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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, atoms or molecules have electrostatic attraction; when atoms or molecules are pulled apart from each other gain electromagnetic potential energy, tension is produced. 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. 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:〔''Physics for Scientists and Engineers with Modern Physics'', Section 5.7. Seventh Edition, Brooks/Cole Cengage Learning, 2008.〕 either acceleration is zero and the system is therefore in equilibrium, or there is acceleration and therefore a net force is present in the system. ==Tension in one-dimensional continua== Tension in a string is a non-negative scalar. Zero tension is slack. A string or rope is often idealized as one dimension, having length but being massless with zero cross section. If there are no bends in the string (as occur with vibrations or pulleys), then tension is a constant along the string, equal to the magnitude of the forces applied by the ends of the string. By Newton's Third Law, these are the same forces exerted on the ends of the string by the objects to which the ends are attached. If the string curves around one or more pulleys, it will still have constant tension along its length in the idealized situation that the pulleys are massless and frictionless. A vibrating string vibrates with a set of frequencies that depend on the string's tension. These frequencies can be derived from Newton's Laws. Each microscopic segment of the string pulls on and is pulled upon by its neighboring segments, with a force equal to the tension at that position along the string. tension where is the position along the string. If the string has curvature, then the two pulls on a segment by its two neighbors will not add to zero, and there will be a net force on that segment of the string, causing an acceleration. This net force is a restoring force, and the motion of the string can include transverse waves that solve the equation central to Sturm-Liouville theory: where is the force constant per unit length (force per area ) are the eigenvalues for resonances of transverse displacement on the string.,〔A. Fetter and J. Walecka. (1980). Theoretical Mechanics of Particles and Continua. New York: McGraw-Hill.〕 with solutions that include the various harmonics on a stringed instrument. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tension (physics)」の詳細全文を読む スポンサード リンク
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