A solid sphere of radius R and made of a material of bulk modulus K is completely immersed in a liquid in a cylindrical container. A massless piston of area A floats on the surface of the liquid. When a mass M is placed on the piston to compress the liquid, the fractional change in the radius of the sphere, is given by
Pressure exerted by the piston on the liquid when a mass M is placed on the piston, This pressure is exerted by the liquid equally in all directions. Therefore, the surface of the sphere experiences a force P per unit area. The stress on the sphere is
Now, the volume of the sphere is
Due to stress, the change in the volume of the sphere is
By definition, bulk modulus