"A kinetic energy recovery system (KERS) is "a system that is designed to recover kinetic energy from the car during braking, store that energy and make it available to propel the car". Such a system is basically an efficient CVT gearbox joined to a flywheel that rotates when the cars undergo braking. Such systems are being developed for Formula One and they have been included in the FIA regulations in December 2006."
This was from Wikipedia, and the concept is very simple, it uses the conservation of angular momentum, since for a rotating wheel you have the energy:
E=1/2 I w^2
where E is the kinetic rotational energy, I is the moment of inertia, w is the angular momentum.
With this, you can upon breaking star rotating such a frictionless wheel and save the energy, and then restore it back ;)
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