SSC
'The total energy per unit volume of a flowing ideal fluid remains constant along a streamline.' This is:
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Answer: Option A — Bernoulli's principle
Detailed Solution
Bernoulli's principle was stated by the Swiss mathematician and physicist Daniel Bernoulli in his book "Hydrodynamica" (1738). It states that for an ideal fluid (incompressible and non-viscous) flowing steadily, the sum of the pressure energy, kinetic energy and potential energy per unit volume remains constant at every point along a streamline:
P + ½ρv² + ρgh = constant
where P is the pressure, ρ is the density of the fluid, v is its speed, g is the acceleration due to gravity and h is the height. Bernoulli's principle is an application of the law of conservation of energy to flowing fluids.
An important consequence is that where the speed of a fluid is high, its pressure is low, and where the speed is low, its pressure is high (at the same height).
Applications and examples of Bernoulli's principle:
Lift on aircraft wings: the wing (aerofoil) is shaped so that air flows faster over the curved upper surface than below it. This creates lower pressure above and higher pressure below, producing an upward lift force.
Atomisers and sprayers: fast-moving air over the top of a tube lowers the pressure, drawing liquid up and spraying it.
Roofs blown off in storms: fast winds over a roof reduce the pressure above it, while the pressure inside the house remains higher, pushing the roof upward.
Two boats or trains moving side by side close together are pulled towards each other, because the water or air between them moves faster and its pressure falls.
A spinning cricket ball or football swings or curves (the Magnus effect).
The Venturi meter, which measures the speed of flow in pipes, and the Bunsen burner and carburettor work on this principle.
When a fast train passes, people standing close to the platform edge can be pulled towards the train.
Why not the other options?
- ✗Pascal's law deals with pressure in a confined fluid at rest.
- ✗Stokes' law gives the viscous drag on a small sphere moving through a fluid.
- ✗Archimedes' principle deals with buoyancy.
Quick tip
Remember: Bernoulli's principle (1738) = faster fluid, lower pressure (P + ½ρv² + ρgh constant); explains aircraft lift, sprayers, roofs blown off in storms and the Venturi meter.
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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.