Forces and couples generated by various elements
Forces and couples generated by various elements: The simplest element is a string that can apply a tension. However a string can only pull by the tension generated in it but not push. For example, a string holding a weight W will develop a tension T = W in it so that the net force on the weight is a tension T pulling the weight up and weight W pulling it down. Thus if the weight is in equilibrium, T = W . This is shown in figure 1.
The second kind of force that is applied when two elements come in contact is that applied by a surface. A smooth surface always applies a force normal to itself. The forces on a rod and on a box applied by the surface are shown in figure 2. Thus as far as the equilibrium is concerned, for an object on a smooth surface, the surface is equivalent to a force normal to it.
Imagine what would have happened had the force by the surface not been normal. Then an object put on a surface would start moving along the surface because of the component of the force along the surface. By the same argument if there is a smooth surface near an edge, the force on the surface due to the edge (and by Newton's IIIrd law the force on the edge due to the surface) will be normal to the surface. See figure 3.
On the other hand if the surface is not smooth, it is then capable of applying a force along the surface also. This force is due to friction.