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)
section(mn . Consider the equilibrium of a body particle. External
forces P and P are applied to it and internal forces of elasticity
n n  1
that lead to the center of gravity of the body section as the main

vector R and main moments M (fig. 1.9, b). Choosing a
coordinate system so that the axis x coincides with the normal to
the section, we divide the main vector and the main moment into
six components: N - longitudinal force; Q and Q - shear forces;
x y z
M , M and M - moments of forces (fig. 1.9, c). These
x z y
components are called internal force factors.

a) b)

c)
Figure 1.9

The equation of equilibrium for the considered body:

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