A force acting perpendicular to a surface is called?

Thrust. A force acting perpendicular to a surface is called thrust — pressure is thrust divided by the area it acts on. Read more: Pressure and Fluid Statics.

Pressure and Fluid Statics

Water boils below 100°C on a tall mountain, not because it’s colder there, but because the air presses down less. This article covers pressure and fluid statics.

MCQ Questions

General Science

Pressure and Fluid Statics

Pressure is a scalar — it acts equally in every direction at a point

Pressure

SI unit: pascal

P = F / A

Archimedes’ Principle

Buoyancy

Upward force equals the weight of fluid displaced

Pascal’s Principle

Hydraulic systems

Enclosed fluid pressure transmits equally — brakes, lifts

Atmospheric Pressure

Sea level

76 cm of mercury

📑 Contents

1Pressure, Archimedes, and Pascal’s Principle
2Atmospheric Pressure and Water’s Odd Behaviour
3Test Yourself
4Pressure in Practice
5Exam Point of View
6Related Topics

✊ Must Know

Pressure, Archimedes, and Pascal’s Principle

PressurePressure is force applied perpendicular to a surface, divided by that surface’s area. Its SI unit is the pascal. This perpendicular force is also called thrust.
Same Force, Different PressureSpreading the same force over a larger area lowers the pressure. A wider base sinks less into soft ground, and a sharp blade cuts more easily than a blunt one.
Pressure in LiquidsPressure at any point in a liquid acts equally in all directions, and increases with depth.
Archimedes’ PrincipleA body immersed in a fluid experiences an upward buoyant force equal to the weight of fluid it displaces.
Floating vs. SinkingWhether an object floats or sinks depends on comparing its density with the fluid’s density.
Pascal’s PrinciplePressure applied to an enclosed fluid transmits equally throughout it. Hydraulic brakes and lifts use this principle.

📘 Good to Know

Atmospheric Pressure and Water’s Odd Behaviour

Standard PressureStandard atmospheric pressure at sea level equals a mercury column about 76 cm high, measured with a barometer.
Water’s Density QuirkWater reaches its maximum density at 4°C, an unusual property compared to most substances.
Sea Water vs. Fresh WaterIt is easier to swim in sea water than fresh water, since seawater’s higher density provides more buoyant support.
Boiling Point, PreciselyBoiling point is the exact temperature where a liquid’s vapour pressure catches up to the surrounding atmospheric pressure — not just “when it gets hot enough.”

Test Yourself

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🌟 Great to Know

Pressure in Practice

Pressure Is a ScalarPressure is a scalar quantity, even though force is a vector, because pressure describes an effect distributed equally in every direction at a point.
Diving and DepthA deep-sea diver feels ear pain from rapidly increasing water pressure with depth, illustrating pressure’s practical, physical consequences.
Hydraulics EverywhereHydraulic systems, from car brakes to industrial lifts, rely entirely on the simple but powerful idea that enclosed fluid pressure transmits undiminished.
Boiling at AltitudeLower air pressure at high altitude means water’s vapour pressure needs to climb less far to match it, so water boils below 100°C on a tall mountain, and takes longer to cook food.

📝 Exam Point of View

Exam Point of View

NDA-NA-I 2026, Q59A floating body stays stable only when the metacentre sits above the centre of gravity — the centre of gravity’s position relative to the centre of buoyancy alone doesn’t decide it. View this question on the full NDA-NA-I 2026 paper →
NDA-NA-I 2018, Q83A barometer measures atmospheric pressure — not an ammeter (current), potentiometer (voltage), or lactometer (milk purity). View this question on the full NDA-NA-I 2018 paper →
NDA-NA-2 2025, Q65The exact definition of boiling point: when a liquid’s vapour pressure equals the surrounding atmospheric pressure — not simply “when it’s hot.” View this question on the full NDA-NA-2 2025 paper →
CAPF ACS 2018, Q4Lowering atmospheric pressure lowers a liquid’s boiling point — the reason water boils below 100°C on a mountain. View this question on the full CAPF ACS 2018 paper →
The PatternBoiling point and atmospheric pressure questions repeatedly test the same mechanism — vapour pressure catching up to surrounding pressure — from different angles.
NDA-NA-I&II 2020, Q74Liquid pressure at the base of a beaker depends only on the liquid height and density (P = ρgh) — never on the container base area. Test yourself on this question
NDA-NA-2 2017, Q109An object weighed underwater reads lighter than in air, since the buoyant force from Archimedes principle partly cancels its weight — the actual mass never changes. Test yourself on this question
NDA-NA-2 2025, Q57Pascal and N/m^2 are the same unit of pressure — pressure itself is a scalar with no direction, and it increases (not decreases) with depth in a liquid. Test yourself on this question

GenSci0010 — Units, Measurement and Scientific Instruments — covers the barometer and other measuring instruments this article applies to fluid pressure.
GenSci0018 — Elasticity — another mechanics topic on how materials respond to applied stress.
CAPF ACS 2018 GA Paper — contains the boiling-point-and-pressure PYQ cited above.
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