1. The problem statement, all variables and given/known data Blocks A and B weigh 400 and 200 kN, respectively. They rest on a 30degrees inclined plane and are attached to a post hinged at O by cords parallel to the plane, as shown in Fig. E06-1.3. The post is held perpendicular to the plane by force P. Assume that all surfaces are smooth, determine the value of P. 2. Relevant equations [tex]\sum F = 0[/tex] [tex]\sum M = 0[/tex] 3. The attempt at a solution I think my free-body diagram has too many forces. I not sure which one is considered as internal or external. For block B, my forces are tension 1, weight of B, normal contact force by a on b. For block A, my forces are tension 2, weight of A, normal contact force by grd on a, and normal contact force by B on A. For the post, my forces are tensions 1 and 2, force P, and a reaction force by grd on post at point O. Then i take my x axis as along the slope. And i find sum of Fx = 0, sum of Fy = 0 and sum of torque abt pt O = 0 But i can't really solve it. And i'm very unsure about my free body diagram. Are the normal forces internal forces cos they hold the system together? ( But if so, why for some diagrams, like those end supports, with pins or rollers, there is reaction force and it is an external force? Or reaction force and normal force are diff? ) I'm quite confused.. Can help me pls? Thanks!!