SSC
The loss in weight of a body immersed in a fluid is equal to the weight of the fluid displaced. This is known as:
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Answer: Option B — Archimedes' principle
Detailed Solution
Archimedes' principle states that when a body is wholly or partially immersed in a fluid (a liquid or a gas), it experiences an upward force equal to the weight of the fluid displaced by it. This upward force is called buoyant force or upthrust. Because of this force, the body appears to lose some of its weight in the fluid, and the apparent loss in weight is equal to the weight of the fluid displaced.
The principle was discovered by the Greek mathematician and scientist Archimedes of Syracuse (about 287-212 BCE). According to a famous story, King Hiero II asked Archimedes to find out whether his new crown was made of pure gold or mixed with silver, without damaging it. While stepping into a bath, Archimedes noticed that the water level rose, and realised that he could compare the volume of water displaced by the crown with that displaced by an equal weight of pure gold. He is said to have run through the streets shouting "Eureka!" ("I have found it!").
Law of flotation (a consequence of Archimedes' principle):
- If the weight of a body is greater than the buoyant force, it sinks.
- If the weight equals the buoyant force, it floats (partly or fully submerged).
- A body floats if its density is less than that of the fluid, and sinks if its density is greater.
Applications:
Ships made of steel float because their hollow shape displaces a large volume of water, whose weight equals the ship's weight. A ship floats lower in fresh (river) water than in denser sea water.
Submarines dive and rise by filling their ballast tanks with water or air.
Hot-air balloons and hydrogen balloons rise because they displace air heavier than themselves.
A hydrometer, based on this principle, measures the density (relative density) of liquids, such as milk (lactometer) and battery acid.
Icebergs float with about 90% of their volume below water, because ice is slightly less dense than sea water.
Why not the other options?
- ✗Pascal's law deals with the transmission of pressure in a confined fluid.
- ✗Newton's third law states that every action has an equal and opposite reaction.
- ✗Hooke's law states that stress is proportional to strain within the elastic limit.
Quick tip
Remember: Archimedes' principle = upthrust equals the weight of fluid displaced ("Eureka", the gold crown); explains floating ships, submarines, balloons and hydrometers/lactometers.
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Properties of Matter & Fluids — Preparation
This question tests your understanding of Properties of Matter & Fluids, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the properties of solids and fluids: elasticity, pressure and its applications, Pascal's law, Archimedes' principle and buoyancy, floating and sinking, density, surface tension, capillary action, viscosity and Bernoulli's principle. Exams test everyday examples: a hydraulic lift works on Pascal's law, ships float because of Archimedes' principle, raindrops are spherical because of surface tension, and kerosene rises in a lamp wick by capillary action. Also know why it is easier to swim in sea water (greater density and buoyancy). Practise the properties of matter questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.