Tamil Nadu Class 9 Science Fluids — Samacheer Kalvi

Quick Summary

Fluids (liquids and gases) exert pressure in all directions. This chapter covers the behaviour of fluids at rest and in motion, including important principles by Archimedes, Pascal, and Bernoulli. These concepts appear frequently as 3-mark and 5-mark questions.

Key Definitions

• Fluid: A substance that can flow — includes both liquids and gases.
• Pressure: Force per unit area. P = F/A. SI unit: Pascal (Pa) = N/m².
• Buoyancy (Upthrust): Upward force exerted by a fluid on a submerged or floating body.
• Density: Mass per unit volume. ρ = m/V. SI unit: kg/m³.
• Relative Density (Specific Gravity): Ratio of density of substance to density of water at 4°C.
• Streamline Flow: Flow where each fluid particle follows a smooth path.
• Turbulent Flow: Irregular, chaotic fluid flow.

Pressure in Fluids

• Pressure increases with depth: P = ρgh
• Pressure acts equally in all directions at any point in a fluid (Pascal's law).
• Atmospheric pressure at sea level = 101,325 Pa ≈ 1 atm.

Archimedes' Principle

• Buoyant force = weight of displaced fluid = ρ_fluid × V_displaced × g
• Floating condition: Weight of body = Buoyant force (object floats when its density < fluid density).
• Sinking condition: Weight of body > Buoyant force (object sinks when its density > fluid density).
• Applications: ships, submarines, hot air balloons, hydrometer.

Pascal's Law

• P applied = P transmitted (throughout the confined fluid).
• Hydraulic systems: Small force on small piston → large force on large piston.
• F₁/A₁ = F₂/A₂
• Applications: hydraulic press, hydraulic brakes, hydraulic jack, dentist's chair.

Bernoulli's Theorem

• P + ½ρv² + ρgh = constant (for ideal fluid in streamline flow).
• As fluid velocity increases, pressure decreases.
• Applications: aircraft wings (lift force), spray gun, Bunsen burner, carburetor, venturimeter.

Important Formulae

Important Questions

One Mark

• State the SI unit of pressure.
• Define buoyancy.
• Which law forms the basis of the hydraulic press?
• A body floats in water. What can you say about its density?
• Name two applications of Bernoulli's theorem.

Short Answer (3 marks)

• State and explain Pascal's law with one application.
• A solid body weighs 500 N in air and 400 N in water. Find the buoyant force and comment on its density relative to water.
• Explain why a ship made of iron floats while an iron nail sinks.

Long Answer (5 marks)

• State Archimedes' principle. Describe an experiment to verify it. Give three practical applications.
• Explain Bernoulli's theorem and discuss three applications in daily life.

Common Mistakes

• Confusing Archimedes' principle (buoyant force = weight of displaced fluid) with weight of the body.
• Applying Pascal's law only to liquids — it applies to all confined fluids including gases.
• Forgetting that pressure at a depth depends on the height of fluid above, not the shape of the container.

Revision Checklist

• [ ] Know formulae: P = F/A, P = ρgh, Buoyant force = ρVg
• [ ] Can state and explain Archimedes' principle
• [ ] Know Pascal's law and its applications
• [ ] Know Bernoulli's theorem and its applications
• [ ] Practised numerical problems on pressure and buoyancy