How much sand is needed to slow the rotation of a disk?

[sskc248]

Homework Statement

a disk of radius 40 cm is rotating about an axis through its centre. the moment the inertia of the disk is 0.015 kg m^2 and it is turning at 3 rev/s. sand falls on the disk at a distance of 20 cm from the axis and builds a 20 cm radius sand ring on the disc. How much sand must fall on the disc to slow the speed of revolution to 2 rev/s? Use the law of conversation of angular momentum

Homework Equations

The Attempt at a Solution

Angular Momentum L=mvr I know the 40 cm
Moment of Inertia I=mr^2 so mass = .375
How do I approach this problem in what way? what do the rev/s have to do with this problem?

 

[dauto]

No, the moment of inertia of a disk isn't I=mr². that would be the case if all the mass of the disk was at the radius r. But you don't need to calculate the disk mass. Calculate the angular momentum of the disk.