Find mass of uniform solid disk?

[saturn67]

you need to design an industrial turntable that is 48.0 cm in diameter and has a kinetic energy of 0.300 J when turning at 31.0 rpm

i found that inertia of the turntable about the rotation axis
is = 5.71×10^-2 kg m^2

If your workshop makes this turntable in the shape of a uniform solid disk, what must be its mass?

it try to use I=mr^2
so mass = I/r^2

but that don't work

 

[Astronuc]

Try I = 1/2 mr²

http://hyperphysics.phy-astr.gsu.edu/hbase/icyl.html#icyl2

Also make sure that one uses r instead of d=0.48 m.

 

[saturn67]

Woot it work thank you