PA04 — Properties of Matter, Fluids & Sound

✈ Physics – PA04 · AFCAT General Awareness AFCAT Level

This chapter covers why ships float, how hydraulic machines work, why sound doesn’t travel in space, and what makes your voice sound different in a crowded hall. AFCAT tests Archimedes’ principle, Pascal’s law, and basic sound characteristics — all Class 10 level and highly scoring.

📌 AFCAT Focus: Archimedes’ principle (buoyancy), Pascal’s law (hydraulic machines), speed of sound (330 m/s in air; fastest in solids), pitch vs loudness, echo minimum distance (17 m), and Doppler effect (qualitative: siren pitch changes as ambulance passes).

1. Pressure & Pascal's Law

Pressure Formulae:

● Pressure P = F/A [Unit: Pascal (Pa) = N/m²]
● Pressure increases with depth: P = ρgh (ρ = fluid density, h = depth)

Pascal's Law: Pressure applied to an enclosed fluid is transmitted equally in all directions.
● Application: Hydraulic press, hydraulic brakes, hydraulic jack
● Principle: F₁/A₁ = F₂/A₂ → Small force on small piston = large force on large piston

2. Archimedes’ Principle & Buoyancy

Fig. 1 — Archimedes' Principle and the Three Conditions of Floatation

💡 Ship Floats Because: A steel ship is hollow — it displaces a large volume of water. The average density of the ship (steel + air inside) is LESS than water. So buoyant force ≥ weight → floats. If water floods the ship, average density increases and it sinks.

3. Surface Tension & Viscosity

💧 Surface Tension

Force per unit length at a liquid surface
Caused by cohesive forces between surface molecules
Decreases with temperature and on adding soap/detergent
Examples: water droplets are spherical (minimum surface area); insects walk on water; soap bubbles; capillary rise in plants
Cohesion = force between like molecules (water-water)
Adhesion = force between unlike molecules (water-glass)

🐛 Viscosity & Terminal Velocity

Viscosity = resistance to flow of a fluid (internal friction)
High viscosity: honey, glycerine; Low: water, air
Liquids: viscosity decreases with temperature (honey flows faster when warm)
Gases: viscosity increases with temperature
Terminal velocity: when viscous drag + buoyancy = weight → constant velocity
Parachute: large area → large drag → low, safe terminal velocity

4. Sound Waves

Fig. 2 — Speed of Sound in Different Media and Sound Characteristics

Echo & Doppler Effect:

● Echo condition: Reflected sound must return after at least 0.1 second
    Minimum distance for echo = 330 × 0.1 / 2 = 16.5 m ≈ 17 m

● Doppler Effect: Change in apparent frequency due to relative motion
    Source approaching → pitch appears higher
    Source moving away → pitch appears lower
    Example: Ambulance siren sounds higher-pitched as it approaches, lower as it passes

📝 AFCAT PYQs — Properties of Matter & Sound

Q1. Archimedes' principle is related to: AFCAT PYQ

(a) Pressure of gases(b) Buoyancy of liquids(c) Surface tension(d) Viscosity

✔ Answer: (b) Buoyancy of liquids

Archimedes' Principle: Any body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. If buoyant force ≥ weight → floats. Used to explain why ships float, submarines dive, hot air balloons rise, and to find the density of irregular objects.

Q2. The pitch of sound depends on its: AFCAT PYQ

(a) Amplitude(b) Wavelength(c) Frequency(d) Speed

✔ Answer: (c) Frequency

Pitch is the psychological perception of how high or low a sound is — it is directly determined by frequency. High frequency = high pitch (shrill). Low frequency = low pitch (bass). Amplitude determines loudness; waveform determines quality. This is the most directly tested sound characteristic in AFCAT.

Q3. Hydraulic machines work on the principle of: AFCAT PYQ

(a) Archimedes' Principle(b) Bernoulli's Principle(c) Pascal's Law(d) Boyle's Law

✔ Answer: (c) Pascal's Law

Pascal's Law: Pressure applied to an enclosed fluid is transmitted equally in all directions. Hydraulic press, hydraulic brakes, and hydraulic jacks use this — a small force applied on a small piston creates pressure transmitted to a large piston, giving a proportionally larger force.

Q4. Sound travels fastest through: AFCAT PYQ

(a) Vacuum(b) Air(c) Water(d) Steel

✔ Answer: (d) Steel

Sound speed in media: Solid > Liquid > Gas. In steel: ~5100 m/s; in water: ~1500 m/s; in air: ~332 m/s. Sound cannot travel at all through vacuum (needs a medium). This ordering is because solids are most rigid and transmit vibrations most efficiently. Sound CANNOT travel in space (vacuum).

🧠 Quick Memory Chart — PA04

💧 Buoyancy

Floats: ρ_obj < ρ_fluid
Sinks: ρ_obj > ρ_fluid
Neutral: ρ_obj = ρ_fluid
Archimedes: F_b = ρVg
Ship floats: avg density < water

🎵 Sound

Speed: Solid > Liquid > Gas
In air: 330–346 m/s
Cannot travel in vacuum
Pitch ↔ Frequency
Echo: min 17 m distance

⚡ Doppler & Viscosity

Approaching → higher pitch
Moving away → lower pitch
Viscosity of liquid ↓ with heat
Terminal velocity: drag = weight
Parachute: big area → small velocity

📝 Practice Exercise

E1. The minimum distance from a wall needed to hear an echo (speed of sound = 330 m/s) is:

(a) 33 m(b) 17 m(c) 66 m(d) 10 m

E2. A piece of ice floats in water with 90% submerged. The density of ice is approximately:

(a) 1000 kg/m³(b) 900 kg/m³(c) 500 kg/m³(d) 100 kg/m³

E3. The loudness of sound depends on its:

(a) Frequency(b) Wavelength(c) Amplitude(d) Speed

Answers: E1 → (b) 17 m [d = v×t/2 = 330×0.1/2 = 16.5 ≈ 17 m] | E2 → (b) 900 kg/m³ [Fraction submerged = ρ_obj/ρ_fluid = 0.9 → ρ_ice = 900 kg/m³] | E3 → (c) Amplitude [Greater amplitude = more energy = louder sound]

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