Pressure, Fluids and Properties of Matter PYQs – Physics, SSC & Railway

Correct Answer:
Correct Answer: Option 1 – Because there is no pressure difference between the two ends
Explanation:
Fluid flows from a region of higher potential (pressure/height) to lower potential. In a perfectlyhorizontal tube, both ends are at the same height, so there isno pressure difference(no potential gradient) to drive the flow, and the water remains static.
Important Facts:
• Fluid flow requires a pressure or potential difference between two points.
• Gravity does act on the water, but it contributes equally at both ends of a horizontal tube.

Correct Answer: Lactometer — Option (b)
A Lactometer works on the principle of Archimedes (law of buoyancy/principle of flotation). About Lactometer and Archimedes' Principle: Archimedes' Principle: A body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced; Lactometer: An instrument used to check the purity (adulteration) of milk; It is a type of hydrometer specially calibrated for milk; Working principle: When a lactometer is immersed in milk, it floats at a level determined by the milk's density; Pure milk has a specific gravity of about 1.026-1.034; Adulterated milk (watered-down) has a lower density; The lactometer sinks deeper in less dense (watered) milk and floats higher in denser (pure) milk; This floating behavior is governed by Archimedes' principle of buoyancy; A scale on the lactometer reads the percentage of purity; Other instruments using Archimedes' Principle: Hydrometer: Measures specific gravity of liquids; Densitometer: Measures density of liquids; Ships and submarines: Use buoyancy (Archimedes' principle); About other options: Thermometer: Works on the principle of thermal expansion (liquids expand on heating); Barometer: Measures atmospheric pressure — works on pressure principle (Evangelista Torricelli's principle — atmospheric pressure balances a column of mercury); Ammeter: Measures electric current — works on electromagnetic principle (galvanometer working).
Option (a) Thermometer — Incorrect: A thermometer works on the principle of thermal expansion — liquids (mercury, alcohol) expand when heated and contract when cooled; the rise and fall of the liquid in the narrow tube indicates temperature; Archimedes' principle (buoyancy) is NOT involved in the thermometer's working.
Option (c) Barometer — Incorrect: A barometer measures atmospheric pressure and works on the principle that atmospheric pressure supports a column of mercury (invented by Torricelli in 1643); the working principle is about atmospheric pressure balancing mercury column, NOT Archimedes' buoyancy principle. The candidate selected this incorrect option.
Option (d) Ammeter — Incorrect: An ammeter measures electric current and works on the principle of electromagnetic force (a current-carrying conductor in a magnetic field experiences a force — PMMC principle or D'Arsonval galvanometer principle); it has nothing to do with Archimedes' principle of buoyancy.

Correct Answer: Less — Option (b)
Pressure is defined as Force per unit Area: P = F/A. When a person stands on sand, their body weight (force) is distributed over a smaller area (the area of their feet). When the same person lies down on sand, the same body weight is distributed over a much larger area (the entire surface area of the body in contact with sand). Since force remains constant (body weight) but area increases significantly, the pressure (P = F/A) decreases. This is the same principle behind why snowshoes help people walk on snow — larger surface area reduces pressure and prevents sinking.
Option (a) Greater — Incorrect: The pressure decreases when lying down because the same force is distributed over a larger area.
Option (c) Uniform — Incorrect: While lying down distributes pressure more evenly, the net pressure per unit area is less, not the same as when standing.
Option (d) Zero — Incorrect: Pressure cannot be zero as long as the person's weight (force) acts on the sand; lying down reduces pressure but does not eliminate it.

Correct Answer: The weight of the fluid displaced — Option (c)
Archimedes' Principle states that when a body is immersed (fully or partially) in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the body. Mathematical expression: Buoyant Force ( F_b ) = Weight of fluid displaced = ρ_fluid × V_displaced × g; where ρ_fluid = density of the fluid, V_displaced = volume of fluid displaced, g = acceleration due to gravity. Key implications: If the buoyant force exceeds the weight of the object, it floats; if the weight exceeds the buoyant force, it sinks; if they are equal, the object is in neutral buoyancy. Applications: Ship design, submarines (which control their depth by adjusting buoyancy), hot air balloons, hydrometers (used to measure the density of liquids), and determining the purity of gold (as Archimedes famously did for King Hiero II's crown).
Option (a) Its own weight — Incorrect: The buoyant force is NOT equal to the object's own weight (unless the object is in neutral buoyancy/floating); the buoyant force equals the weight of the FLUID DISPLACED, which may be more, less, or equal to the object's own weight.
Option (b) The volume of the body — Incorrect: The buoyant force is not equal to the volume of the body; volume is part of the calculation (buoyant force = density of fluid × volume displaced × gravity), but the buoyant force itself equals the WEIGHT (not volume) of fluid displaced. The candidate selected this incorrect option.
Option (d) The density of the fluid — Incorrect: The density of the fluid alone does not equal the buoyant force; density is one factor in calculating the buoyant force (along with volume displaced and gravity), but Archimedes' principle specifically states the buoyant force equals the WEIGHT of the fluid displaced.

Topic: Fluid Dynamics - Laminar vs Turbulent Flow
When tap is slightly open → water flows smoothly (laminar flow)
When tap is opened wider → outflow speed increases significantly
At high velocities →Reynolds number increases
High Reynolds number → flow becomes chaotic and irregular (turbulent)
Turbulence causes loss of smoothness and visible disturbances
Answer: Option 2 → The outflow speed increases, causing turbulence.

Pascal's Law: Pressure applied to a fluid in a
closed container is transmitted equally in all
directions and to all surfaces.
Hydraulic Lift: Small force on small piston →
pressure increase → large force on large piston.
Hydraulic Brakes: Brake pedal pressure →
transmitted through fluid → brake pads activate.
Answer: Option 3 → Pascal's law

Formula: Pressure = ρ × g × h
Given: h = 20 m, ρ = 1000 kg/m³ (1 kg/ litre = 1000 kg/m³), g = 9.8 m/s²
P = 1000 × 9.8 × 20 = 1,96,000 Pa
Answer: Option 2 → 1,96,000 Pa

Pressure is defined as force applied per unit area.
When the same force acts on a smaller area, pressure increases because pressure = force / area.
Thus, force acting on a smaller area exerts a larger pressure compared to a larger area.
Correct option is Option 3.
Final answer: Option 3

Archimedes' Principle is used to measure buoyancy of objects in fluid.
It applies to hydrometers which measure liquid density.
Lactometers use it to test milk density.
Submarines are designed using buoyancy concepts based on this principle.
Speedometers measure speed, unrelated to buoyancy or Archimedes' Principle.
Hence, Speedometers are NOT an application of Archimedes' Principle.
Final answer: Option 4

When a solid object is immersed in water, it experiences an upward buoyant force.
This force reduces the object's apparent weight when measured in water compared to in air.
Mass remains unchanged, as it is intrinsic to the object.
Density does not change just by immersion.
Buoyant force acts upwards, it does not decrease but causes the apparent weight to decrease.
Hence, the quantity that decreases is the apparent weight.
Final answer: Option 1

According to Archimedes' Principle, the buoyant force on an object equals the weight of the fluid displaced. The buoyant force depends on the density of the fluid, the volume of the displaced fluid, and gravitational acceleration. A denser fluid exerts a greater buoyant force.
Refractive index, colour , and charge of a fluid have no effect on the magnitude of buoyant force.

The relationship between thrust, pressure, and area is: Pressure = Force (Thrust) / Area. This means Pressure equals Thrust divided by Area. Equivalently, Thrust = Pressure × Area.
Option 1 (Thrust = Pressure - Area) and Option 2 (Pressure = Thrust + Area) are mathematically incorrect. Option 4 (Thrust = Pressure / Area) inverts the correct formula. Hence, Pressure = Thrust / Area is correct.

Correct Answer: Pascal
The SI unit of pressure is Pascal (Pa), named after French mathematician and physicist Blaise Pascal. 1 Pascal = 1 Newton per square metre (1 Pa = 1 N/m2). It measures force per unit area. Other commonly used units of pressure include: atmospheres (atm), bar, torr, and mmHg.
Why other options are wrong:
- Ampere (A): SI unit of electric current.
- Watt (W): SI unit of power (energy per unit time).
- Newton (N): SI unit of force, not pressure.

We are not crushed under atmospheric pressure because the pressure inside our body — maintained by blood pressure, fluid pressure, and air in body cavities — exactly balances the external atmospheric pressure. This equilibrium of internal and external pressures keeps the body stable. Atmospheric pressure is not low (it is about 1 atm = 101,325 Pa); gravity does not pull pressure away; our physical strength has nothing to do with withstanding pressure.

Pressure = Force / Area. When force is kept the same and the area is reduced, the pressure increases. This is the principle behind sharp-edged tools like knives and needles — a smaller area concentrates the force and increases pressure. For example, a knife with a thin blade exerts much more pressure than a blunt blade with the same force applied.

Correct Answer: Pressure
Pressure is defined as the force applied per unit area of a surface. Formula: Pressure (P) = Force (F) / Area (A). The SI unit of pressure is Pascal (Pa), where 1 Pa = 1 N/m2. Examples: atmospheric pressure, blood pressure, water pressure.
Why other options are wrong:
- Force density: Refers to force per unit volume, not per unit area.
- Weight: Is the gravitational force on an object (W = mg); not force per unit area.
- Work: Is force multiplied by displacement (W = F x d); not force per unit area.

Pressure is defined as the force applied per unit area. The correct formula is: Pressure (P) = Force (F) / Area (A) SI unit of pressure is Pascal (Pa), where 1 Pa = 1 N/m². Why others are wrong: - Option : Multiplying force and area gives no standard physical quantity. - Option : Area divided by force is the inverse of pressure, which has no standard meaning. - Option : This is not a standard formula for any physical quantity.

By the continuity equation, when a fluid flows through a narrowing section, its speed increases.
By Bernoulli's principle, an increase in speed of a fluid is accompanied by a decrease in pressure.
Hence speed increases and pressure decreases.

Correct Answer: Option 4 — N m⁻²
Pressure is defined as the thrust (force acting normally) per unit area:
P = F/A
So its unit must be the unit of force divided by the unit of area — that is newton per square metre, N m⁻². This unit is given the special name pascal (Pa), so 1 Pa = 1 N m⁻².
Because area sits in the denominator, the same force spread over a smaller area produces a much larger pressure. That is why a sharp knife cuts easily, why a nail is pointed, and why a camel's broad feet stop it from sinking into sand. Pressure is a scalar quantity even though force is a vector, since it acts equally in all directions inside a fluid.
❌ Why the other options are incorrect
✗ N m — newton × metre is the unit of work / energy (joule) and also of torque, not of pressure.
✗ N m⁻¹ — newton per metre is the unit of surface tension and of spring constant, not pressure.
✗ N — the newton alone is the unit of force; pressure needs force *divided by area*.
📌 SSC & Railway Exam Facts
• Pressure P = F/A; SI unit pascal (Pa) = N m⁻²; named after Blaise Pascal.
• 1 atmosphere = 1.013 × 10⁵ Pa = 760 mm of Hg = 1.013 bar.
• Pressure in a liquid at depth h: P = hρg; it depends on depth and density, not on the shape of the vessel.
• Pascal's law: pressure applied to an enclosed fluid is transmitted equally in all directions — used in hydraulic lifts and brakes.
• Atmospheric pressure is measured by a barometer; pressure decreases with altitude.

Correct Answer: Option 2 — 20 N
📖 Given
• Pressure, P = 10 Nm⁻²
• Area, A = 2 m²
• To find: thrust F
✍️ Formula
Pressure = Thrust / Area, i.e. P = F/A, therefore F = P × A
🔢 Solution
• F = P × A
• F = 10 × 2
• F = 20 N
So a thrust of 20 N acts on the body. Thrust is simply the force acting perpendicular to a surface, and pressure is that thrust spread over each square metre of the surface.
❌ Why the other options are incorrect
✗ 5 N — this comes from dividing the pressure by the area (10 ÷ 2) instead of multiplying; that gives pressure per unit area, which is meaningless here.
✗ 2 N — this is just the area value written with the unit of force; area is not thrust.
✗ 10 N — this is the pressure value carried over as if it were the force; it ignores the area altogether, i.e. it assumes A = 1 m².
📌 SSC & Railway Exam Facts
• P = F/A; SI unit of pressure is the pascal (Pa) and 1 Pa = 1 N/m².
• Thrust is a force (unit newton), while pressure is force per unit area — a vector and a scalar respectively.
• For a fixed force, smaller area means larger pressure — this is why a knife is sharpened and a nail is pointed.
• Pressure in a liquid at depth h: P = hρg; it acts equally in all directions.
• Atmospheric pressure = 1.013 × 10⁵ Pa = 1 atm = 760 mm of Hg, measured with a barometer.

Correct Answer: Option 1 — Undiminished in all directions.
This is the statement of Pascal's law, given by Blaise Pascal. It says that the pressure applied to an enclosed (confined) fluid is transmitted equally and undiminished in all directions, and it acts at right angles to every surface the fluid touches.
A liquid has no fixed shape, so its molecules can pass a push on to their neighbours in every direction. That is why squeezing one end of a sealed tube of toothpaste makes the paste come out at the other end with the same pressure.
Because pressure is force per unit area (P = F/A), a small force on a small piston can be turned into a very large force on a large piston. This is the working principle of the hydraulic press, hydraulic lift and hydraulic brakes.
❌ Why the other options are incorrect
✗ In one direction only. — this describes a solid, where a push is passed along only the direction it is applied.
✗ In two directions only. — a fluid has no such restriction; pressure acts sideways, upwards and downwards alike.
✗ In three directions only. — pressure is transmitted in every direction, not in some fixed count of directions.
📌 SSC & Railway Exam Facts
• Pascal's law — pressure in a confined fluid is transmitted equally and undiminished in all directions.
• Pressure P = F/A; SI unit pascal (Pa), and 1 Pa = 1 N/m².
• Pressure due to a liquid column: P = hρg — it depends on depth, not on the area of the base.
• Applications: hydraulic lift, hydraulic press, hydraulic brakes.
• Atmospheric pressure at sea level ≈ 1.013 × 10⁵ Pa, measured with a barometer.

Correct Answer: Option 1 — 10 N
📖 Given
• Mass of the metal box, m = 1 kg
• Acceleration due to gravity, g = 10 m/sec²
• The box rests on a horizontal table surface
✍️ Formula
Thrust = weight of the box = mg (for a body resting on a horizontal surface)
🔢 Solution
• Thrust = m × g
• Thrust = 1 × 10
• Thrust = 10 N
Thrust is the force acting perpendicular (normal) to a surface. Here the box presses down on the table with its full weight, so the thrust on the table is exactly 10 newtons.
❌ Why the other options are incorrect
✗ 100 N — this comes from wrongly taking the mass as 10 kg, or from multiplying by 10 one extra time.
✗ 1 N — this is just the numerical value of the mass; the mass has not been multiplied by g.
✗ 1000 N — this would need a mass of 100 kg; it is 100 times the correct answer.
📌 SSC & Railway Exam Facts
• Thrust is the force acting normal to a surface; SI unit newton (N).
• Pressure = thrust/area, P = F/A; SI unit pascal (Pa), and 1 Pa = 1 N/m².
• The same thrust on a smaller area gives a larger pressure — that is why a sharp knife cuts better.
• Atmospheric pressure at sea level ≈ 1.013 × 10⁵ Pa = 1 atm, measured by a barometer.
• Archimedes' principle: the upthrust on a body in a fluid equals the weight of the fluid displaced.

Correct Answer: Option 3 — less than the density of the liquid
When a body is placed in a liquid, two forces act on it: its weight acting downwards and the upthrust (buoyant force) acting upwards. By Archimedes' principle, the upthrust equals the weight of the liquid displaced.
If the density of the object is less than the density of the liquid, then even when only a part of the body is submerged, the liquid displaced already weighs as much as the whole body. The upthrust therefore balances the weight, and the body floats with a part of it above the surface. If the densities were equal, the body would just float fully submerged, and if the object were denser, the upthrust could never match its weight and it would sink. This is why a ship of iron floats — its hollow shape makes its average density less than that of water — while a solid iron nail sinks.
❌ Why the other options are incorrect
✗ equal to the density of the liquid — then the body neither rises nor sinks; it stays completely submerged at rest anywhere in the liquid, which is not ordinary floating.
✗ exactly twice the density of the liquid — a body twice as dense as the liquid is far heavier than the upthrust it can get, so it sinks straight to the bottom.
✗ greater than the density of the liquid — the weight exceeds the maximum possible upthrust, so the body sinks; this is the opposite of floating.
📌 SSC & Railway Exam Facts
• Archimedes' principle: upthrust = weight of the fluid displaced.
• Floats if ρ(object) < ρ(liquid); fully submerged & stationary if equal; sinks if greater.
• Density ρ = mass/volume; SI unit kg/m³; density of water = 1000 kg/m³ = 1 g/cm³.
• Relative density has no unit; it is measured with a hydrometer.
• Ice floats on water because ice (920 kg/m³) is less dense than water; about 1/10 of an iceberg stays above the surface.

Correct Answer: Option 4 — 1.01 × 10⁵ Pa
Pressure is the force acting per unit area, P = F/A. Its SI unit is the pascal (Pa), where 1 Pa = 1 N/m².
The atmosphere (atm) is the pressure exerted by the column of air of the earth's atmosphere at sea level. Measured with a mercury barometer, it supports a mercury column of 760 mm (76 cm). Converting that to SI units gives
1 atm = 1.01 × 10⁵ Pa ≈ 101.3 kPa ≈ 1.013 bar.
Because gases are studied at such pressures, 1 atm is the reference pressure used in STP (standard temperature and pressure) along with 273 K.
❌ Why the other options are incorrect
✗ 1.01 × 10⁶ Pa — this is 10 atm, ten times too large; it comes from mis-writing the power as 6 instead of 5.
✗ 1.01 × 10⁻⁶ Pa — a negative power makes the value almost zero, close to a high vacuum, not atmospheric pressure.
✗ 1.01 × 10⁻⁵ Pa — again a near-vacuum value; the correct exponent is positive 5.
📌 SSC & Railway Exam Facts
• 1 atm = 1.01 × 10⁵ Pa = 101.3 kPa = 1.013 bar = 760 mm of Hg.
• 1 bar = 10⁵ Pa; 1 Pa = 1 N/m².
• Pressure is measured by a barometer (atmospheric) and a manometer (gas).
• The mercury barometer was invented by Evangelista Torricelli.
• At STP, temperature = 273 K (0 °C) and pressure = 1 atm; molar volume of a gas = 22.4 L.

Correct Answer: Option 3 — 500 Pa
📖 Given
• Thrust (force acting normally on the surface), F = 50 N
• Area of the surface, A = 1000 cm²
• 1 cm² = 10⁻⁴ m², so A = 1000 × 10⁻⁴ = 0.1 m²
• To find: pressure, P
✍️ Formula
Pressure = Thrust / Area, i.e. P = F/A
🔢 Solution
• Convert the area: A = 1000 cm² = 1000 × 10⁻⁴ m² = 0.1 m²
• P = 50/0.1
• P = 500 Pa
In plain language: a force of 50 N spread over just one-tenth of a square metre presses on that surface with 500 pascal, i.e. 500 newton on every square metre.
❌ Why the other options are incorrect
✗ 5 Pa — obtained by dividing 50 N by 10 instead of by 0.1 m²; a unit-conversion error of a factor of 100.
✗ 20 Pa — this has no basis in the given data; it comes from mixing up 1000 and 50 by subtraction or a wrong ratio.
✗ 30 Pa — again unrelated to the data; the only correct route is F ÷ A with A converted into m².
📌 SSC & Railway Exam Facts
• P = F/A (or Thrust/Area); SI unit pascal (Pa), where 1 Pa = 1 N/m², named after Blaise Pascal.
• 1 m² = 10⁴ cm², so 1 cm² = 10⁻⁴ m² — this conversion decides the answer here.
• Pressure decreases as area increases — that is why a sharp knife cuts easily, a camel walks on sand, and school bags have broad straps.
• Atmospheric pressure at sea level = 1.013 × 10⁵ Pa = 1 atm ≈ 760 mm of Hg; measured by a barometer.
• Pressure in a liquid at depth h: P = hρg, and it acts equally in all directions (Pascal's law).

Correct Answer: Option 2 — thrust
The force acting on a body perpendicular (normal) to a surface is called thrust. Since it is a force, it is a vector quantity and its SI unit is the newton (N). A block resting on a table, for example, presses on the table with a thrust equal to its own weight.
The effect of this thrust depends on the area over which it is spread. Thrust acting on unit area is called pressure: Pressure = Thrust/Area, with SI unit pascal (Pa) = N/m². This is why a sharp knife, a thin nail or a needle — all having a very small area — produce a very large pressure from the same thrust, while a camel's broad feet reduce the pressure on soft sand.
❌ Why the other options are incorrect
✗ pressure — pressure is thrust per unit area (P = F/A) and is measured in pascal; it is the *effect* of the force, not the force itself.
✗ buoyancy — buoyancy, or upthrust, is the upward force exerted by a fluid on a body immersed in it; here the force need not come from a fluid at all.
✗ weight — weight is the force of gravity on a body, W = mg, and always acts vertically downwards; it is perpendicular to a surface only when the surface happens to be horizontal.
📌 SSC & Railway Exam Facts
• Thrust — the force acting perpendicular to a surface; SI unit newton (N).
• Pressure = Thrust/Area; SI unit pascal (Pa), and 1 Pa = 1 N/m².
• Pressure inside a liquid at depth h is P = hρg, and it acts equally in all directions.
• Archimedes' principle — the upthrust on a body equals the weight of the fluid displaced by it.
• Atmospheric pressure ≈ 1.013 × 10⁵ Pa = 76 cm of mercury = 1 atm, measured by a barometer.

Correct Answer: Option 1 — The iron rod would float on water surface.
When any object is placed in a fluid, two forces act on it: its weight (W = mg) acting vertically downwards, and the upthrust or buoyant force acting vertically upwards. By Archimedes' principle, the upthrust equals the weight of the fluid displaced by the object.
The behaviour of the object is decided by comparing these two forces:
Upthrust > Weight → the object floats; Upthrust = Weight → it just floats, fully submerged; Upthrust < Weight → it sinks.
The question specifically supposes that the upthrust is greater than the downward force. In that condition the net force on the rod is upwards, so the rod is pushed up and floats on the water surface — exactly as a ship made of iron does, because its hollow shape displaces a large volume of water.
❌ Why the other options are incorrect
✗ The iron rod would move downwards — a downward motion needs weight greater than upthrust, the opposite of the given condition.
✗ The iron rod would sink slowly — sinking of any kind requires a net downward force; here the net force is upward.
✗ The iron rod would sink faster — this would need an even larger weight compared with upthrust, which contradicts the given condition.
📌 SSC & Railway Exam Facts
• Archimedes' principle: upthrust = weight of fluid displaced.
• An object floats if its density is less than the density of the liquid; iron sinks in water because its density (7.8 g/cm³) exceeds water's (1 g/cm³).
• Upthrust increases with the volume of the object immersed and the density of the liquid.
• A hydrometer and a lactometer work on the principle of flotation; a ship floats because of its hollow shape.
• Ice floats on water because the density of ice (0.92 g/cm³) is less than that of water; about 1/10 of an iceberg stays above the surface.

Correct Answer: Option 4 — Both 'A' and 'B' are correct.
Statement A is correct. The weight of a body is the force with which the Earth attracts it, W = mg. Since the gravitational pull of the Earth is directed towards its centre, weight always acts vertically downwards. Its SI unit is the newton (N) and it is measured with a spring balance.
Statement B is correct. Thrust is the force acting on a surface in a direction perpendicular (normal) to it. Its SI unit is also the newton (N). Thrust acting per unit area is called pressure, P = Thrust/Area, whose SI unit is the pascal (Pa), where 1 Pa = 1 N/m².
Since both statements match the standard definitions, the correct choice is that both A and B are correct.
❌ Why the other options are incorrect
✗ Both 'A' and 'B' are incorrect — wrong; both definitions are exactly as given in the textbook.
✗ 'A' is correct and 'B' is incorrect — B is also correct; thrust is by definition the perpendicular force on a surface.
✗ 'A' is incorrect and 'B' is correct — A is also correct; weight always acts vertically downwards towards the Earth's centre.
📌 SSC & Railway Exam Facts
• W = mg; mass is constant everywhere but weight changes with g.
• Weight on the Moon is 1/6 of that on the Earth because the Moon's g is about 1.63 m/s².
• Pressure = Thrust/Area; SI unit pascal (Pa) = N/m², named after Blaise Pascal.
• A sharp knife cuts better and a camel walks easily on sand because pressure decreases as the area increases.
• Atmospheric pressure at sea level = 1.013 × 10⁵ Pa = 1 atm ≈ 760 mm of Hg.