A 400-N force P is applied at point A of the bell crank shown. (a) Compute the moment of the force P about O by resolving it into components along line OA and in a direction perpendicular to that line. (b) Determine the magnitude and direction of the smallest force Q applied at B that has the same moment as P about O.
Fig. P3.4

Figure shows the force P components along line OA and in a direction perpendicular to that line.
(a)
Resolve the force P into components along line OA and in a direction perpendicular to that line
Let
&
are components along line OA and in a direction perpendicular to that line
Find the component of the force P along the line OA.
Find the component of the force P in a direction perpendicular to line OA.
Find the moment of the force P about O using the equation.
Therefore, the moment of the force about O is
.
(b)
Figure shows equivalent loading with force Q applied at B:
Find the direction of the smallest force.
For the smallest force Q applied at B, the force should be perpendicular to the line OB
Hence,
Therefore, the direction of the smallest force is
.
Find the magnitude of the smallest force Q applied at B.
Moment about point O is same on both cases
Therefore, the magnitude of the smallest force Q applied at B is
.