When the shear force diagram passes through the zero axis, the corresponding bending moment at the same location will be...?Question 5Select one:a.equal to zerob.below the zero axis line, i.e., positivec.above the zero axis line, i.e., negatived.a local maximum or minimum
Question
When the shear force diagram passes through the zero axis, the corresponding bending moment at the same location will be...?Question 5Select one:a.equal to zerob.below the zero axis line, i.e., positivec.above the zero axis line, i.e., negatived.a local maximum or minimum
Solution
The answer is d. a local maximum or minimum.
Here's why:
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Shear force is the internal force in a beam or structure that resists changes in shape.
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The shear force diagram is a graphical representation of the variation of shear force along the length of the beam.
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When the shear force diagram crosses the zero axis, it means that the shear force at that point is zero.
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The bending moment at a section of a beam is the sum of the moments about that section of all external forces acting to one side of that section.
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The bending moment is maximum or minimum where the shear force is zero. This is because the rate of change of bending moment is equal to the shear force. So, when the shear force is zero, the bending moment is not changing, which means it is at a maximum or minimum.
So, when the shear force diagram passes through the zero axis, the corresponding bending moment at the same location will be a local maximum or minimum.
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To solve this problem, we need to use the concepts of shear stress in beams and the location of the shear center. ### Part (i): Shear Stress at Points A, B, and the Neutral AxisThe shear stress (\(\tau\)) in a beam is given by: \[ \tau = \frac{VQ}{It} \] where: - \(V\) is the shear force. - \(Q\) is the first moment of area about the neutral axis. - \(I\) is the second moment of area (moment of inertia). - \(t\) is the thickness of the section at the point where the shear stress is being calculated. Given: - \(V = 11 \, \text{kN} = 11000 \, \text{N}\) - \(I = 5.626 \times 10^6 \, \text{mm}^4\) - Thickness at point A (\(t_A\)) = 4 mm- Thickness at point B (\(t_B\)) = 4 mm#### Shear Stress at Point ATo find \(Q\) at point A, we need to consider the area above point A: \[ Q_A = \text{Area} \times \text{distance from centroid of area to neutral axis} \] The area above point A is a rectangle with height 150 mm and width 4 mm: \[ \text{Area} = 150 \, \text{mm} \times 4 \, \text{mm} = 600 \, \text{mm}^2\] The distance from the centroid of this area to the neutral axis is: \[ \text{Distance} = \frac{150 \, \text{mm}}{2} = 75 \, \text{mm} \] So, \[ Q_A = 600 \, \text{mm}^2 \times 75 \, \text{mm} = 45000 \, \text{mm}^3\] Now, calculate the shear stress at point A: \[ \tau_A = \frac{11000 \, \text{N} \times 45000 \, \text{mm}^3}{5.626 \times 10^6 \, \text{mm}^4 \times 4 \, \text{mm}} = \frac{495000000 \, \text{N} \cdot \text{mm}}{22504 \, \text{mm}^4} = 21998.2 \, \text{N/mm}^2 = 22 \, \text{MPa} \] #### Shear Stress at Point BTo find \(Q\) at point B, we need to consider the area to the left of point B: \[ Q_B = \text{Area} \times \text{distance from centroid of area to neutral axis} \] The area to the left of point B is a rectangle with height 150 mm and width 4 mm: \[ \text{Area} = 150 \, \text{mm} \times 4 \, \text{mm} = 600 \, \text{mm}^2\] The distance from the centroid of this area to the neutral axis is: \[ \text{Distance} = \frac{150 \, \text{mm}}{2} = 75 \, \text{mm} \] So, \[ Q_B = 600 \, \text{mm}^2 \times 75 \, \text{mm} = 45000 \, \text{mm}^3\] Now, calculate the shear stress at point B: \[ \tau_B = \frac{11000 \, \text{N} \times 45000 \, \text{mm}^3}{5.626 \times 10^6 \, \text{mm}^4 \times 4 \, \text{mm}} = \frac{495000000 \, \text{N} \cdot \text{mm}}{22504 \, \text{mm}^4} = 21998.2 \, \text{N/mm}^2 = 22 \, \text{MPa} \] #### Shear Stress at the Neutral AxisAt the neutral axis, the shear stress is maximum. The first moment of area \(Q\) at the neutral axis is the sum of the first moments of the areas above and below the neutral axis. \[ Q_{\text{neutral}} = \frac{1}{2} \times \text{Area} \times \text{distance from centroid
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