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Physics
How are objects deformed?

When an object is deformed, the position of its particles changes. If a body is compressed,
the distance between its molecules decreases as the repulsive forces become greater than
the attraction forces trying to return the molecules to their initial position. If molecules do
return to their initial position, this deformation is called an elastic deformation. Elastic
materials include rubber, steel, wood, cork and PVC (a common material widely used in
every aspect of modern life).
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Sometimes an object may not be able to return to its initial conditions, depending on
the magnitude of forces acting on the body, the type of material and the new positions
of particles. In this case, the material is said to have undergone a plastic deformation.
Examples of plastic (malleable) materials include wet clay, dough, lead, plastic and melflex
(a material used in shoe production). In the image below you can see the position of
molecules in 3 conditions:

elastic elastic elastic elastic
forces forces forces forces

Non-deformed condition Compressed condition Extended condition

REMEMBER! When the shape or size of objects changes, this is called deformation (from
the Latin deformatio, meaning ‘distortion’).

Types of deformation: extension, compression, bending, torsion and shear.

Elastic deformations are those that disappear after the external forces stop acting.
Plastic deformations are those that do not disappear after the external forces stop acting.

INV Explore the extension of an elastic band under the effect of a suspended load.

1. Determine the length of the band and the height at which one end should be fixed so
that a load attached to the other end can fall freely without hitting the ground.
2. Draw the approximate positions of the molecules in the band when it is not deformed
and when it is subject to some deformation. Write explanations to accompany your
images, describing the state of the object.

OVER TO YOU!

1. Provide three examples of different kinds of deformation in daily life.
2. Explain why each example is an elastic or plastic deformation.

EXT Different civilizations have traditionally used different materials to build bridges
over rivers or other geographical features. The choice of materials depended on
the purpose to which they were being put and their availability. Find some images
of bridges. What mechanical properties do you think the basic materials used in
bridge-building should have?

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