Viscosity is the property by which liquids?

Oppose the relative motion between their own internal layers. Viscosity describes a liquid's internal friction — its resistance to relative motion between its own internal layers. Read more: Viscosity and Bernoulli's Principle.

Viscosity and Bernoulli’s Principle

Stand near a speeding train, and you feel pulled toward it. Fast-moving air simply pushes back less. This article covers viscosity and Bernoulli’s principle.

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GenSci0020 · General Science

Viscosity & Bernoulli’s Principle

How fluids resist motion, and how speed and pressure trade off

Viscosity

Internal frictionResists layers sliding past each other

Bernoulli’s principle

Speed up, pressure downConservation of energy in flow

Streamline flow

Same steady pathEvery particle follows the last

Viscosity

Viscosity is a liquid’s resistance to relative motion between its own layers.
It decreases as temperature rises — why machine parts can jam in cold weather.
A falling sphere’s terminal velocity depends on its size and the liquid’s viscosity and density.
Engine oils are formulated to keep working across a range of temperatures.

Bernoulli’s Principle

Based on conservation of energy: faster-moving fluid exerts lower pressure.
A venturimeter and an atomiser both put this pressure-drop to work.
Standing close to a fast train, you can feel pulled toward the low-pressure air rushing past.
Explains aircraft lift, and the design of carburettors and spray bottles.

The Same Effect, A Real Hazard

Two cars passing at high speed risk swerving toward each other — the same low-pressure effect that pulls a bystander toward a passing train.

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📑 Contents

1Viscosity, Bernoulli’s Principle, and Streamline Flow
2The Train Effect and Terminal Velocity
3Test Yourself
4Fluids in Practice
5Exam Point of View
6Related Topics

✊ Must Know

Viscosity, Bernoulli’s Principle, and Streamline Flow

ViscosityViscosity is the property by which a liquid resists relative motion between its layers, like an internal friction.
Viscosity and TemperatureA liquid’s viscosity generally decreases as temperature rises, which is why machine parts can jam in cold winter weather.
Bernoulli’s PrincipleBernoulli’s principle is based on the conservation of energy in a flowing fluid. Faster-moving fluid exerts lower pressure.
Venturimeters and AtomisersA venturimeter measures fluid flow rate. An atomiser sprays a fine mist. Both work using Bernoulli’s principle.
Streamline FlowIn streamline flow, any particle passing through a fixed point always has the same velocity. That velocity matches every earlier particle that passed through the same spot.

📘 Good to Know

The Train Effect and Terminal Velocity

The Train EffectA person standing close to a fast-moving train can feel pulled toward it, since the air rushing past has lower pressure than the still air behind them.
Terminal VelocityThe terminal velocity of a small sphere falling through a viscous liquid depends on the sphere’s size, and the liquid’s viscosity and density.
Two Cars, Same EffectTwo cars passing at high speed risk swerving toward each other, for the same Bernoulli-principle reason as the train platform effect.
Surface TensionSurface tension, a related liquid property, also decreases as temperature rises — heat gives molecules enough energy to overcome the attraction that holds a liquid’s surface together.

Test Yourself

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🌟 Great to Know

Fluids in Practice

Bernoulli in EngineeringBernoulli’s principle explains a wide range of everyday and engineering phenomena, from aircraft lift to the design of carburettors and spray bottles.
Engine Oil DesignViscosity’s temperature dependence is a practical engineering concern, which is why engine oils are formulated to perform across a range of temperatures.
Streamline vs. TurbulentUnderstanding streamline versus turbulent flow helps explain why smooth, aerodynamic shapes reduce drag in vehicles and aircraft.

📝 Exam Point of View

Exam Point of View

NDA-NA-2 2025, Q66Surface tension decreases as temperature increases, the same direction as viscosity — not the reverse, and not unchanged. View this question on the full NDA-NA-2 2025 paper →
NDA-NA-II 2016, Q103Streamline flow means velocity is constant at a fixed POSITION over time, not that any single particle’s own speed never changes as it moves. View this question on the full NDA-NA-II 2016 paper →
Exam Wisdom: Position vs. ParticleWatch for questions that swap “constant at a position” with “constant for a particle” — streamline flow only guarantees the former.
Exam Wisdom: Temperature DirectionViscosity and surface tension both fall as temperature rises — a useful pattern to recall when a question tests either property indirectly.

GenSci0019 — Pressure and Fluid Statics — the still-fluid counterpart to this article’s moving-fluid concepts.
GenSci0018 — Elasticity — another materials-response topic, covering solids instead of fluids.
NDA-NA-II 2016 GA Paper — contains the streamline-flow PYQ cited above.
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