The force F is given in Cartesian vector form as F=60i-30j+20k. The moment of F about point A is -100i-100j-140k. Members DF and BH are identified as zero force members since truss members cannot sustain tangential loads.

1. help please
Write force F in Cartesian vector form What is the moment of the force F about point A Identify
zero force members
Solution
1)
A=(0.3.2)
and B=(6,0,4)
so AB=(6,-3,2)
so unit vector along AB=(6,-3,2)/(magnitude)
magnitude=sqrt(36+9+4)=7
so F=70*(6i-3j+2k)/7=60i-30j+20K
2)
B=(2,5,6)

2. A=(1,-1,1)
so AB=(1,6,5)=r
torque=r*F=(i+6j+5k)*(30i+40j-5K)
=-100i-100j-140K
3) DF, BH should be zero force member since truss member cannot sustain tangential load
note