A wooden rod of a uniform cross section and of length 120 cm is hinged at the bottom of the tank which is filled with water to a height of 40 cm. In the equilibrium position, the rod makes an angle of 600 with the vertical. The centre of buoyancy is located on the rod at a distance (from the hinge) of
OP is the portion of the rod immersed in water.
The centre of buoyancy is at the centre of the immersed part of the rod. So, the required distance is 40 cm.