NDA Physics · Teaching notes
Fluid Mechanics and Properties of Matter — NDA Physics
Fluid Mechanics is the toughest chapter in NDA Physics — about 23 PYQs across 2017–2026 and the bank's highest HARD share (~30%). It rewards a clean grasp of two foundations and one famous principle. It teaches in two movements that follow the physics: (1) Pressure and Surface Tension — what pressure is (force per unit area), how it grows with depth in a liquid (P = rho g h), Pascal's transmission of pressure through an enclosed fluid (the hydraulic press), the difference between gauge and absolute pressure, and surface tension (the skin of a liquid, capillary rise, and how it falls as temperature rises); (2) Buoyancy, Density and Flotation — density and relative density first (the make-or-break foundation), then Archimedes' principle (the upthrust equals the weight of displaced fluid), why things float or sink (compare densities), how to combine densities by mixing equal volumes versus equal masses, apparent weight loss when submerged, and the stability of a floating body (centre of gravity, centre of buoyancy, metacentre). The single biggest pool is flotation and density (16 q) — nail the density comparison and Archimedes, and you own most of the chapter's hardest marks. Drill the formula, re-derive every step, walk out with the marks.
Subtopic notes
Pressure and Surface Tension
7 PYQsPressure is force spread over an area (P = F/A); inside a liquid it grows only with depth and density (P = rho g h) and is transmitted undiminished through an enclosed fluid (Pascal). Surface tension is the elastic skin of a liquid surface, and it weakens as temperature rises.
Open note
Buoyancy, Density and Flotation
16 PYQsDensity (mass per unit volume) decides everything here: a body floats when its average density is less than the fluid's, and Archimedes' principle says the upward buoyant force equals the weight of the fluid the body displaces.
Open note
PYQ weightage by concept
11 concepts · 23 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
11 concepts · 23 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Pressure — force spread over an area | 2 | 9% |
| Pressure in a liquid — P = rho g h | 2 | 9% |
| Atmospheric pressure, gauge vs absolute, and the pascal | 2 | 9% |
| Surface tension — the skin of a liquid | 1 | 4% |
| Pascal's principle — the hydraulic pressfoundation | — | — |
| Concept | PYQs | Share |
|---|---|---|
| Float or sink — the density comparison | 6 | 26% |
| Density and relative density | 3 | 13% |
| Archimedes' principle and the buoyant force | 3 | 13% |
| Combining densities — equal volumes vs equal masses | 2 | 9% |
| Apparent weight loss on submersion | 1 | 4% |
| Stability of a floating body — the metacentre | 1 | 4% |
Formula & revision sheet
10 formulas · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
10 formulas · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (5)
- Pressure — force spread over an area · Pressure
- Pressure in a liquid — P = rho g h · Pressure due to a liquid column
- Pascal's principle — the hydraulic press · Hydraulic press
- Atmospheric pressure, gauge vs absolute, and the pascal · Absolute pressure in a liquid open to air
- Surface tension — the skin of a liquid · Surface tension
Watch out for (6)
- Pressure rises when area shrinks→ Pressure — force spread over an area
- Pressure is NOT the same at all points→ Pressure in a liquid — P = rho g h
- Shape and base area do not matter→ Pressure in a liquid — P = rho g h
- Force is multiplied, energy is not→ Pascal's principle — the hydraulic press
- Barometer vs manometer vs thermometer→ Atmospheric pressure, gauge vs absolute, and the pascal
- Temperature LOWERS surface tension→ Surface tension — the skin of a liquid
Formulas (5)
- Density and relative density · Density and relative density
- Combining densities — equal volumes vs equal masses · Mixture density (equal masses)
- Archimedes' principle and the buoyant force · Buoyant force (Archimedes)
- Apparent weight loss on submersion · Apparent weight in a fluid
- Float or sink — the density comparison · Fraction submerged of a floating body
Watch out for (6)
- Relative density has no unit→ Density and relative density
- Equal volumes vs equal masses give DIFFERENT averages→ Combining densities — equal volumes vs equal masses
- Upthrust = weight of displaced FLUID, not of the body→ Archimedes' principle and the buoyant force
- Mass is unchanged; only apparent weight drops→ Apparent weight loss on submersion
- It is AVERAGE density that decides flotation→ Float or sink — the density comparison
- Stability is about M above G, not G below B→ Stability of a floating body — the metacentre