### Non-Stiff String

Let's now assume the string is perfectly flexible (zero stiffness) so that the direction of the force vector is given by the unit vector tangent to the string. (To accommodate stiffness, it would be necessary to include a force component at right angles to the string which depends on the curvature and stiffness of the string.) The magnitude of at any position is the rest tension plus the incremental tension needed to stretch it the fractional amount

*Young's modulus*(stress/strain--the ``spring constant'' for solids--see §B.5.1), then

where no

*geometrical*limitations have yet been placed on the magnitude of and , other than to prevent the string from being stretched beyond its elastic limit.

The four equations (B.31) through (B.35) can be combined
into a single *vector* wave equation that expresses the
propagation of waves on the string having three displacement
components. This differential equation is *nonlinear*, so that
*superposition* no longer holds. Furthermore, the three
displacement components of the wave are *coupled* together at all
points along the string, so that the wave equation is no longer
*separable* into three independent 1D wave equations.

To obtain a linear, separable wave equation, it is necessary to assume
that the *strains*
,
, and
be
*small compared with unity*. This is the same assumption
(
) necessary to derive the
usual wave equation for transverse vibrations only in the -
plane.

When (B.35) is expanded into a Taylor series in the strains, and when only the first-order terms are retained, we obtain

(B.36) |

This is the

*linearized*wave equation for the string, based only on the assumptions of elasticity of the string, and strain magnitudes much less than unity. Using this linearized equation for the force , it is found that (B.31) separates into the three wave equations

(B.37) | |||

(B.38) | |||

(B.39) |

where is the longitudinal wave velocity, and is the transverse wave velocity.

In summary, the two transverse wave components and the longitudinal
component may be considered *independent* (*i.e.*, ``superposition''
holds with respect to vibrations in these three dimensions of
vibration) provided powers higher than 1 of the strains (relative
displacement) can be neglected, *i.e.*,

**Next Section:**

Wave Momentum

**Previous Section:**

String Tension