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Two thin discs each of mass M and radius r are attached as shown in figure, to form a rigid body. The rotational inertia of this body  about an axis perpendicular to the plane of disc B and passing through its centre is :

  1. 2 M r squared
  2. 3 M r squared
  3. 4 M r squared
  4. 5 M r squared

The correct answer is: 5 M r squared


    I equals I subscript B plus I subscript A equals 1 half M r squared plus left parenthesis 1 half M r squared plus M left parenthesis 2 r right parenthesis squared right parenthesis equals 5 M r squared

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