Question

A car is designed to get its energy from a rotating flywheel (solid disk) with a radius of 1.50 m and a mass of 600 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 4,000rev/min.

(a) Find the kinetic energy stored in the flywheel.

(b) If the flywheel is to supply energy to the car as a 15.0-hp motor would, find the length of time the car could run before the flywheel would have to be brought back up to speed.

Answer 1

a)

kinetic energy stored in the fly wheel is

         K.E = 1/2 I?²     .=1/2((1/2) MR²) w^2

                                = (1/2)(1/2) (600 kg)(1.5 m)²(4000 rev / min) ( 2 pi/ 60 rad))^2

                       =59157600 J

(b)

power P = 15 hp

             = 746*15 W

             = 11190 W

power P = energy/time

therefore ,

             time t = energy/power

                      = 59157600 J/ 11190 W

                      = 5286.64s( 1h/ 3600 s)

                     =1.46 hr