Picture a fifty-foot-diameter concrete barrel rolling through downtown LA

Brian has a question about storing flywheel power:

You can't just hook up a bunch of NiCds to a charger and expect to do anything useful with them. But what's wrong with setting up giant, house-sized flywheels made of concrete? You could spin them up using the power produced during the off-peak hours, and the cities could drain from them during peak hours. It would even out the supply side (like a capacitor) and reduce the need for power plants to have to respond quickly to fluctuations in demand. And it would probably reduce by at least half the amount of power generation infrastructure we'd need to have running, as more and more of these flywheels could be built to store up power in little unobtrusive buildings that could even be dressed up to look like cute little houses with white picket fences and everything. And just think—when Jehovah's Witnesses come to the door, they'd be answered by a GIANT SPINNING PIECE OF CONCRETE. Converting that kind of power would be really fun to watch.

And Steven Den Beste shows up with the madd math skilz:

Just to pull a number out of my ear, let's assume that the rim mass of the wheel is 50 metric tons, or 50,000 kilograms. The formula for kinetic energy is:

e = 1/2*m*v^2

v = sqrt((2*e)/m)

So the rim velocity turns out to be 23.7 km/s. That's 63 times the speed of sound. It's also twice the escape velocity of the earth. That's really cooking.

Pulling another number out of my ear, let's assume that the radius of the flywheel is 10 meters. Then the circumference is about 63 meters, which means the flywheel would rotate 375 times per second. What kind of bearing can spin that fast, for hours (or weeks) at a time with negligible energy loss, supporting that much weight, without failing? I don't think anyone knows how to design such a thing.