Example Problem 15

This is the working of problem F6-22 from Engineering Mechanics Statics (14th Edition) by Hibbeler. This direct instruction supports a Statics course offered at the College of Southern Maryland.

[demetrius]

Please explain why this comes out as a positive Cx? Based on your last equation: 0 = (-250)(1.5) + (125)(1.5) - (Cx)(2) this becomes: 0 = (-375)+(187.5)-(2Cx) ==> 2(Cx) = -187.5 ==> Cx = -93.75 N. So the reaction force on the top of member ABC is in the negative x direction. Now, the reaction force at C based on the other member is in the opposite direction, so maybe you're reporting the absolute value of Cx because the direction is meaningless in the context of point C as it depends on the perspective, but you don't say one way or the other why the negative is being ignored. Physically, the negative makes sense as Cy is acting to rotate the member about B in the opposite direction as force Ay over the same distance. However Cy is half that of Ay, so it needs some extra help to counter Ay in the opposite direction (negative X). Ay = CW, Cy = CCW so Cx should be CCW.