An understanding of force and how it affects an object or material is important in construction of any kind. The three primary types of mechanical forces are pulling, pushing and shear. Tension, or tensile force, is an example of a pulling force and is typically measured in pounds (lbs) or Newtons (N). Tension force plays a role in many physics, mechanical engineering, and civil engineering applications.
An effective way to help understand this mechanism is to use the example of a rope or cable. A rope cannot be used to push an object across a flat surface. Strings, ropes, cables and chains are used in instances where a pull force is required. Hanging a weight onto the end of rope will pull on the rope. The pull force created by the weight is called the tension force.
In this example, tension force will act on opposite ends of the rope and pull it tight. The force is applied in the direction of the rope. Objects on both ends of the rope will experience a pulling force equal to the tension force. Similarly, mechanical components used to support and reinforce bridges and buildings are commonly exposed to this type of force. These include objects such as cables, wires, support columns and beams.
Tension loading will cause a material to elongate or stretch. A very flexible object, such as a rubber band, will stretch a great deal when a tension force is applied. Less flexible materials, like plastic and steel, also elongate when pulling force is applied, but in much smaller amounts.
Force and motion are related through Newton’s First Law of Motion. This law states that a body will remain at rest, or in uniform motion, unless it is compelled to change that state by a force applied to it. A tension force will cause an object to move through pulling action. Newton’s law is illustrated using a simple example of a child's wagon at rest on a flat surface. The wagon will remain at rest until an external force is applied to the handle, causing the wagon to move.
When a force is applied, it will cause internal stresses in the material. If the force is high enough, the internal stress may become excessive and cause permanent elongation or complete failure to occur. Understanding the stress created by an applied tension force is very important when selecting materials for mechanical engineering and design applications. The applied force must be sufficiently low to avoid internal stresses that can cause permanent deformation or failure to occur.