7.3. Internal Forces

External forces create internal forces.

Internal forces do not effect equilibrium.

Internal forces are related to stress and strain.

Looking at a section of the cable, where F is the tensile force on the cable we note that the cable is in equilibrium.

If we look at a plane perpendicular (normal) to the axis of the cable we see the cross-sectional area and the internal force P.

The normal stress in the cable is

\[ \sigma = \frac{P}{A} \]

Where \(\sigma\) denotes stress, \(P\) is the internal force, and \(A\) is the cross-sectional area \(P\) is applied over.