[Thorondor]

The main issue is that when objects spin really, really fast, they tend to explode due to centrifugal force.

The tensile stress on a spinning round, homogeneous object is p * r^2 * w^2, where p is density, r is radius, and w is angular velocity. Using your numbers for a steel cylinder with density 8 g/cm^3 gives a tensile stress of (8 g/cm^3) * (5 mm)^2 * (2pi*1 MHz)^2 = about 8 TPa which vastly exceeds the tensile strength of steel or any other known material. Using cows connected by ropes would be even worse because the enormous centrifugal force would be borne by only a small rope.

[dexwiz]

There are some great examples on youtube of people demonstrating this with skateboard wheels and compressed air.

[sfink]

That makes sense. With (strong) steel maxxing out at around 1Gpa, that's 4 orders of magnitude less. The rotation frequency gets squared, so... 10Khz? Huh: "The fastest rotation achievable by standard motors is of the order of 10 kHz". (How does a "standard motor" achieve 10 KHz without ripping apart, then? I don't think many standard motors are 5mm or smaller.)

Thinking about this makes me imagine a potter's wheel for shaping a ductile metal. It spins really fast, but you can only reshape in the outwards direction, and the resistance goes up dramatically towards the center. Oh, and if anything flakes off, you're dead. But before you die, you could probably make some pretty artwork.

[jmb99]

I’m not an expert, but I don’t believe there are that many motors that spin at 600k RPM (= 10kHz), and definitely no large ones. That is absurdly fast. Modern turbochargers are some of the fastest-spinning off-the-shelf things your average person can buy, and they normally top out below 200k RPM (and only the smallest/lightest turbines can spin at 200k RPM). If there does exist a 500k+ RPM electric motor, I would be surprised if it was larger than a few mm.