Some materials spring back — Physical education, 7–10 years
Elastic materials change shape when pushed or pulled, then return when the force stops. This is different from a material that stays bent or stretched.
A spring-back shape
Stretch a rubber band and it becomes longer; let go and it pulls back towards its old shape. Squash a sponge and it rises again. This spring-back behaviour is called elasticity, as long as the material has not been stretched or squeezed too far.
Why materials spring back
When we bend, stretch or squash an elastic object, we give its parts a changed arrangement. The material pushes or pulls back because its parts tend to return to their earlier arrangement. People learned to use this useful behaviour in springs, shoes and protective equipment.
Testing a rubber band
A rubber band is 8 centimetres long before pulling. With a small weight, it becomes 11 centimetres, so it stretched 3 centimetres. Remove the weight and it returns to 8 centimetres. If a much heavier weight leaves it at 14 centimetres, the change may be permanent.
Not every bend springs back
It is tempting to call anything that changes shape elastic. A paper clip bends back a little, so it may seem like a rubber band, but bending it repeatedly can leave it bent. Elasticity has limits: a force that is too large can damage the material instead of making a temporary change.
Elasticity around us
Elasticity helps a trampoline push a jumper back up and lets a shoe sole bend while walking. Bicycle suspension uses springs to soften bumps. These objects must be designed for the forces they meet; an elastic part that is overloaded can stop working safely.
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