# 18.2: The Intrinsic Equation to the Catenary

We consider the equilibrium of the portion AP of the chain, A being the lowest point of the chain (Figure XVIII.1).

It is in equilibrium under the action of three forces: The horizontal tension \( T_{0}\) at A; the tension \( T\) at P, which makes an angle \( \psi\) with the horizontal; and the weight of the portion AP. If the mass per unit length of the chain is \( \mu\) and the length of the portion AP is \( s\), the weight is \( \mu s\text{g}\). It may be noted than these three forces act through a single point.

Clearly,

\[ T_{0}=T\cos\psi \label{18.2.1}\]

and

\[ \mu s\text{g}=\ T\sin\psi, \label{18.2.2}\]

from which

\[ (\mu s\text{g})^{2} + T_{0}^{2}=\ T^{2} \label{18.2.3}\]

and

\[ \tan\psi=\frac{\mu\text{g}s}{T_{0}} \label{18.2.4}\]

Introduce a constant \( a\) having the dimensions of length defined by

\[ a=\frac{T_{0}}{\mu\text{g}}. \label{18.2.5}\]

Then Equations \( \ref{18.2.3}\) and \( \ref{18.2.4}\) become

\[ T\ =\ \mu\text{g}\sqrt{s^{2}\ +\ a^{2}} \label{18.2.6}\]

and

\[ s\ =\ a\tan\psi. \label{18.2.7}\]

Equation \( \ref{18.2.7}\) is the *intrinsic equation* of the catenary.