Chapter 9 — Mechanical Properties of Fluids

Chapter 9 of the Class 11 Physics NCERT textbook, "Mechanical Properties of Fluids", covers pressure, streamline flow, Bernoulli's principle, viscosity, and surface tension. The free NCERT PDF is available to download.

• Fluids and pressure — Fluids differ from solids by their ability to flow. Pressure, the normal force per unit area, spreads through a fluid by Pascal's law and rises with depth, underpinning the behaviour of liquids and gases at rest.
• Flow and Bernoulli's principle — For steady streamline flow, mass conservation links speed to pipe area, and Bernoulli's principle ties together pressure, kinetic, and potential energy along a streamline—explaining how faster flow accompanies lower pressure.
• Viscosity and surface tension — Viscosity is a fluid's internal resistance to motion, governing drag and terminal velocity. Surface tension, the extra energy at a liquid's surface, explains spherical droplets, capillary rise, and wetting behaviour.

1. 01Pressure is a scalar quantity defined as normal force per unit area (P = F/A), measured in pascals (Pa); 1 atm = 1.01 × 10⁵ Pa
2. 02Pascal's law: pressure in a fluid at rest is uniform at the same height and transmitted undiminished in all directions throughout an enclosed fluid
3. 03Pressure increases with depth in a fluid according to P = Pa + ρgh, where ρ is density, g is gravity, and h is depth below surface
4. 04Continuity equation (Av = constant) conserves mass in incompressible fluid flow; velocity increases where cross-sectional area decreases
5. 05Bernoulli's equation (P + ½ρv² + ρgh = constant) applies to streamline flows and relates pressure changes to velocity and elevation changes
6. 06Viscosity η quantifies resistance to fluid motion; Stokes' law gives viscous drag F = 6πηav on a sphere; terminal velocity vt = 2a²(ρ−σ)g / 9η
7. 07Surface tension is force per unit length at liquid-air interfaces, causing droplets to be spherical and water to rise in narrow capillaries by height h = 2S cosθ / ρga