3rd post

Stresses are the physical demands that a body or object must withstand when one or more external forces are applied to it

Basic stresses 

Compression is caused by forces acting on a body that tend to flatten it or reduce its lenght or thickness. It happens in strcutures 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 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 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 right. this is what happens to a shelf when we put losts of heavy books on it

Torsion 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 subjectd to torsion

Shear is caused by two equal forces applied in oppsite 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.

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

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

ballon : plastic
empty drinks can : rigid
window frame : rigid
clay : elastic
catapult : rigid
door : rigid