Mechanical Properties of Fluids for NEET UG
Liquids and gases push, flow, stick and resist motion. Pressure, Bernoulli's principle, viscosity and surface tension explain blood pressure, aircraft lift and why insects can walk on water.
PhysicsClass 1160 MCQs4 lessons13 flashcards4 memory tricks
Practise Mechanical Properties of Fluids free
What you'll learn
- Pressure in fluids and Pascal's law
Calculate pressure at a depth and explain how a hydraulic lift multiplies force. - Flowing fluids: continuity and Bernoulli
Use A₁v₁ = A₂v₂ and Bernoulli's equation to explain speed and pressure changes. - Viscosity and terminal velocity
Use Stokes' law to find the terminal velocity of a sphere falling through a fluid. - Surface tension and capillarity
Explain surface tension and calculate the rise of liquid in a capillary tube.
Key formulas
| Pressure | P = F/A |
|---|---|
| Pressure at depth h | P = P₀ + ρgh |
| Hydraulic lift | F₂/A₂ = F₁/A₁, so F₂ = F₁ × A₂/A₁ |
| Equation of continuity | A₁v₁ = A₂v₂ |
| Bernoulli's equation | P + ½ρv² + ρgh = constant along a streamline |
| Speed of efflux (Torricelli) | v = √(2gh) |
| Stokes' law (drag on a sphere) | F = 6πηrv |
| Terminal velocity | v_t = 2r²(ρ − σ)g / 9η |
Memory tricks
Where a fluid flows faster its pressure is lower (P + ½ρv² + ρgh = constant). Fast wind over a roof or umbrella lowers the pressure above, so the higher pressure below pushes it up.
h = 2T cos θ / (rρg), so h ∝ 1/r. Water wets glass (θ < 90°) and rises; mercury doesn't wet glass (θ > 90°) and is depressed.
Viscous drag 6πηrv grows with speed until it balances weight minus buoyancy. Terminal velocity v ∝ r², so bigger drops fall faster.
Pressure applied to an enclosed fluid is transmitted equally, so F₂ = F₁ × A₂/A₁.
Try 3 NEET-style questions
Q1. Detergents help in washing clothes mainly because they
- lower the surface tension so water wets the cloth
- raise the surface tension of the water
- greatly increase the viscosity of water
- make the water denser than the oil stains
Show answer
Answer: (1) lower the surface tension so water wets the cloth
Detergents reduce the surface tension of water, so it spreads and soaks into the fabric better and helps lift away oily dirt.
Q2. Water flows through a horizontal pipe. Its speed is 2 m/s at one section and 4 m/s at a narrower section. The pressure difference between the two sections is
- 1000 Pa
- 2000 Pa
- 6000 Pa
- 12,000 Pa
Show answer
Answer: (3) 6000 Pa
By Bernoulli's equation for a horizontal pipe, P₁ − P₂ = ½ρ(v₂² − v₁²) = ½ × 1000 × (16 − 4) = 6000 Pa.
Q3. Water wets clean glass because the angle of contact between water and glass is
- obtuse (greater than 90°)
- acute (less than 90°)
- exactly 180°
- exactly 90°
Show answer
Answer: (2) acute (less than 90°)
For water on glass, the angle of contact is acute, so water spreads (wets) and rises in a glass capillary. For mercury on glass, it is obtuse, so mercury does not wet glass.
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