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UPPCS Physics Gap Sheet: Fluids and Material Properties.
UPPCS has asked 19 of these. 10 have not been asked yet. Lock the first list, then read the second: it holds what UPPCS has not used yet.
First, the concept
Fluid pressure acts in every direction and increases with depth. Buoyancy depends on the weight of displaced fluid. A hollow metal ship can float because its average density, including its air spaces, is low.
Surface tension makes a liquid surface behave like a stretched film. Viscosity resists flow within a fluid. Capillary rise depends on attraction between the liquid and the tube, so different liquids behave differently.
✅ Already asked (19)
Pressure and buoyancy
- Archimedes principle · Buoyant force equals the weight of fluid displaced.
- Floating iron ship · Its hollow shape lowers its average density below that of water.
- Ship from river to sea · It rises slightly because seawater is denser than river water.
- Bucket lifted from water · It feels heavier as the supporting buoyant force decreases.
- Floating ice melting · In fresh water, melting floating ice leaves the water level unchanged.
Surface effects
- Small spherical drops · Surface tension tends to minimise surface area; large falling drops can deform.
- Soap and cleaning · Soap lowers water's surface tension and helps wet dirty surfaces.
- Wet brush hairs · Surface tension pulls the hairs together when a wet brush leaves water.
- Pressure in a soap bubble · Internal pressure exceeds external pressure; smaller bubbles have greater excess pressure.
Bulk properties and flow
- Water at 4°C · Pure water at ordinary pressure has maximum density near 4°C.
- Density and sample size · At fixed conditions, increasing the amount of a uniform substance does not change its density.
- Viscosity of warm oil · Liquid viscosity generally decreases as temperature rises.
- Bernoulli principle · In steady ideal flow along a horizontal streamline, faster flow has lower pressure.
- Elasticity of steel · Steel has a higher Young's modulus than rubber and strains less under the same stress.
- Fountain pen in a plane · Lower outside pressure can let trapped air expand and push ink out.
More fluid applications
- Oil spreading on water · Spreading depends on the balance of air-water, air-oil and oil-water surface tensions, not density alone.
- Syringe · Pulling the plunger reduces internal pressure and draws fluid in.
- Submarine buoyancy · Taking water into ballast tanks increases average density; expelling it helps ascent.
- Compressibility · Gases are much more compressible than ordinary liquids because their particles are farther apart.
❌ Not asked yet (10)
Items marked 'asked in other exams' have come in UPSC or other state exams, not yet in UPPCS.
🔒 10 items not asked yet
Pressure and buoyancy: 2 · Surface effects: 3 · Bulk properties and flow: 1 · More fluid applications: 4
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⚠️ Density is not the same as viscosity. An iron ship floats by buoyancy; a gently placed needle can be supported by surface tension.
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