A force exerted on a body can cause a change in either the shape or the motion of the body. The unit of force in SI system is the newton (N) and CGS system is dyne. No solid body is perfectly rigid and when forces are applied to it, changes in dimensions occur. Such changes are not always perceptible to the human eye since they are negligible. For example, the span of a bridge will sag under the weight of a vehicle and a spanner will bend slightly when tightening a nut. It is important for civil engineers and designers to appreciate the effects of forces on materials, together with their mechanical properties of materials.
There are three main types of mechanical forces that can act on a body. They are:
1. Tensile force
2. Compressive force and
3. Shear force
Tensile force that tends to stretch a material, as shown in the figure 1 below.
Figure 1: Tensile force
For example,
1. Rubber bands, when stretched, are in tension.
2. The rope or cable of a crane carrying a load is in tension.
3. When a nut is tightened, a bolt is under tension.
A tensile force will increases the length of the material on which it acts.