Question

An object is suspended with a string it gets stretched.when the object is completely immersed in water, the extension of string decreases.Explain the reasons of this change.

Answer 1

Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.
let suppose a mass of object "M" when an object is suspended with a string only TENSION force (T)acted on the string which equals to T-Mg=0
but in the case when the object is completely immersed in water a buoyant force(B) also acted that is T + B -Mg = 0
or, T = Mg- B and B is equal to the weight of the fluid that the body displaces. Due to this extension of string decreases.