A square frame ABCD is formed by four identical rods each of mass ‘m’ and length ‘T’. This frame is in X – Y plane such that side AB coincides with X – axis and side AD along Y – axis. The moment of inertia of the frame about X – axis is
A square frame ABCD is formed by four identical rods each of mass ‘m’ and length ‘T’. This frame is in X – Y plane such that side AB coincides with X – axis and side AD along Y – axis. The moment of inertia of the frame about X – axis is
  1. 2ml2/3
  2. 4ml2/3
  3. 5ml2/3
  4. ml2/12

 Solution: The correct answer is 

The inertia moments of rods AD and BC are as follows:  \frac{1}{3}m{{T}^{2}}

Because rod AB is on the X-axis, its moment of inertia is zero, and so r=0.
Because every element in rod CD is at a distance T from the X-axis, its moment of inertia is as follows: m{{T}^{2}}.

Hence, I=2\times \frac{1}{3}m{{T}^{2}}\times m{{T}^{2}}

=> I=\frac{5}{3}m{{T}^{2}}