Forces make things move, but they can also stretch things,
squeeze them, and change their shape. A rubber ball changes shape when you use
force to squeeze it, but it returns to its original shape when you stop
squeezing. Materials that do this have elasticity. They are made up of
particles called molecules that can stretch apart. Other materials, such as
modelling clay, change shape easily when a force is applied, but they do not
return to their original shape when the force is no longer applied. These
materials have PLASTICITY.
A trampoline is made of stretchy rubber fastened to a metal frame by
metal springs. When you land on a trampoline, you stretch the rubber and the
springs. Both rubber and springs are elastic. As they return to their original
shape, they pull back upwards and push you into the air.
Materials have plasticity when they are easily moulded
into shape and do not return to their original shape when the moulding force is
removed. When we talk about plastics, we usually mean various colourful
materials that have been made out of chemicals produced from oil. In fact, the
word plastic applies to any material that can be easily moulded into different
shapes. Even metals can be plastic because, if heated, they soften and can be
shaped.
