Question

Which of the following is not an application of Archimedes principle?
(A) Determining the relative density of a substance
(B) In hydrometers
(C) In lactometers
(D) None

Answer 1

Archimedes’ principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the centre of mass of the displaced fluid. A hydrometer is an instrument used for measuring the relative density of liquids.

Complete Step by Step Solution:
Let us consider the various options given in the questions and check whether Archimedes’ Principle is applicable to them.
Relative density is the ratio of the density of a substance to that of a standard substance. Archimedes’ Principle can be used in calculating the density or specific gravity of an object. For an object denser than water, the object can be weighed in air and then weighed when submerged in water. When the object is submerged, it weighs less because of the buoyant force pushing upward. The object’s specific gravity is then the object’s weight in air divided by how much weight the object loses when placed in water.
So Option A is eliminated.
A hydrometer is an instrument used for measuring the relative density of liquids. Hydrometer consists of lead shots which makes them float vertically on the liquid. The lower the hydrometer sinks, the lesser is the density of the liquid. Hence, we can eliminate Option B as well.
Lactometer is a device used to check the purity of milk. It also works on the Archimedes' Principle that a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced. If the milk sample is pure, then the lactometer floats on it and if it is adulterated or impure, then the lactometer sinks.

Hence the correct answer is option D.

Note: Archimedes’ principle, physical law of buoyancy, discovered by the ancient Greek mathematician and inventor Archimedes, stating that anybody completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force, the magnitude of which is equal to the weight of the fluid displaced by the body. The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface for an object partially submerged in a liquid. The weight of the displaced portion of the fluid is equivalent to the magnitude of the buoyant force. The buoyant force on a body floating in a liquid or gas is also equivalent in magnitude to the weight of the floating object and is opposite in direction; the object neither rises nor sinks.