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torsion, bending and shear (potential energy, due to transverse
forces are usually neglected).
2
2
2
2
N dx M dx M dx M dx
y
U   x   x   z   . (2.2)
l 2EF l 2GJ k l 2EJ z l 2EJ y
In addition to the actual work, external and internal forces
that are applied to the rod, can perform the so-called potential
(virtual) work on the relevant possible movements.

Figure 2.1

Work is called possible when it is performed by the force
on such a moving that is not depend on it. Small moving is called
possible which does not depend on forces that perform work on
it.
Note when the system provides a possible move from the
state of equilibrium, we consider that module strength and
direction remain constant. Therefore, when calculating the
possible work we should not take half and full product
corresponding forces and displacements. For example, consider a
system with two flat load conditions (fig. 2.1).

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