Properties of matter: Class 9 Physics

Key points

Matter is anything that has mass and occupies space. It exists as solid, liquid and gas. The kinetic molecular model says that matter is made of tiny particles that are always moving and that attract each other. The state of matter depends on how strongly these particles are held together and how fast they move.

The three states compare as follows:

• Solid: particles are packed close in a fixed pattern and only vibrate about fixed positions. A solid has a definite shape and volume.
• Liquid: particles are close but can slide past each other. A liquid has a definite volume but takes the shape of its container.
• Gas: particles are far apart and move freely at high speed. A gas has no fixed shape or volume and fills its container.

Density is the mass per unit volume of a substance: ρ = m/V. Its SI unit is kg/m³. Also 1 g/cm³ = 1000 kg/m³. The density of water is 1000 kg/m³. Density helps identify materials. A body with higher density than a liquid sinks in it, and a body with lower density floats.

Pressure is the force acting normally on a unit area: P = F/A. Its SI unit is the pascal (Pa), equal to 1 N/m². For the same force, a small area gives a high pressure. This is why a knife has a sharp edge, why nails have pointed tips and why heavy vehicles use wide tyres or tracks.

A liquid at rest exerts pressure on the base and the walls of its container. At depth h in a liquid of density ρ, the pressure is P = ρgh. It increases with depth and with density, and does not depend on the shape of the container. Therefore dams are built thicker at the bottom.

The air around the Earth has weight and presses on everything. This is atmospheric pressure. At sea level it is about 101 325 Pa, which is roughly 1 × 10⁵ Pa. It is measured by a barometer. In a mercury barometer the air pressure holds up a column of mercury about 760 mm high. As we go up, atmospheric pressure falls, because the layer of air above becomes thinner.

Pascal's law: pressure applied to an enclosed fluid is transmitted equally and undiminished to every part of the fluid and to the walls of the container. The hydraulic press and the hydraulic jack use this law. A small force F₁ on a small piston of area A₁ makes a pressure F₁/A₁. The same pressure acts on a large piston of area A₂ and gives a force F₂ = F₁ × A₂/A₁. Car brakes and garage lifts work in this way. The work done is not increased, since the small piston moves a longer distance.

Archimedes' principle: when a body is partly or wholly immersed in a fluid, it feels an upward force equal to the weight of the fluid displaced. This upward force is called upthrust or buoyant force. Upthrust = ρ_fluid × V_displaced × g. The apparent weight in the fluid equals the true weight minus the upthrust.

Floatation: a body floats when its weight is balanced by the upthrust. A body floats if its average density is less than that of the liquid. A steel ship floats because its hollow shape displaces a large volume of water, so its average density is less than that of water. A submarine floats or sinks by taking water into its tanks or pushing it out. A hydrometer measures the density of liquids, and it floats higher in denser liquids.

Elasticity is the property of a body by which it returns to its original shape and size after the deforming force is removed. Steel and rubber are elastic. A body that does not regain its shape, such as clay or putty, is plastic. The elastic limit is the largest deformation from which a body can still recover fully. Beyond it the body is permanently changed.

Hooke's law: within the elastic limit, the extension of a spring or wire is directly proportional to the applied force: F = kx. The constant k is the spring constant, in N/m. A graph of force against extension is a straight line through the origin up to the elastic limit.

Stress is the force per unit area acting on a body: stress = F/A, in N/m² or Pa. Strain is the change in length divided by the original length: strain = ΔL/L, and it has no unit. Within the elastic limit, stress divided by strain is a constant called Young's modulus: Y = (F/A)/(ΔL/L). It measures how stiff a material is. Steel has a much larger Young's modulus than rubber.

Study tip: For density and pressure numericals, change all lengths to metres and masses to kilograms first. Remember that 1 g/cm³ equals 1000 kg/m³. For explanation questions give the law, the formula and one daily-life use. Draw a neat labelled diagram for the hydraulic press and for a floating body.

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Common mistakes

• Not converting g/cm³ or cm² to SI units before using the formulas.
• Saying a body sinks because it is heavy. It sinks because its density is greater than that of the liquid.
• Using the shape of the container in P = ρgh. Only depth, density and g matter.
• Confusing mass and weight when using Archimedes' principle. Upthrust equals the weight of the fluid displaced.
• Giving strain a unit. Strain is a ratio and has no unit.

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