What is the Mass of the Uniform Disk Used as a Cam on a Vertical Shaft?

[Soaring Crane]

A uniform disk is attached at the rim to a vertical shaft and is used as a cam. The disc has a diameter of 38 cm. The moment of inertia of the disk about the axis of the shaft is 7.5*10^-3 kg*m/s. The shaft rotates uniformly about its axis at 70 rpm. The mass of the disc is closest to:

a.0.12 kg
b. 0.092 kg
c. 0.14 kg
d. 0.18
e.0.16 kg

I don’t know why the angular velocity is given. This is what I managed to do:

Using parallel axis theorem,

I_p = I_cm + M*d^2 = 0.5*M*r^2 + M*r^2 = 1.5*M*r^2

M = (I_p)/(1.5*r^2) = (0.0075 kg*m/s)/(1.5*.19 cm^2) = 0.1385 kg

Thanks.

 

[Soaring Crane]

Is this the right track?

Thanks again.

 

[Doc Al]

Looks good to me.