3RD POST

STRESS 

The basic stresses that a body may have to support are compression, tension, bending, torsion and shear .

Basic stresses:

Compression: This is caused by forces acting on a body that tend to flatten it or reduce its length or thickness. It happens in structures that need to support weights. For example, when we lean on a table, it undergoes compressive stress; columns are also subject to compressive stress.

Tension (or traction) : This is caused by forces acting on a body that tend to stretch it. For example , when two people pull on an object from opposite directions.

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 centre, as shown in the figure to the rights. This is what happens 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.For example, when two people hold  a towel at either end and twist it to wring it out ; a crank is also subjected to torsion.

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. For example, when we cut paper with a pair of scissors, or the point where a beam rests on a column.

RIGIDITY

If a material is rigid, it does not change shape when force is applied to it. By contrast, if the material does change shape, we say that it is deformable.

Exercise

12. Think about the following objects and state whether they are rigid, plastic or elastic: ballon, empty drinks can, window frame, clay, catapult, door.

Rigid                                  Plastic                    Elastic

window frame             empty drinks can          ballon

catapult                                                             clay

door