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way in which a material flows when subjected to various types of stress is a branch of science called rheology. Rheology is important to many of the factors that surround us in life. Common are the oils with which our cars are lubricated. Another is the non-drip paints that we use to decorate our houses. There are a number of different behaviours. Materials that have a linear relationship between stress and strain are Newtonian,[link widoczny dla zalogowanych], the most famous example of which is water. A material that gets more fluid with time at a constant sheer rate is called thixotropic. Quicksand is an example. A third type of behaviour is called pseudoplastic; that is a material that gets more fluid with greater rate of sheer. This type of behaviour is typical of paint.
Viscoelestic materials combine the rheological properties of thixotropy and pseudoplasticity described above with elasticity. Their overall behaviour is to exhibit time dependent strain. A perfect example is a door closing mechanism that combines a spring which is attempting to close the door,[link widoczny dla zalogowanych], with a damper that is resisting the door being closed.
With a memory foam mattress, if you press your hand into it,[link widoczny dla zalogowanych], it will gradually mould itself to the shape of your hand and when you take your hand away its impression will remain. This is its viscous behaviour. Then over time it will return to its original shape. This is its elastic behaviour.