Rotational Kinetic Energy of a flywheel

[Djbari]

Problem: An experimental flywheel, used to store energy and replace an automobile engine, is a solid disk of mass 200.0 kg and radius 0.40 m. (a) What is it's rotational inertia? (my calculation says 16 kg.m) (b) When driving at 22.4 m/s (50 mph), the fully energized flywheel is rotating at an angular speed of 3160 rads/s. What is the initial rotational kinetic energy of the flywheel?

My thinking was to find the KE (rotational) = 1/2 I w^2 but the number is huge.
1/2 (16)(3160). Am I on the right track? Thank you.

 

[alphysicist]

Hi Djbari,

Problem: An experimental flywheel, used to store energy and replace an automobile engine, is a solid disk of mass 200.0 kg and radius 0.40 m. (a) What is it's rotational inertia? (my calculation says 16 kg.m) (b) When driving at 22.4 m/s (50 mph), the fully energized flywheel is rotating at an angular speed of 3160 rads/s. What is the initial rotational kinetic energy of the flywheel?

My thinking was to find the KE (rotational) = 1/2 I w^2 but the number is huge.
1/2 (16)(3160). Am I on the right track? Thank you.

What did you get? If that's really what you multiplied together ( 1/2 (16)(3160) ) then you answer would be too small, since you did not square the angular speed.

 

[Djbari]

I did have the 3160 squared and got 79,884,800...??

 

[alphysicist]

I did have the 3160 squared and got 79,884,800...??

If they want the kinetic energy when it is spinning at 3160 rad/second, then that sounds right to me. Remember this flywheel has the mass of several people, spread out over a diameter of .8m, and is spinning at about 500 revolutions per second.

 

[Djbari]

ok. I had no clue. Thank you for explaining. I will find out the solution tonite in class.