Stresses and rigidity

Stresses: they are the physical demands that a body or object must withsand when one or more external forces are applied to it.

The basic stresses are:

Compression: this is caused by forces acting on a body that tend to flatten it or reduce its lenght or thickness. It happends in structures that need to support weights.

Tension: this is caused by forces acting on a body that tend to strecht it.

Bending: this is caused by forces acting on an element that make it curve or bend. It is done by supporting the two ends and applying force in the center. This is what happends to a shelf when we put lots of heavy books on it.

Torsion: this is caused by forces acting on a body that make it twist.

Shear: this is caused by two equal forces applied in opposite directions that act on lines of action close to each other. Shear stress will tend to cut the material if it cannot twist or bend.

Rigidity: it is an esential characteristic for any strong structure 

Exercise. Think about the following objects and state whether they are rigid, plastic or elastic:

BALLOON: Elastic.

EMPTY DRINKS CAN: Plastic.

WINDOW FRAME: Rigid.

CLAY: Rigid. 

CATAPULT: Elastic.

DOOR: Rigid.