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 Dyne. The unit of force in USCS is pound-force. 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. Compression force
3. Shear force
Tensile force that tends to stretch a material.
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.
2. Compression force
Compression force (compressive force) occurs when a physical force presses inward on an object, causing it to become compacted. In this process, the relative positions of atoms and molecules of the object change. This change can be temporary or permanent depending on the type of material receiving the compressive force. There can also be different results depending on the direction or position on the object that the compressive force is applied.
3. Shear force
Shear forces are unaligned forces pushing one part of a body in one specific direction, and another part of the body in the opposite direction. When the forces are aligned into each other, they are called compression forces. An example is a deck of cards being pushed one way on the top, and the other at the bottom, causing the cards to slide. Another example is when wind blows at the side of a peaked roof of a house - the side walls experience a force at their top pushing in the direction of the wind, and their bottom in the opposite direction, from the ground or foundation.
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