A vessel filled with water is kept on a weighing pan and the scale adjusted of zero. A block of mass M and density `rho` is suspended by a masselss spring of spring constant k. This block is submerged inside into the water in the vessel. What is the reading of the scale /

Explanation

When the block, suspended by the massless spring, is submerged into the water, then the upthrust exerted by the water on the block will compress the spring (upwards).
At equilibrium, all the forces acting on the block will sum upto zero. So, weight of the block will be equal to the sum of spring force and upthrust due to water. `Mg+kx=Vp_(w)g`
where x=compression produced in the spring
V=Volume of block
`p_(w)="density of water"`
Reading on the scale (Mad of yesael + mass of water inside vessel + mass of water displaced by the block) g
`=m_(v)+m_(w)+Vp_(w)g`
The scale was adjusted to zero when the vessel was filled with water and was kept on the weighing pan. So, the final reading on the scale when the block is submerged inside the vessel is of only the water displaced
`Vp_(w)g=(M/p)p_(w)g=(p_(w))/(p) Mg`