To drive a typical car at 40 mph on a level road for one hour requires about 3.2

Question

To drive a typical car at 40 mph on a level road for one hour requires about 3.2x10^7 J of energy. Suppose one tried to store this much energy in a spinning flywheel which was then coupled to the wheels of the car. A large flywheel cannot be spun too fast or it will fracture. If one used a flywheel of radius .60 m and mass 400 kg, what frequency of revolution would be required to store 2.0x10^7 J? Incidentally, 2500 rpm is about the maximum feasible rate of revolution with present materials technology for such a flywheel.

A)455 Hz
B)227 Hz
C)66.7 Hz
D)119 Hz
E) 688 Hz

Explanation / Answer

radius R = 0.60m mass = 400kg E = 2x107J Max rate of revolution = 2500 rpm The Rotational Kinetic energy E = 1/2I?2 = ( 1/2)*(1/2)mR2 *( 2pf )2 Orf = ( E / p2 MR2)1/2 Substituting the given values, we get f = 119Hz Therefore, the answer is (e)

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