Ask a question

Physics Problem

A 15-g CD with a radius of 7.5 cm rotates with an angular speed of 40 rad/s. What is its kinetic energy?

1 Answer by Expert Tutors

Tutors, sign in to answer this question.
Grigori S. | Certified Physics and Math Teacher G.S.Certified Physics and Math Teacher G.S.

Let's write whatt is given, and put "m" for mass, "R" for the radius of the disk, and "w" for the angular speed. We have:

m = 15g = 0.015 kg,  R = 0.075m, w = 40 rad/sec

If KE is the kinetic energy of a rotaional motion, then

                                                                   KE = Iw^2/2,

where  "I" stands for the moment of inertia

which is (for a disk) equal to         

                                                              I = mR^2/2.  

Thus we have, after substitution of all numbers,

                                                             KE = m(Rw)^2  = 0.0675 J.