PROBLEM 9.108 For a given area the moments of inertia with respect to two rectangular centroidal xand yaxes are xI=640 4in and yI=280 4in , respectively. Knowing that after rotating the xand yaxes about the centroid 60° clockwise the product of inertia relative to the rotated axes is −180 4in , use Mohr’s circle to determine (a) the orientation of the principal axes, (b) the centroidal principal moments of inertia. SOLUTION Have ()( )444ave11640 in280 in460 in22xyIII=+=+=4640 in280 in180 in−=−=Also have 4180 in , 2120 , xyxyIθ′′=−°>Letting the points
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