CDS Physics · Free notes

Fluids and Properties of Matter: CDS Physics

Fluid Mechanics and Properties of Matter has 9 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 2 EASY, 6 MODERATE and 1 HARD. Buoyancy is the biggest share, and three of its questions came in 2025 and 2026. Most of them need one idea used carefully: the buoyant force depends on the liquid and the volume under it, not on the body's own mass, and pressure depends on area as well as force. Learn Archimedes' principle as an equation, and the sums become one line each.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

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Subtopic notes

Pressure, Buoyancy and Elasticity
9 PYQs
Pressure as force per area and how it grows with depth, the buoyant force and why bodies float, the density of a mixture, and Hooke's law for springs and wires.

Formula & revision sheet

3 formulas · 1 reference tables · 8 gotchas across all subtopics — the exam-eve cheat-sheet

Pressure, Buoyancy and Elasticity

Formulas (3)

Pressure and pressure at a depth · Pressure $P = \dfrac{F}{A} \qquad P = P_0 + \rho g h$
Buoyancy and flotation · Buoyant force and flotation $F_b = \rho_{\text{liquid}}\,V_{\text{sub}}\,g \qquad W_{\text{apparent}} = W - F_b \qquad \dfrac{V_{\text{sub}}}{V} = \dfrac{\rho_{\text{body}}}{\rho_{\text{liquid}}}$
Density of a mixture · Density of a mixture $\rho = \dfrac{m_1 + m_2}{V_1 + V_2} \qquad \rho_{\text{equal volumes}} = \dfrac{\rho_1 + \rho_2}{2} \qquad \rho_{\text{equal masses}} = \dfrac{2\rho_1\rho_2}{\rho_1 + \rho_2}$

Reference tables (1)

Hooke's law and springs4 rows

Part of the stress-strain curve	What happens
Up to the proportional limit	Straight line; Hooke's law holdsQ CDS 2019 (I): Hooke's law is valid only in the proportional region of the stress-strain curve.
Proportional limit to elastic limit	Returns to shape, but not in proportion
Beyond the elastic limit	Permanent stretch (plastic), then fracture
Spring constant k	Set by wire thickness, coil diameter and materialQ CDS 2016 (II): the spring constant depends on the thickness, the diameter and the material.

Watch out for (8)

The weight stays the same; the area changes
→ Pressure and pressure at a depth
Moving liquid is Bernoulli; still liquid is Pascal
→ Pressure and pressure at a depth
The body's own mass does not set the buoyant force
→ Buoyancy and flotation
Floating means equal, not greater
→ Buoyancy and flotation
g cancels in floating
→ Buoyancy and flotation
Equal volumes average, equal masses do not
→ Density of a mixture
Proportional limit comes before elastic limit
→ Hooke's law and springs
No single factor fixes the spring constant
→ Hooke's law and springs

PYQ weightage by concept

4 concepts · 9 PYQs — where the marks actually sit, so you know what to drill first

Pressure, Buoyancy and Elasticity9 PYQs · 100%

Concept	PYQs	Share
Buoyancy and flotation	4	44%
Pressure and pressure at a depth	2	22%
Hooke's law and springs	2	22%
Density of a mixture	1	11%

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