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A rubber band will stretch if you pull it
Elastic is used to keep some of your clothes on
Rubber car tyres give you a comfortable ride
Fishing line will break if tugged too hard
You can sleep well on a bed that has good springs in it
Many ballpoint pens have springs in them
Bouncing on a trampoline is fun
If you drop a piece of putty on a concrete floor it does not bounce
A glass beaker will shatter if it is dropped but an aluminium one will bounce
Playing squash with a golf ball would be difficult

All these effects are connected to the elastic properties of materials.

Some materials will stretch easily while others are very strong. Some are flexible while others are brittle – they break without "giving". The use of different materials for different jobs depends on the elastic properties of a material.

One effect of a force on an object is to change its shape or size. The property of the material of the object that governs just what change will take place is called the ELASTICITY of the material.

Some materials are affected a lot by a force e.g., rubber, while others change their size very little e.g., steel. We are thinking here about pieces of equal size, of course.


Another important property of materials is their hardness. The following table shows the relative hardness of a number of materials. Ones which are hard will scratch ones that are softer.

Substance Hardness Substance Hardness Substance Hardness
Agate 6-7 Chromium 9 Penknife blade 6.5
Aluminium 2-3 Concrete 6-7.5 Sand 10.5
Amber 2-2.5 Copper 2.5-3 Silicon carbide 13
Asbestos 5 Diamond 15 Silver 2.5-2.7
Brass 3-4 Finger nail 2.5 Steel (mild) 4-5
Calcium 1.5 Glass 4.5-6.5 Tungsten carbide 8.5
Carborundum 9-10 Marble 3-4 Wood 1.5-4.5

Elastic and plastic materials

You can divide materials into two main types when you consider their elastic properties:
(a) those which will go back to their original length when the force is removed after stretching, these are called ELASTIC materials such as steel and rubber
(b) those which stay stretched after the force is removed- these are called PLASTIC materials such as putty or plasticene

We use elastic materials in our lives in a variety of ways:
Elastic in our clothes, elastic bands, springs in watches, beds, ball point pens, cars and trampolines to name just a few. You can probably think of others.

Hooke's Law

The more force that is put on to a piece of elastic or a spring the more it will stretch.
If you plot the extension against the force applied to a copper wire or a steel spring you will get a graph like the one drawn in Figure 1.

As the force is increased so the length of the sample increases and so the extension gets bigger. You will find that for every Newton increase in the force the length of the sample will increase by the same amount.

The extension is directly proportional to the force applied.

Force = constant x extension

This is known as Hooke’s Law.

If you stretch an elastic material too much it will not go back to its initial size and if it is stretched even more it will break.

Example problems
If a load of 5 N extends a spring by 4 cm then a load of 15 N will extend it by 12cm.

Remember that it is the extension that is proportional to the force and NOT the total length of the spring.

You can compare how much different materials will stretch using the following table. This shows the relative extensions for samples of the same length and diameter but of different materials using the same force.

Material Extension Material Extension
Diamond 1 mm Concrete 59 mm
Steel 5 mm Beech 67 mm
Copper 8 mm Oak 91 mm
Cast iron 9 mm Pine 83 mm
Aluminium 14 mm Plastic 500 mm
Granite 20 mm Nylon 500 mm
Lead 56 mm Rubber 500 m
Bone 56 mm 100 100
© Keith Gibbs 2013