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Tension is often used to label the pull exerted on an object by a string, cable, wire, thread, rod or similar device. It is also used to refer to the force transmitted through (in, by, on) a string to imply the same amount of force is applied to objects at either end. Tension may also refer to the state or stress of an object being stretched.
Tension is not a fundamental force; it results from electromagnetic interactions among particles of an object being extended or compressed. Therefore, tension is conceived to "reside" in the device being either stretched or compressed, hence the often-used term, "tension in the string." Cohesive forces bind particles to resist separating when an object is stretched. The totality of these forces are experienced as tension.
Real strings and rods have weight and can deform. These properties complicate a situation and may distract from the concept being taught. Simplifying assumptions or approximations are usually inserted into introductory textbook questions. An ideal string is a one-dimensional continuous object, while an ideal rod is a similar three-dimensional object. Ideal strings or rods have no mass and do not stretch, compress or deform. Further, when a force is applied to one end of the string or rod, the same force is transmitted to the other end.