Forces

Forces

Structures are designed to withstand forces acting on them, but sometimes structures collapse or break down. For example, bridges are designed to carry the weight of traffic or trains crossing them. If a bridge collapses, a terrible accident could occur.

Chairs are designed for people to sit on, and although they can be made for use by even the heaviest person, they sometimes collapse if a person rocks about, or swings back on them.

Forces are not all the same. Forces can press, pull, twist or bend.

TENSION

If we take a material and try to pull it apart from either end, we are putting it into tension.

A tow rope has to resist the forces of tension.

Tension is a pulling force.

COMPRESSION

If we take a material or structure and try to squash it, we can say that it is in compression. Imagine an elephant sitting on a stool. The legs of the stool would be compressed. Compression is a squashing force.

TORSION

When you twist the top of a bottle or jar to undo it, you are using a torsion force. Torsion is a turning or twisting force. When you use a spanner to tighten or undo a nut, you are using torsion.

SHEAR

When you use a pair of scissors to cut a piece of paper you are using a shear force.

Shear is a force that is created when two opposite forces try to cut a material. If you cut a piece of metal with tin snips or bench shears, then the metal is said to be in shear; it is being sheared. Garden shears and lawn mowers use shear force.

BENDING

Bending is a force which is trying to bend a structure. If you were to build a bookshelf supported at either end by brackets and then load it with heavy books in the middle, the shelf would bend.

Bending is not quite as simple as this. If we look closely at the shelf, we will find that the top half of the shelf is being squashed (in compression), while the bottom half is stretched (in tension).