GCSE SCIENCE COURSEWORK:

HOOKE'S LAW

INTRODUCTION

As it is known, different materials have different properties. They act differently under different circumstances. There are a number of properties of matter which can be explained in terms of molecular behaviour. Among these properties is elasticity.

Intermolecular forces: these are electromagnetic forces between molecules. The strength and direction of these forces differ in accordance to the separation of the molecules.

Materials are often subjected to different forces. Forces can be distorting, that is they can alter the shape of a body. Two distorting forces I shall look at are tension and compression.

Tension/tensile stress, more generally referred to as stretch, happens when external forces (larger red arrows) act on a body so that different parts of that body are pulled to go in different directions. In most materials, the intermolecular force (smaller aqua arrows) of attraction shows resistance to these external forces, so that once the external forces have abated, the body resumes its original shape/length.

Compression/compressive stress, more generally referred to as squashing, happens when external forces act on a body of material so that different parts of that body are pushed in towards the centre of the body. In most materials, the intermolecular force of repulsion acts against these external forces, so that when the distorting force is removed, the molecules return to their original arrangement and spacing.

Materials that do this are known to have the property of elasticity. In short, elasticity is the ability of a material to return to its original shape and size after distorting forces (i.e. tension and/or compression) have been removed. Materials which have this ability are elastic; those which do not have this ability are considered plastic.

This always happens when the distorting force is below a certain size (which is different for each material). This point where the body will no longer return to its original shape/size (due to the distorting force becoming too large) is known as the elastic limit (which differs from material to material). As long as the distorting force is below this size, the body that is under the external forces will always return to its original shape.

As the body is put under more and more stress (distorting force), the body strains (deforms, extends) more and more. Right up to the elastic limit, the body will continue straining, in accordance to the size of the stress. This is where Hooke's Law comes in.

Hooke's Law states that, when a distorting force is applied to an object, the strain is proportional to the stress. For example, if the load/stress is doubled, then the extension/strain would also double. However, there is a limit of proportionality (which is often also the elastic limit), only up to which Hooke's Law is true.
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Since the strain is proportional to the stress for different materials where Hooke's Law is true, then there should be a fixed ratio of stress to strain for a given elastic material. This ratio is known as its Young's Modulus. Young's Modulus can be calculated from the stress and the strain of an object under tensile/compressive stress.

e = change in length/extension of object, in cm

p = original length of object, in cm

a = cross-sectional area of object, in cm2

f = size of force applied, in newtons

For example, ...

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