Rotational kinetic energy and moment of inertia

[court2011]

1. The problem statement, all variables and given/known data

A 16 g CD with a radius of 6.0 cm rotates with an angular speed of 42 rad/s.
(a) What is its kinetic energy?

2. Relevant equations

k=1/2Iw^2

3. The attempt at a solution
k=1/2(0.016x0.0036)(1764)=0.05 J

[ideasrule]

Careful. The moment of inertia of a disk is not MR^2. That's the moment of inertia for a hoop, or for a point mass.

[court2011]

ohhh so should i have 1/2mr^2

[ideasrule]

Yup.

[court2011]

kk sweet thanks! i got it right :)