Page 173 - Standard Handbook Of Petroleum & Natural Gas Engineering

158 General Engineering and Science

a, = -0.3621 ft/s2
When looking for the velocities of points on a rigid body, the method of
instantaneous centers can often be used. If the velocity of two points on the body are
known, those points and all other points on the body can be considered to be rotating
with the same angular velocity about some motionless central point. This central
point is called the instantaneous center of zero velocity. The instantaneous center
generally moves through space as a function of time and has acceleration. It does not
represent a point about which acceleration may be determined.
Example 2-7

Link AB of length 2 ft (see Figure 2-14a) is sliding down a wall with point A moving
downward at 4 ft/s when 0 is 30". What is the angular velocity of the link and linear
velocity of point B?
Because v, (Figure 2-14b) is parallel to the vertical wall, it is rotating about a
point on a line through A perpendicular to the wall. Likewise B is rotating about a

(b)

Figure 2-1 4. Diagram for Example 2-7.