Explain why displacement method is unsuitable for determining the volume of solids

Displacement method of volume measurement is no suitable for the objects that do not get immersed into the water completely because of the hindrance in accuracy of the measurement.
Explanation:
When we measure the volume of a solid by water displacement method then we expect that the solid must get immersed completely below the free surface of the water which leads to the displacement of the volume of the water equal to the volume of the immersed body.
But when the object does not sinks below the surface of the water as in case of wood, coal, ice etc. then we have to tie a dense heavy object to the floating body to make it immerse under the water surface. In this case the volume of the denser object must be object must be known while we ignore the volume occupied by the string which leads to a marginal error in the volume measurement.

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