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Hi everyone,

I was given a statics problem to determine some forces in a truss. The attached image has the specific problem on it. The correct answer was supplied so I know what I should be getting, and I was able to get the correct answer using the method of joints starting from Pulley C. However, when I try to solve the problem using the method of sections, using the cut I have shown on the attached image, I get the wrong answer. I'm not sure what I'm doing wrong, but I would like to understand the correct way to apply the method of sections in this case. The following is what I did to try and determine the forces in two force member AB:

Take moments about point D assuming Tab is in tension with CCW being the positive direction:

Tab*3 + 1500*sin(15)*4+1500*cos(15)*(3+0.5)-1500*(4+0.5) = 0

Solving for Tab:

Tab = -41.99 [lbf] (the negative indicates compression)

The correct answer for Tab is supposed 33.47 [lbf] Tension.

I'm not sure what I'm doing wrong. I would appreciate it if someone could help me figure out where my error so I will have a better understanding of these types of problems.

Thanks,

Engineering Student

I was given a statics problem to determine some forces in a truss. The attached image has the specific problem on it. The correct answer was supplied so I know what I should be getting, and I was able to get the correct answer using the method of joints starting from Pulley C. However, when I try to solve the problem using the method of sections, using the cut I have shown on the attached image, I get the wrong answer. I'm not sure what I'm doing wrong, but I would like to understand the correct way to apply the method of sections in this case. The following is what I did to try and determine the forces in two force member AB:

Take moments about point D assuming Tab is in tension with CCW being the positive direction:

Tab*3 + 1500*sin(15)*4+1500*cos(15)*(3+0.5)-1500*(4+0.5) = 0

Solving for Tab:

Tab = -41.99 [lbf] (the negative indicates compression)

The correct answer for Tab is supposed 33.47 [lbf] Tension.

I'm not sure what I'm doing wrong. I would appreciate it if someone could help me figure out where my error so I will have a better understanding of these types of problems.

Thanks,

Engineering Student