I am trying to understand why v = (T/mu)^(1/2) Equation 16.18 EDIT: T is tension, mu is the mass divided by the lenght of the string, v is the speed of the wave on the string. According to my book: "Let us use a mechanical analysis to derive Equation 16.18. Consider a pulse mov- ing on a taut string to the right with a uniform speed v measured relative to a sta- tionary frame of reference as shown in Figure 16.11a (page 474). Instead of staying in this reference frame, it is more convenient to choose a different inertial refer- ence frame that moves along with the pulse with the same speed as the pulse so that the pulse is at rest within the frame. This change of reference frame is permit- ted because Newton’s laws are valid in either a stationary frame or one that moves with constant velocity. In our new reference frame, shown in the magnified view of Figure 16.11b, all elements of the string move to the left: a given element of the string initially to the right of the pulse moves to the left, rises up and follows the shape of the pulse, and then continues to move to the left. Both parts of Figure 16.11 show such an element at the instant it is located at the top of the pulse." I don't understand why all elements of the string move to the left when the reference frame is moving at the same speed. I thought the motion would only be up and down.