Stress and strain describe how materials respond when forces are applied to them. Stress is the force exerted per unit area, showing how much load a material is experiencing internally. Strain measures the resulting deformation — the fractional change in length compared to the material’s original length. Studying these quantities allows you to understand elastic behaviour, where deformation is reversible, and plastic behaviour, where permanent changes occur. The relationship between stress and strain, often expressed through Young’s modulus, helps compare how stiff or flexible different materials are. This topic forms the basis of analysing structural behaviour, material strength, and failure, making it an essential part of mechanics in A-level Physics.

Density and pressure are fundamental quantities used to describe how matter behaves in fluids and solids. Density measures how much mass is contained in a given volume, allowing you to compare how compact different substances are and to predict how they will interact — for example, whether an object will float or sink. Pressure is the force exerted per unit area, and it explains how forces are distributed in fluids and gases. In this topic you examine how pressure varies with depth in a liquid, how atmospheric pressure changes with height, and how density and pressure link together in fluid statics. These ideas are essential for understanding buoyancy, hydraulic systems, and many practical applications in engineering and geophysics.