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Class 9–10 · Physics
States of matter and pressure
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States of matter and pressure
Lessons in this chapter
Lessons
1Particles and states
Solid particles vibrate near fixed positions; liquid particles move past neighbours; gas particles are widely separated. This explains shape, volume and compressibility. Heating usually increases average microscopic kinetic energy.
Key relationship
solid: fixed shape and volume; liquid: fixed volume; gas: neither
Worked example
Air in a closed syringe compresses much more readily than water.
Next lesson →2Elasticity and Hooke's law
Elastic materials recover their shape after force is removed. Within its linear range, a spring's extension is proportional to force. Stress is force per area and strain is fractional extension. Beyond limits, permanent deformation may occur.
Key relationship
F = kx; stress = F/A; strain = ΔL/L
Worked example
k = 200 N/m, x = 0.03 m ⇒ F = 6 N
Next lesson →3Density and relative density
Density is mass per volume. Relative density compares it with a reference, commonly water at a specified temperature for solids and liquids. The ratio has no unit. Consider both mass and volume when comparing objects.
Key relationship
ρ = m/V; relative density = ρobject/ρreference
Worked example
540 g / 200 cm³ = 2.7 g/cm³ = 2700 kg/m³
Next lesson →4Pressure on a surface
Pressure is perpendicular force divided by area. The same force over a smaller area produces higher pressure. A sharp blade concentrates force, while a wide strap spreads it.
Key relationship
p = F⊥/A; 1 Pa = 1 N/m²
Worked example
100 N / 0.02 m² = 5000 Pa
Next lesson →5Liquid and atmospheric pressure
Hydrostatic pressure increases with depth and density, independent of container shape. At equal depth in one connected stationary liquid, pressure is equal. Absolute pressure adds surface pressure to the liquid's gauge pressure. A barometer measures atmospheric pressure.
Key relationship
gauge p = ρgh; absolute p = p₀+ρgh
Worked example
Water at 2 m: 1000 × 9.8 × 2 = 19600 Pa gauge
Next lesson →6Pascal's principle
A pressure change applied to confined fluid is transmitted throughout it. A larger output piston gives greater force, but travels less far. An ideal hydraulic machine conserves work rather than creating energy.
Key relationship
F₁/A₁ = F₂/A₂; A₁s₁ = A₂s₂
Worked example
A₂ = 10A₁; F₁ = 50 N ⇒ F₂ = 500 N; 10 cm input travel ⇒ 1 cm output
Next lesson →7Buoyancy and Archimedes' principle
Greater pressure below an immersed object produces an upward resultant. Buoyant force equals the weight of displaced fluid. Apparent weight is true weight minus buoyancy. The principle also applies in gases.
Key relationship
Fb = ρfluid g Vimmersed; apparent weight = mg−Fb
Worked example
Water, immersed volume 0.002 m³ ⇒ Fb = 19.6 N
Next lesson →8Floating and sinking
A freely floating object displaces fluid of equal weight. Its average density includes hollow spaces, explaining steel ships. Moving it to denser fluid reduces the immersed volume needed for equilibrium.
Key relationship
floating: Vimmersed/Vtotal = ρobject/ρfluid
Worked example
Wood 600 kg/m³ in water 1000 kg/m³ ⇒ 60% of volume immersed
Next lesson →9Surface tension, viscosity and diffusion
Surface tension resists increasing liquid surface area. Viscosity resists relative motion between fluid layers. Diffusion is net spreading from higher to lower concentration through random particle motion. These are distinct from density and pressure.
Key relationship
surface tension unit N/m
Worked example
A carefully placed needle may be supported by water's surface; honey flows slowly because of high viscosity.
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Same force, half area: pressure?
p = F/A ⇒ double
2. Twice the depth in one liquid: gauge pressure?
p = ρgh ⇒ double
3. Buoyancy in floating equilibrium equals what?
Fb = weight = mg
4. Output piston area is five times input: ideal force ratio?
F₂/F₁ = A₂/A₁ = 5
5. Fraction submerged for 800 kg/m³ wood in water?
800/1000 = 0.8 = 80%
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.