In physics, tension describes the pulling force exerted by each end of a string, cable, chain, or similar one-dimensional continuous object, or by each end of a rod, truss member, or similar three dimensional object. At the atomic level, tension is produced when atoms or molecules are pulled apart from each other and gain electromagnetic potential energy. Each end of a string or rod under tension will pull on the object it is attached to, to restore the string/rod to its relaxed length.Tension is the opposite of compression. Although not physics terms, slackening and tensioning are used when talking about fencing, for example.In physics, although tension is not a force, it does have the units of force and can be measured in newtons (or sometimes pounds-force). The ends of a string or other object under tension will exert forces on the objects to which the string or rod is connected, in the direction of the string at the point of attachment. These forces due to tension are often called "tension forces." There are two basic possibilities for systems of objects held by strings: either acceleration is zero and the system is therefore in equilibrium, or there is acceleration and therefore a net force is present.