Engineering Statics

C-5


The 80 N horizontal force P acts on a bell crank as shown. Replace P with an equivalent force-couple system at B.

The moment about B of force is P times the distance perpendicular to P from B to P.
The force P at A can thus be replaced by a force at B of 80 N plus 
a 4 N-m counterclockwise moment at B.  Now the system is equivalent 
with the same force as the original system and a moment which 
compensates for the movement of P from A to B.
We know that the sum of the forces in the y-direction must be equal 
to zero, hence the forces at C and D must be equal and opposite.  To 
cause a counterclockwise moment, the force at C must be down and the 
force at D must be up.
The 4 N-m moment at B is equal to the magnitude of the force at D (or C) times the perpendicular distance between them.  Hence both forces can be 
found.