my mind went to first, but I couldn't find any resources that describe this behavior mathematically.

I'm also curious what defines the steady state speed.

> For a twice larger "spin resistor", then, the equilibrium torque would be twice as large, and intuitively, the chain speed would be half as fast.

For the twice-larger resistor, the torque is the same, and the speed is half as fast. Can you explain why you think the torque is twice as large?

To move the 2x resistor at the same speed as the 1x resistor, you need twice the torque.

Maybe my misunderstanding is how the speed would be independent of torque.

My thought was that the 2x resistor has 2x the coefficient of kinetic friction, which means 2x force and 2x frictional torque. In order for the system to be at equilibrium, meaning constant speed, the battery torque must match the friction torque.

I think the chain just wouldn't ever move at constant speed. It would be slowing the whole time as the energy stored in the battery is spent in the resistor.

That's a good point, but I think for an approximation, we can assume its constant. It gets up to speed very fast and then stops equally fast. Presumably this is because of the constant-torque spring they're using? In any case, by observation it does seem that for most of the time, the speed isn't changing.

Yea

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