## Overview This topic introduces the behaviour of different solids under stress and introduces the concepts of stress, strain and Young modulus. The work done deforming a solid is related to the strain energy stored. The behaviour under stress for metals, brittle materials and rubber are compared. ## Working Scientifically The specified practical work in this topic gives learners the opportunity to know and understand how to use a wide range of experimental and practical instruments, equipment and techniques appropriate to the knowledge and understanding included in this specification; to safely and correctly use a range of practical equipment and materials; to make and record observations; to present information and data in a scientific way; to use appropriate analogue apparatus to record a range of measurements of length and to interpolate between scale markings; to use callipers and micrometers, using digital or vernier scales. ## Mathematical Skills There are a number of opportunities for the development of mathematical skills in this unit. These include recognising and making use of appropriate units in calculations; using an appropriate number of significant figures; making order of magnitude calculations; identifying uncertainties in measurements and using simple techniques to determine uncertainty; translating information between graphical, numerical and algebraic forms; determining the slope and intercept of a linear graph; understanding the possible physical significance of the area between a curve and the x-axis and to be able to calculate it or estimate it by graphical means; calculating areas of triangles, circumferences and areas of circles and volumes of cylinders. ## How Science Works There are opportunities within this topic for learners to use theories, models and ideas to develop scientific explanations; to use appropriate methodology, including ICT to answer scientific questions and solve scientific problems; to communicate information and ideas in appropriate ways using appropriate terminology; to consider applications and implications of science and evaluate their associated benefits and risks. Learners can investigate the behaviour of different solids under stress, for example, metals, rubber, polythene and consider such practical applications as adding carbon to rubber tyres, pre-stressing concrete and the behaviour of building materials under compression and tension. ### Learners should be able to demonstrate and apply their knowledge and understanding of: (a) Hooke’s law and use F kx = where the spring constant k is the force per unit extension (b) the ideas that for materials the tensile stress, F A  = and the tensile strain, l l   = and the Young modulus, E   = when Hooke’s law applies (c) the work done in deforming a solid being equal to the area under a forceextension graph, which is 1 2 Fx if Hooke’s law is obeyed (d) the classification of solids as crystalline, amorphous (to include glasses and ceramics) and polymeric (e) the features of a force-extension (or stress-strain) graph for a metal such as copper, to include • elastic and plastic strain • the effects of dislocations, and the strengthening of metals by introducing barriers to dislocation movement, such as foreign atoms, other dislocations, and more grain boundaries • necking and ductile fracture (f) the features of a force-extension (or stress-strain) graph for a brittle material such as glass, to include - elastic strain and obeying Hooke’s law up to fracture - brittle fracture by crack propagation, the effect of surface imperfections on breaking stress, and how breaking stress can be increased by reducing surface imperfections (as in thin fibres) or by putting surface under compression (as in toughened glass or prestressed concrete) (g) the features of a force-extension (or stress-strain) graph for rubber, to include - Hooke’s law only approximately obeyed, low Young modulus and the extension due to straightening of chain molecules against thermal opposition - hysteresis [[Specified Practical Work]] - Determination of Young modulus of a metal in the form of a wire - Investigation of the force-extension relationship for rubber