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Equation 2-82 may also be used to analyze constrained sliding
contact between two links in a mechanism. In this case, we choose
points A and B as coincident points, one on each link, for the instant
under consideration. In contrast to the previous example, in this case,
the two points are on different bodies so they are not a fixed distance
apart.

56 Instantaneous Center of Zero Velocity

The velocity of any point B located on a rigid body can be
obtained in a very direct way by choosing the base point A to be a
point that has zero velocity at the instant considered. In this case
v A = 0, and therefore the velocity equation v B = v A + ω r / BA ,
×
becomes v B = ω r / BA . For a body having general plane motion, point
×
A so chosen is called the instantaneous center of zero velocity (IC),
and it lies on the instantaneous axis of zero velocity, Fig. 2-26. This
axis is always perpendicular to the plane of motion, and the
intersection of the axis with this plane defines the location of the IC.
Since point A coincides with the IC, then v B = ω r / B IC and so point B
×
moves momentarily about the IC in a circular path; in other words,
the body appears to rotate about the instantaneous axis. The

Fig. 2-26.

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