be defined with respect to the link's LNCS. Note that these kinematic and applied force
data must be available for all positions of the linkage for which a force analysis is desired.
In the following discussion and examples, only one linkage position will be addressed.
The process is identical for each succeeding position and only the calculations
must be repeated. Obviously, a computer will be a valuable aid in accomplishing the
task.
Link 2 in Figure 11-2b shows forces acting on it at each pin joint, designated F 12
and F32. By convention their subscripts denote the force that the adjoining link is exerting
on the link being analyzed; that is, F 12 is the force of 1 on 2 and F 32 is the force of3
on 2. Obviously there is also an equal and opposite force at each of these pins which
would be designated as F 21 and F 23, respectively. The choice of which of the members
of these pairs of forces to be solved for is arbitrary. As long as proper bookkeeping is
done, their identities will be maintained.
When we move to link 3, we maintain the same convention of showing forces acting
on the link in its free-body diagram. Thus at instant center !z3 we show F 23 acting
on link 3. However, because we showed force F32 acting at the same point on link 2,
this introduces an additional unknown to the problem for which we need an additional
equation. The equation is available from Newton's third law: