String Tension

The tension of a vibrating string is the force $F$ used to stretch it. It is therefore directed along the axis of the string. A force $F$ must be applied at the endpoint on the right, and a force $-F$ is applied at the endpoint on the left. Each point interior to the string is pulled equally to the left and right, i.e., the net force on an interior point is $F + (-F) = 0$. (A nonzero force on a massless point would produce an infinite acceleration.)

If the cross-sectional area of the string is $S$, then the tension is given by the stress on the string times $S$.