Trouble Understanding Moment Calculations?

Permanence · Aug 6, 2015

Two-panel truss diagram from Example 6.3: (a) a triangular truss with pin support at A, 400 N load down at B, 600 N load right at D, spans 3 m and 3 m, height 4 m; (b) free-body diagram with reactions Ay at A and Cy, Cx at C, dimensions 3 m, 6 m and 4 m.

Textbook page titled Example 6.3 Continued showing truss support reaction equations, a joint A free-body diagram with forces F_AB and F_AD and a 600 N load, and a 3-4-5 slope triangle.

Hi,

I am having trouble understanding how the signs were obtained in the moment calculation.

When I imagine the 600 N force acting on the moment arm, it moves CW which would give it a negative value. When I solve the problem I end up reviving Ay and Cy values that are swapped with the book.

billy_joule · Aug 6, 2015

Permanence said:

When I imagine the 600 N force acting on the moment arm, it moves CW which would give it a negative value.

The 600N force is to the right, that's counter clockwise about point C.

Permanence · Aug 6, 2015

I thought that I'm suppose to draw a line down from C to the line of action of the force. Then apply the force to that line, and imagine (ignore that)

As I'm writing my response I see that it is CCW. I have no idea why I thought it was CW and was so insistent upon that. Thank you for the reply.

billy_joule · Aug 6, 2015

You're welcome

Trouble Understanding Moment Calculations?

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