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The absolute position vector r of particle B is measured to the
B
common origin O of the fixed X, Y, Z reference frame. The time
derivatives of the absolute position r give absolute velocity and
B
absolute acceleration

Fig. 2-21

2
dr d r
v = B , a = B .
B
dt B dt 2
Relative motion. The movement of the particle B relation to the
origin A of the moving x, y, z reference frame is named relative
motion.
The position of B measured relative to A is denoted by the
relative position vector r / BA , what is changed by two reason:
movement of the particle and movement of the x, y, z frame. To
eliminate the last reason of a change it is necessary to stop the moving

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