two uniform solid cylinders, each rotating about its central (longitudinal) axis, have the same mass of 1.25 kig and rotate with the same angular speed of 235 rad/s, but they differ in radus. what is the rotational kinetic energy of a) the smaller cylinder, of raidus 0.25 m, and b) the larger cylinder, of radius 0.75?



i started part a..
K=(1/2)Iw^2 [w is omega]
I= sum (mr^2)
i don't know how you do the sum or whatever..
i just put in the mass they gave me and did
K=(1/2)(1.25)(0.25^2)(2.35)^2
and i got 4,314.45 J
the correct answer from the back is 1.1 kJ
help?

sfij29

nevermind.
figured it out.
i forgot to note that because of the shape and how it was rotating, (1/2)mr^2 could be used for I.