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The second one can be mitigated for the most part by using two flywheels that rotate in opposite directions.

Right. Except the smallest synchronization error between the two flywheels becomes disaster.

Why ? They don't need to be synchronized at all, as long as they're roughly spinning at the same speeds in opposite directions there is 0 effect. The only effects you'd be left with would be from the difference between the two rotational speeds. If you stop one and the other continues that's a different matter.

Besides, keeping two wheels synchronized is not the hardest thing to do, a car contains a lot of moving, synchronized parts.

And the first is a problem with anything that stores energy. Gas can explode. Batteries can explode. Etc.

I think an accident causes gasoline to burn more often than explode. Even when the tank explodes I don't think it would be as dangerous as a fly wheel shattering with shrapnel flying everywhere.

Flywheels running at high speed are not made of cast iron or other things that turn in to 'shrapnel', they're made of wound filaments of carbon fibre bound with resin, usually they are designed with failure in mind. The bigger problem is dealing with the conversion of the energy in the wheel in to heat.

If we have a flywheel falling on a movable object on the floor I imagine this movable object can attain quite high speeds.

@jacquesm Does the heat problem confine it to providing only bursts of energy?

I meant on destruction of a running wheel, not on normal use.

Except some things are more dangerous than others.

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