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..........

Three identical balls are placed on a frictionless horizontal surface touching each other. They stick to each other because of adhesive. Another ball of same radius and mass m is placed over the void created by the three balls. Find the forces applied by the balls kept on the floor to each other if the system remains in equilibrium.

A car loaded with water having total mass 1000 kg moves on a straight horizontal Road starting from rest under the action of a force of 100 Newton. The water spills throw a small hole in the bottom at a rate of 0.1 KG per second the velocity of cart after 300 seconds is nearly.

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