NEET · Physics
Own the mechanical solids-and-fluids portion of official Unit 7: elasticity, fluid pressure, Pascal law, viscosity, Stokes/terminal velocity, streamline/turbulent flow, Bernoulli and surface tension/capillarity. Thermal content is carved out to Thermal Properties.
In short
This Rank Sarthi route owns the mechanical part of official Unit 7. It covers stress-strain and elastic moduli, fluid pressure and Pascal law, viscosity and terminal velocity, flow/Bernoulli relationships, surface tension, excess pressure and capillary rise. Temperature, expansion, calorimetry, phase change and heat transfer belong to Thermal Properties.
Subject faculty
Also in Physics: Kumar Gandharva Saxena
Stress describes applied force per area; strain describes relative deformation. In the linear elastic region, modulus ratios relate stress to strain. Hooke-law language must remain inside the regime where stress is proportional to strain.
Pressure in a fluid acts normally to surfaces. Hydrostatic pressure change follows density, gravity and depth in the simple constant-density model. Pascal's principle describes pressure transmission in a confined fluid and supports hydraulic applications.
Viscosity describes internal resistance to relative motion in fluid layers. Stokes' drag and the familiar terminal-speed formula apply to a small sphere in the low-Reynolds-number/laminar regime with the associated idealizations. They are not universal drag laws.
Bernoulli's relation applies along a streamline for an ideal steady incompressible non-viscous flow model. Surface tension acts tangentially along a liquid surface and creates pressure differences across curved interfaces. Drop and soap-bubble formulas differ because a soap bubble has two liquid-air surfaces.
Official Unit 7 continues into heat, temperature, thermal expansion, specific heat, calorimetry, phase change and heat transfer. Those relationships belong to /neet/physics/thermal-properties so this page does not duplicate them.
Normal stress: σ = F_perp/A. Pa = N m^-2. Scalar magnitude for simple normal loading; sign convention should be stated if tension/compression signs are used.
Normal stress
Use when — Pa = N m^-2. Scalar magnitude for simple normal loading; sign convention should be stated if tension/compression signs are used.
Longitudinal strain: ε = ΔL/L. Dimensionless; small-deformation model.
Longitudinal strain
Use when — Dimensionless; small-deformation model.
Young modulus: Y = stress/longitudinal strain. Pa. Linear elastic region only.
Young modulus
Use when — Pa. Linear elastic region only.
Bulk modulus: B = -ΔP/(ΔV/V). Pa. Minus sign reflects decrease in volume for positive pressure increase under the common convention.
Bulk modulus
Use when — Pa. Minus sign reflects decrease in volume for positive pressure increase under the common convention.
Hydrostatic pressure difference: ΔP = ρ g h. Pa. Constant-density fluid, uniform g, depth difference h.
Hydrostatic pressure difference
Use when — Pa. Constant-density fluid, uniform g, depth difference h.
Stokes drag: F_d = 6πηrv. N. Sphere of radius r moving slowly in viscous fluid; low-Reynolds-number Stokes regime.
Stokes drag
Use when — N. Sphere of radius r moving slowly in viscous fluid; low-Reynolds-number Stokes regime.
Terminal speed sphere: v_t = 2r^2(ρ_s-ρ_f)g/(9η). m s^-1. Small sphere, Stokes regime, steady terminal motion, buoyancy included, ρ_s>ρ_f for downward terminal fall.
Terminal speed sphere
Use when — m s^-1. Small sphere, Stokes regime, steady terminal motion, buoyancy included, ρ_s>ρ_f for downward terminal fall.
Bernoulli: P + (1/2)ρv^2 + ρgh = constant. Energy per volume; steady incompressible non-viscous flow along a streamline under gravity.
Bernoulli
Use when — Energy per volume; steady incompressible non-viscous flow along a streamline under gravity.
Excess pressure drop: ΔP = 2T/r. Single spherical liquid-air interface, e.g. droplet; T N m^-1.
Excess pressure drop
Use when — Single spherical liquid-air interface, e.g. droplet; T N m^-1.
Excess pressure soap bubble: ΔP = 4T/r. Soap bubble with two surfaces.
Excess pressure soap bubble
Use when — Soap bubble with two surfaces.
Capillary rise/depression: h = 2T cosθ/(ρ g r). Circular capillary of radius r; static equilibrium and ideal wetting geometry; sign follows cosθ.
Capillary rise/depression
Use when — Circular capillary of radius r; static equilibrium and ideal wetting geometry; sign follows cosθ.
Concept being tested: Stokes terminal speed
Prompt: A small sphere falls through the same viscous liquid under Stokes conditions. If its radius doubles while density difference and viscosity stay the same, how does terminal speed change?
Decisive condition: The terminal-speed formula under the stated model has v_t ∝ r^2.
Method choice: Use the ratio (2r)^2/r^2.
Reasoning/calculation: Terminal speed becomes 4 times the original value.
Unit/sign/condition check: The conclusion is dimensionless ratio; it is valid only while both cases remain in the Stokes regime and other parameters are unchanged.
PI if answered incorrectly: Decision / Selection Error if the proportionality is used outside the model conditions.
Using f=6πηrv at high-speed/turbulent conditions
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Check the Stokes/low-Reynolds model.
Using 2T/r for a soap bubble
Recall gapWhy it happens
Preparation Intelligence classification: Recall Gap.
How it is corrected
A soap bubble has two surfaces, giving 4T/r.
Applying Bernoulli despite strong viscosity/energy loss without qualification
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Check the ideal steady-flow assumptions.
Treating Hooke behavior as valid beyond the linear elastic range
Knowledge gapWhy it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
State the linear-region condition.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
The mechanical solids-and-fluids portion of official Unit 7.
For slow motion of a sphere in the low-Reynolds-number viscous-flow regime.
Because a soap bubble has two liquid-air surfaces.
Official scope is taken from the NEET (UG) 2026 syllabus and official NTA documents; concept structure follows current NCERT contents. No performance, weightage or question-frequency data is asserted.
