A flywheel for energy storage in a hybrid gasoline-electric car is a ring-shaped rotor of mass 49 kg, designed to rotate at 30000 rpm. At that speed it stores 10 MJ of energy (equivalent to about a cup of gasoline).
a) What is the ring's diameter?
b) If the flywheel can supply power at the rate of 25 kW, how long will it take for its stored energy to be exhausted?