1)

Seven identical circular planar discs, each of mass M and radius R are welded symmetrically as show in figure. The moment of inertia of the arrangement about the axis normal to the plane and passing through the point P is

382019798_ammmm.JPG


A) 192MR2

B) 552MR2

C) 732MR2

D) 1812MR2

Answer:

Option D

Explanation:

First we found moment of inertia (MI) of system using parallel axis theorem about centre of mass, then we use it to find moment of inertia about given axis.

Moment of inertia of an outer disc about the axis through centre is

MR22+M(2K)2=MR2(4+12)=92MR2

   38201919_idea.JPG

For 6 such discs,

  Moment of inertia = 6×92MR2=27MR2

 So, moment of inertia of system

           = MR2227MR2=552MR2

Hence,

   Ip=552MR2+(7M×9R2)

    Ip=1812MR2

    Isystem=1812MR2