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and thus the rigid-body assumption is clearly acceptable. On the other
hand, if the problem is one of describing, as a function of time, the
internal wing stress due to wing flutter, then the relative motions of
portions of the wing cannot be neglected, and the wing may not be
considered a rigid body.
The motion of rigid body can be completely specified by
knowing both the motion of body as a single object and the motion of
a point on the body.

Fig. 2-15.

Position. The locations of points A and B on the body are
defined with respect to fixed reference frame (origin at O, Fig. 2-15)
using position vectors r and r . The position of B with respect to A
A
B
is denoted by AB. By vector addition
r B = r A + AB. 2-43

Velocity. A relation between the instantaneous velocities of A
and B is obtained by taking the time derivative of this equation, which
yields
dAB
v B = v A + dt . 2-44

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