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rajan jha Grade: 12
        

A horizontal disc rotating freely about a vertical axis makes 100rpm.A small piece of wax of mass 10kg falls vertically on the disc and adheres to it at a distance of 9cm from the axis.If the number of revolutions per minute is thereby reduced to 90.Calculate the moment of inertia of disc.

6 years ago

Answers : (1)

vikas askiitian expert
510 Points
										

taking wax & disk as a system ....


then there is no external torque acting on this system so angular momentam will remain conserved..


IW = IfWf              ..............1


finally moment of inertia = I + Iwax


Iwax = mr2 =  10(9*10-2)2                                         (r = 9cm = 9*10-2m  &   m =10kg )


                = 0.081 kg-m2


If = I + 0.081           ..........2


putting 2 in 1


 IW = (I+0.081)Wf


 W/Wf = (1+.081/I)                                 {W = 100rpm &  Wf=90rpm (given)}


 100/90 = 1 + 0.081/I


  I = 0.729kg-m2

6 years ago
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