Mechanics of Materials segment choosing

[mastermechanic]

Homework Statement

Homework Equations

Torque = F. d
Distributed Force -> Concentrated Force = w.d

The Attempt at a Solution

Concentrated force is equal to 40kN and I have found the B(y) as -10kN by taking moment with respect to the A point. And from the equilibrium A(y) is 50 kN.

Everything is good so far. But my problem is about to choosing the segment which I will analyze. The book shows the CB and finds the shear force 10 kN. But if I choose AC part, it is 50 kN. How do we decide to choose which segment will be analyzed? I mean what is the key point? If it is arbitrary, what did I miss to take into account and why it is not taken into account while choosing the part CB?

Thanks!

 

[Chestermiller]

Homework Statement

View attachment 210010

Homework Equations

Torque = F. d
Distributed Force -> Concentrated Force = w.d

The Attempt at a Solution

Concentrated force is equal to 40kN and I have found the B(y) as -10kN by taking moment with respect to the A point. And from the equilibrium A(y) is 50 kN.

Everything is good so far. But my problem is about to choosing the segment which I will analyze. The book shows the CB and finds the shear force 10 kN. But if I choose AC part, it is 50 kN. How do we decide to choose which segment will be analyzed? I mean what is the key point? If it is arbitrary, what did I miss to take into account and why it is not taken into account while choosing the part CB?

Thanks!

Show us how you get a shear force of 50kN choosing AC please.

 

[mastermechanic]

Show us how you get a shear force of 50kN choosing AC please.

A(y) is 50 kN so C(y) must be 50 kN. It is weird, it can be any value even 0 depending on the region. So, I should know respect to what we choose the region.

 

[Chestermiller]

View attachment 210011 A(y) is 50 kN so C(y) must be 50 kN. It is weird, it can be any value even 0 depending on the region. So, I should know respect to what we choose the region.

In your force balance on AC, you omitted the downward shear force on the left side of cross section A.

 

[mastermechanic]

In your force balance on AC, you omitted the downward shear force on the left side of cross section A.

50 kN and 40 kN are external forces. So you're saying there must be shear force of 10 kN internal shear force to compansate it, right? I think I understand it thank you!