# The Naked Scientists Forum

### Author Topic: Why can't I make a black hole flywheel?  (Read 2448 times)

#### Geezer

• Neilep Level Member
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##### Why can't I make a black hole flywheel?
« on: 10/11/2009 07:07:34 »
There is a lot of energy available on Earth that could be used if we had a convenient means of storing it. Why can't we build flywheels from super-dense material (black holes might be the ultimate). They would not take up a lot of space, so we could house them in small, near vacuum conditions. Their mass might be a bit tricky for mobile applications of course.

#### Luther

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##### Why can't I make a black hole flywheel?
« Reply #1 on: 17/03/2010 15:48:59 »
The energy stored in a flywheel is proportional to the mass, the square of the radius, and the square of the rotational speed.  To store more energy in a flywheel you don't need to make it more dense, just spin it faster, so making it out of a black hole provides no immediate advantage.  Flywheels are difficult to use for large-scale energy storage for engineering reasons: they lose energy through friction and drag (though if you can get a true vacuum that can be overcome); gyroscopic forces wrench them about if you move them; and the more energy you put into them the more centrifugal forces try to rip them apart.  If any part of them does fail all their stored energy immediately becomes linear kinetic energy, causing high-density matter to burst outward at high speeds, which can be deadly to anyone nearby.

The safe upper limit to energy stored in a flywheel is based on the binding energy of the wheel itself: exceed this and the wheel explodes.  Hence the real question is how strong a material can use to construct the wheel, rather than its density.

Another problem with black holes is we can't get energy out of them; an axle is clearly out of the question as it would be torn apart by the gravity of the hole, and electromagnetic waves can't escape its gravitational field either.

#### Atomic-S

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##### Why can't I make a black hole flywheel?
« Reply #2 on: 25/03/2010 05:58:13 »
Rats; another good idea down the drain.

#### The Naked Scientists Forum

##### Why can't I make a black hole flywheel?
« Reply #2 on: 25/03/2010 05:58:13 »