## Glossary for “Structures – or why things don’t fall down” by J.E. Gordon

` A biscuit is stiff but weak, steel is stiff but strong, nylon is flexible (low E) and strong, raspberry jelly is flexible (low E)`

• stiffness:
• (p 50) the slope of a stress-strain diagram measures the elastic stiffness or floppiness of a given solid. This is also known as Young’s modulus of elasticity or “elastic modulus”.

Stiffness = E = stress / strain [SI: MN/m^2]

The stiffness of a material is the stress for which the length of the material would double (if the material doesn’t break by then).

Stiff (high E) vs Flexible (low E)

• strain:
• (p 49) How far the atoms at any point in a solid are being pulled apart – that is by what proportion the bonds between the atoms are stretched.

Strain = (increase of length) / (original length) [no measurement units – sometimes it is conveyed as %]

• strength
• (p 56) The stress required to break a piece of the material itself. A lot of times we are concerned with the “ultimate tensile stress” which is the “tensile strength” of a material determined by breaking a small test-piece in a testing machine.

Strong (withstands high stress) vs Weak (breaks at low stress)

• stress
• (p 46) A measure of how hard the atoms in a material are being pushed together or pulled apart as a result of external forces. Unlike the concept of pressure stress in a solid is often a directional or one-dimensional affair (for most discussions).

Stress = (load or force applied) / (cross-sectional area) [SI: MN/ m^2]

1 Meganewton ~ 100 tons force