Mechanical Properties of Fluids
Class 11 · Physics
4 ideas in Class 11 Physics, explained in plain words, with games and quizzes to practise.
Pressure in Fluids
Part of: Pressure and Flow
Pressure is push per unit area, acting square to every surface. Resting fluids share pressure across each level, and depth adds weight of the column above. Gauges read the excess over air, and narrow or wide vessels with equal heights push equally below.
Why it matters. Dams, lifts, syringes and clinic gauges all run on these rules. Shape independence surprises almost everyone.
Formulas
- P = F / A
- Depth rule: P = Pa + h rho g
- Hydraulic lift: F2 = F1 x (A2 / A1)
Key words
- Pressure
- : Push per unit area, acting square to every surface.
- Pascal
- : The SI unit of pressure.
- Gauge pressure
- : The excess over atmospheric pressure.
- Absolute pressure
- : Gauge pressure plus atmospheric pressure.
- Pascal's law
- : A push on a confined fluid spreads undiminished to all parts.
Try it in two minutes
- Press a palm on water and feel push grow with depth.
- Fill linked bottles of mixed shapes to one height.
- Compare gauge and absolute readings on a pump.
Try at home. Poke holes at two heights in a bottle and compare jets.
Flow: Continuity and Bernoulli
Part of: Pressure and Flow
Steady streams keep volume flow fixed, so narrow parts run fast. Bernoulli trades pressure for speed and height: faster or higher flow means lower pressure. Tank holes jet like falling bodies, and wings and spinning balls ride the pressure gaps.
Why it matters. Pipes, pumps, wings and curve balls all obey one energy balance. It predicts where pressure drops before gauges do.
Formulas
- Continuity: A1 v1 = A2 v2
- Bernoulli: P + (1/2) rho v^2 + rho g h = constant
- Torricelli: efflux speed = root(2 g h)
Key words
- Volume flow rate
- : Volume crossing a section each second.
- Continuity
- : Narrow parts run fast so volume flow stays fixed.
- Bernoulli's principle
- : Faster or higher flow means lower pressure.
- Efflux speed
- : The jet speed from a tank hole.
- Streamline
- : The fixed path particles follow in steady flow.
Try it in two minutes
- Pinch a hose and watch the jet speed up.
- Float paper over a fast blow and watch it lift.
- Spin a ball and watch its path bend.
Try at home. Drain a bottle from two heights and compare jet reach.
Viscosity
Part of: Stickiness and Skin of Liquids
Viscosity is internal friction between sliding layers. Small spheres settle to a terminal speed where drag plus buoyancy match weight, growing with radius squared and shrinking with stickiness. Past a critical speed, smooth flow breaks into turbulence.
Why it matters. Oils, syrup, rain and pendulums all drag. Terminal speed explains why mist hangs and stones plunge.
Formulas
- Stokes drag F = 6 pi eta r v
- Terminal speed grows with radius squared: v related to r^2
- Viscous force = eta A (speed / gap)
Key words
- Viscosity
- : Internal friction between sliding fluid layers.
- Terminal velocity
- : The steady fall speed where drag plus buoyancy match weight.
- Stokes' law
- : Drag on a small sphere grows with radius, stickiness and speed.
- Turbulence
- : Wild unsteady flow past a critical speed.
- Critical speed
- : The speed past which smooth flow breaks into turbulence.
Try it in two minutes
- Drop a bead in water, then in oil, and compare.
- Stir fast and watch smooth swirls turn wild.
- Time marbles falling through syrup.
Try at home. Race drops of honey and water down a tilted plate.
Surface Tension
Part of: Stickiness and Skin of Liquids
Surface molecules carry extra energy, so liquids shrink their skins. Tension pulls per unit length along the surface. Small drops curve with inside pressure above outside by twice tension over radius; bubbles double that with two skins. Wetting pulls water up narrow tubes.
Why it matters. Sprays, soaps, wicks and leaves all run on skins. Curvature sets pressures that bulk rules miss.
Formulas
- Drop excess pressure = 2 S / R
- Bubble excess pressure = 4 S / R (two skins)
- Capillary rise h = 2 S cos(angle) / (r rho g)
Key words
- Surface tension
- : Pull per unit length along a liquid surface.
- Excess pressure
- : Inside pressure above outside in a curved surface.
- Capillarity
- : Rise or fall of liquid in a narrow tube.
- Angle of contact
- : The angle a liquid edge makes with a solid wall.
- Wetting
- : Spreading on a surface from a low contact angle.
Try it in two minutes
- Blow bubbles and watch them ball up.
- Dip tubes of two widths and compare rise.
- Float a needle on water with tissue help.
Try at home. Compare water beading on wax versus spreading on glass.
What your child will practise
- Flow: Continuity and Bernoulli: Desert Quick Fire (Quick Fire)
- Pressure in Fluids: Space Puzzle (Clue Puzzle)
- Pressure in Fluids: Space Quick Fire (Quick Fire)
- Surface Tension: Ocean Quick Fire (Quick Fire)
- Viscosity: Jungle Puzzle (Clue Puzzle)
- Quiz: Flow: Continuity and Bernoulli (8 questions)
- Quiz: Pressure in Fluids (8 questions)
- Quiz: Surface Tension (8 questions)
- Quiz: Viscosity (8 questions)