What are forces?

When you stand on a ladder, your weight is pressing down on to it.  This is referred to as force.  The force is known as an external force.
In order for the ladder to hold your weight, there is another force inside the ladder that exerts pressure to prevent the ladder from collapsing.  This force is known as an internal force.  These internal forces work against the external force, and prevents the ladder from collapsing.   If the external force is greater than the internal force then the ladder will collapse.  In other words if you are too heavy for the ladder, it will collapse.
A successful structure should be able to withstand all the forces that it is exposed to, without collapsing or toppling over.  We must therefore understand the different kinds of forces which can have an effect on and within a structure.  There are five different types of forces.  Remember that not all forces are the same.  Forces can press, twist, bend and pull.
The five forces that we are going to learn about include tension force, compression force, torsion force, bending force and shear force.