Mechanical Properties of FluidsNEET Physics · Class 11 · NCERT Chapter 9

You can expect 1 to 3 questions from this chapter in NEET 2027. The chapter has high PYQ frequency. Bernoulli's equation, Archimedes' principle, Stokes' law / terminal velocity and surface tension / capillary rise are the favourite topics.

Pascal's law: pressure applied at any point of a confined fluid is transmitted equally in all directions, undiminished. The hydraulic press uses this — a small force on a small piston produces a large force on a large piston, with the multiplication factor equal to A2 over A1.

A body fully or partly submerged in a fluid experiences an upward buoyant force equal to the weight of fluid displaced. If buoyant force exceeds the body's weight, it floats. If equal, it is in equilibrium at any depth. If less, it sinks.

For steady, non-viscous, incompressible flow along a streamline: P plus one half rho v squared plus rho g h equals a constant. Pressure, kinetic energy per unit volume and potential energy per unit volume sum to a constant. Useful for Venturi meters, speed of efflux from a tank, and aircraft lift.

A small sphere falling through a viscous fluid reaches a constant velocity when the viscous drag plus buoyancy balance gravity. v_t equals (2 r squared (rho_s minus rho_f) g) over (9 eta), where r is sphere radius, rho_s is sphere density, rho_f is fluid density and eta is fluid viscosity. Stokes' law gives the drag.

When a thin tube is dipped in a liquid that wets it (low contact angle), surface tension pulls the liquid up the tube. Height h equals (2 T cos theta) over (rho g r), where T is surface tension, theta is contact angle, rho is liquid density and r is tube radius. Mercury (contact angle greater than 90 degrees) shows capillary depression instead.

Surface tension T is the force per unit length acting along the surface of a liquid, perpendicular to any line drawn on it (units N/m). Surface energy is the work needed to increase the surface area by unit amount (units J per m squared). The two are numerically equal — extra molecules at a free surface have extra PE.