The tension of a vibrating string is the force used to stretch it. It is therefore directed along the axis of the string. A force must be applied at the endpoint on the right, and a force 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 . (A nonzero force on a massless point would produce an infinite acceleration.)
If the cross-sectional area of the string is , then the tension is given by the stress on the string times .