Confusion in solution Q;A solid sphere of radius R of a material of bulk modulus B is surrounded by a liquid in a cylindrical container. A massless piston of area A floats on the surface of the liquid .Find the fractional change in the radius of the sphere (dR/R) when a mass M is placed on the piston to compress the liquid? on solving we take BULK MODULUS=Mg/A/delta V/V.....as we take V=4/3pieRcube but why we take delta V=4pieRsquare-deltaR instead of 4/3piedelRcube..........

Confusion in solution

Q;A solid sphere of radius R of a material of bulk modulus B is surrounded by a liquid in a cylindrical container. A massless piston of area A  floats on the surface of the liquid .Find the fractional change in the radius of the sphere (dR/R) when a mass M is placed on the piston to compress the liquid?

on solving we take BULK MODULUS=Mg/A/delta V/V.....as we take V=4/3pieRcube but why we take delta V=4pieRsquare-deltaR instead of 4/3piedelRcube..........