+1832 what is my mistake here?
it is written in the solution manual that FCB is tension??
+33661 You should have FCB = 30*sin(alpha), and since alpha = 36.27° you should end up with a positive value for FCB. (You need to resolve the 30N force parallel and perpendicular to BC - FCB is the parallel component - not resolve FCB into horizontal and vertical components. alpha is the angle between the 30N force and the perpendicular to BC).
The above assumes AB exerts no force on BC and that the 30N is the only force in play. However, if AB exerted a horizontal reaction force on BC, the net effect would still be for a tendency to stretch BC, hence for BC to be in tension.
+33661 In your sketch you have defined the direction of FCB as positive for tension. FCB is trying to stretch BC, so BC is in tension.
+1832 In my calculation I got FCB negative , so I must revesrse its direction ! so it will be compression .
thats how I understand
+33661 You should have FCB = 30*sin(alpha), and since alpha = 36.27° you should end up with a positive value for FCB. (You need to resolve the 30N force parallel and perpendicular to BC - FCB is the parallel component - not resolve FCB into horizontal and vertical components. alpha is the angle between the 30N force and the perpendicular to BC).
The above assumes AB exerts no force on BC and that the 30N is the only force in play. However, if AB exerted a horizontal reaction force on BC, the net effect would still be for a tendency to stretch BC, hence for BC to be in tension.
