A cylindrical block of wood of height 4.2m and mass 100Kg floats vertically in water. The relative density of wood is 0.8 . What height of the block will be seen above the water ?

Let the cross section area be A.

Let the height below water be H.

Density of water = 1

Then the buoyant force will be AH1 .....…(1)

The weight of wood will be =cross section area * height * density of wood=A * 4.2 * 0.8 ...….(2)

Equating 1 and 2 we get

Height below water = H = 3.36 m

So height above water will be 4.2 - 3.36 = 0.84 m