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A pair of forces is needed to change the shapeof a spring. If the spring is being squashed or

shortened, the forces are compressive. If thespring is being stretched, the forces are tensile.The thicker the wire, the greater thecompression and tension forces along its edges.

By plotting a graph of force against extension, the stifness of a springcan be found. Gradient of this graph is the force constant of thespring. If the graph is a straight line then extension x is directlyproportional to the force F :

where kis the force constant of the spring. The SI unit for the forceconstant is Nm-1. A stiffer spring will have a large value for the springconstant.A material obeys Hooke's law if the extension produced in it is proportional to the

applied force (load). This is true as long as the leastic limit of a material is not

exceeded.

After some point the graph will no longer be a straight line. The springhas become permanently deformed - it has reached its elastic limit.

Elastic potential energy orstrain energy is the energy in a deformed solid.

Types of force-extension graphs

Deformations of solidsGLOSSARY

Extension is the increase in length of the spring

Elastic limit the greatest stress that can be applied to a materialwithout causing permanent deformation

Stress forceapplied per unit cross-sectional area of the wireStrain fractional increase in the original length of the wire

Elastic deformation a material is elastic if it returns to its originalshape and size when the force is removed

Plastic deformation a material is plastic if it does not return to itsoriginal shape and size when the force is removed; permanentUltimate tensile stress value of the stress at which the materialbreaks; tells us about the stren th of a material

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Copper, pure iron and gold are ductile materials, which means that they can be drawn into wires. They arealso malleable, which means that they can be reshaped by hammering and bending without breaking.When stretched beyond the point E on the graph they retain a new shape.Rubber does not follow Hookes law and it remains elastic until it breaks. When the stretching force isremoved the rubber sample does not return to its original length by the same path (elastic hysteresis).The are between the two paths on the graph is the work done on the rubber that is not returned when thesrteching force is removed. This work becomes the internal energy in the rubber and causes a rise intemperature.

Glass and cast iron are brittle; it follows Hookes law until it snaps; they show elastic behavior up to thebreaking point.Kevlar is tough; it can withstand shock and impact.

Mild steel is durable; it can withstand repeated loading and unloading.

Diamond is hard; it cannot be easily scratched.