📑 Topics on this page (Jump to Topic):
📌 1. Pressure & Atmospheric Pressure
- Pressure = force / area (Pa). Small area → high pressure: sharp nail, sharp knife; wide area → low pressure: camel's broad feet, tank tracks, wide school-bag straps.
- Pressure in a liquid = hρg — it rises with depth, so a dam wall is thicker at the bottom.
- At sea level atmospheric pressure ≈ 1.013 × 10⁵ Pa = 76 cm of mercury = 1 atm; it falls with height — ink leaks from a fountain pen and noses may bleed on mountains.
- A barometer (Torricelli) measures air pressure: a sudden fall → storm; slow fall → rain; rise → fair weather.
📌 2. Pascal, Archimedes & Floatation
- Pascal's law: pressure applied to an enclosed liquid is transmitted equally in all directions — hydraulic lift, hydraulic brakes, hydraulic press.
- Archimedes' principle: buoyant force on an immersed body = weight of liquid displaced; so objects feel lighter in water.
- Law of floatation: a body floats when its weight = weight of liquid displaced. An iron needle sinks but an iron ship floats (it displaces more volume).
- Swimming in the sea (salt water) is easier — higher density; a ship rises slightly when it moves from a river to the sea. People do not sink in the Dead Sea.
- About 9/10 of ice stays under water (icebergs). A hydrometer measures relative density of liquids; a lactometer checks the purity of milk.
- Water has maximum density at 4 °C (1000 kg/m³) — so lakes freeze from the top and aquatic life survives below.
📌 3. Surface Tension, Capillarity, Viscosity, Bernoulli
| Property | Meaning | Examples |
|---|---|---|
| Surface tension | Liquid surface acts like a stretched membrane | Drops are round, needle floats on water, insects walk on water; falls with heat or soap |
| Capillarity | Rise or fall of liquid in a thin tube | Lamp wick, water in plants, blotting paper, towel; mercury falls |
| Viscosity | Internal friction of a fluid | Honey, glycerine are highly viscous; heating lowers viscosity of liquids, raises it for gases |
| Bernoulli's theorem | Where speed is high, pressure is low | Aeroplane lift, roofs blown off in storms, pull near a fast train, sprayer |
⚠️ Exam Traps:
- Pouring oil on the sea calms waves — surface tension decreases.
- Ice floats because its density is less than water.
- When floating ice melts, the water level in a glass neither rises nor falls.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Hydraulic brakes work on: Practice
Correct answer: (D)
Pressure in an enclosed liquid is transmitted equally in all directions.
Pressure in an enclosed liquid is transmitted equally in all directions.
Q2. A sudden fall in barometer reading indicates: Practice
Correct answer: (A)
Sudden low pressure = storm; a slow fall suggests rain.
Sudden low pressure = storm; a slow fall suggests rain.
Q3. At what temperature is the density of water maximum? Practice
Correct answer: (B)
Water is densest at 4 °C — anomalous expansion.
Water is densest at 4 °C — anomalous expansion.
Q4. A lactometer is used to test: Practice
Correct answer: (A)
It measures the relative density of milk.
It measures the relative density of milk.
Q5. Why does an iron ship float on water? Practice
Correct answer: (D)
Its hollow shape makes the displaced water weigh as much as the ship.
Its hollow shape makes the displaced water weigh as much as the ship.
Q6. When a ship enters the sea from a river, it: Practice
Correct answer: (B)
Salt water is denser, so less volume needs to be displaced.
Salt water is denser, so less volume needs to be displaced.
Q7. Raindrops are spherical due to: Practice
Correct answer: (D)
Surface tension minimises area; a sphere has the least surface area.
Surface tension minimises area; a sphere has the least surface area.
Q8. Oil rises in a lamp wick because of: Practice
Correct answer: (B)
The thin pores of the wick act like capillary tubes.
The thin pores of the wick act like capillary tubes.
Q9. Lift on an aeroplane wing is explained by: Practice
Correct answer: (D)
Faster air above the wing → lower pressure; higher pressure below pushes it up.
Faster air above the wing → lower pressure; higher pressure below pushes it up.
Q10. Why does a fountain pen leak at high altitude? Practice
Correct answer: (C)
Air inside the pen is at higher pressure than outside, pushing ink out.
Air inside the pen is at higher pressure than outside, pushing ink out.
Q11. Why is a dam wall made thicker at the bottom? Practice
Correct answer: (C)
P = hρg — pressure is greatest at the bottom.
P = hρg — pressure is greatest at the bottom.
Q12. Normal atmospheric pressure at sea level equals how many cm of mercury? Practice
Correct answer: (A)
1 atm = 76 cm (760 mm) Hg ≈ 1.013 × 10⁵ Pa.
1 atm = 76 cm (760 mm) Hg ≈ 1.013 × 10⁵ Pa.
Q13. Why can a camel walk easily on sand? Practice
Correct answer: (C)
Spreading weight over a larger area lowers pressure.
Spreading weight over a larger area lowers pressure.
Q14. Adding soap to water makes its surface tension: Practice
Correct answer: (D)
Soap/detergent lowers surface tension, helping remove dirt.
Soap/detergent lowers surface tension, helping remove dirt.
Q15. On heating, the viscosity of liquids: Practice
Correct answer: (C)
Warm honey flows easily; in gases it is the opposite — viscosity rises.
Warm honey flows easily; in gases it is the opposite — viscosity rises.
Q16. Tin roofs get blown off in a storm because: Practice
Correct answer: (B)
Bernoulli — fast wind lowers pressure above; higher pressure below lifts the roof.
Bernoulli — fast wind lowers pressure above; higher pressure below lifts the roof.
Q17. When an ice cube floating in a glass of water melts, the water level: Practice
Correct answer: (B)
Floating ice displaces exactly the water it forms on melting.
Floating ice displaces exactly the water it forms on melting.
Q18. The formula for pressure is: Practice
Correct answer: (A)
P = F/A, unit pascal.
P = F/A, unit pascal.
Q19. In a capillary tube, mercury: Practice
Correct answer: (A)
Mercury does not wet glass (cohesion dominates), so it is depressed.
Mercury does not wet glass (cohesion dominates), so it is depressed.
Q20. A person floats easily in the Dead Sea because: Practice
Correct answer: (C)
Very high salinity gives a large buoyant force.
Very high salinity gives a large buoyant force.