Two point masses A of mass M and B of mass 4M are fixed at the ends of a rod of length l and of negligible mass. The rod is set rotating about an axis perpendicular to its length with a uniform angular speed. The work required for rotating the rod will be minimum when the distance of axis of rotation from the mass A is at
45l
W=12Iω2
Let x is the distance of CM from A
I=Mx2+4M(l−x)2
And if I is minimum, W will be minimum
∴dIdx=2Mx+4M×2(l−x)×(−1)
=2Mx−8M(l−x)
dIdx=10Mx−8Ml
∴10Mx−8Ml=0
x=45l
Which is the position of center mass of this system.