Q.

A thin wire of length L and uniform linear mass density ρ is bent into a circular loop with centre at O as shown. The moment of inertia of the loop about the axis XX ' is

1 ρL38π2  
2 ρL316π2  
3 5ρL316π2  
4 3ρL38π2  

Ans.

(4)

About the diameter of the circular loop (ring)

I=12MR2

Using parallel axis theorem

Moment of inertia of the loop about XX' axis

Ixx'=MR22+MR2=32MR2

Here mass M=Lρ and radius R=L2π;

∴ Ixx'=32(Lρ)(L2π)2=3L3ρ8π2