53 thoughts on “Homework 27.B”

  1. For this problem, you do not need to make a cut between A and C, since there is no force or moment acting at C. You can solve the problem by cutting between A and B, as well as cutting between B and D.

    1. Does not matter, the reaction forces only work against the force P and create moments to work against the applied moments M and 2M at A and B, respectively.

    1. Christian,
      That is correct. The only sections that need to be considered are A to B and B to D. There is no need to consider the segments of A to C or C to B because there are no forces acting at C.

    2. That is correct, for the problem given, we would only have to consider that there would be two different sections for the beam that we would have to take into account.

  2. Are we considering the moments M and 2M as the moments caused by the reactions at points A and B? Or do we still have to calculate the moments caused by the reactions at those points?

    1. Hi Tessa, since there are no forces acting at point C, it would not be significant to make a cut there. The only cuts that are significant in this problem are between A and B, and B and D.

    1. As point moments do not depend on the distance that they act from a particular point when it comes to summing the moments of a system around said point, the moment on the arm at point A does need to be included when performing the calculations to find the moment around point A.

    1. Hi Eunice. I think for the problems we were given the bending moment just so happened to end at zero, but this is not always the case. Example 9.B.1 in the lecture book is a good example of a problem where the bending moments start and end at values other than zero. Also, double check your starting value for this problem because I think you'll find that it doesn't start at zero.

  3. Don't forget both moments in your equilibrium equations. I at first thought you didn't include one of the moments, but both are needed to get the correct calculation.

  4. I have a bit of a conceptual question on problems involving the shear force V like this problem. Sometimes when I solve, assuming that V points downward, I will get a negative number for V. In a normal force diagram setting, this would indicate that the force is actually towards the other direction (pointing upward in this case). However, it seems through the practice problems we did in class that this is not the case. Would anyone be able to explain the reason for the difference in methods? Thanks!

    1. Yes, I believe this is correct. You will still need to manipulate the signs when calculating your moment equations as the moments are clockwise like you mentioned.

    1. Yes, you do not need to make a cut between AB. The segments that you should have after making the cuts are 0<x<6 and 6<x<9 since L is 9ft. It is crucial to note the direction of the shear forces and the moments provided in the diagram when setting up your equations for each segment.

  5. When I did my sketch for my M(x), I am getting a jump (not continuous) at point B (6ft). Is this correct? I thought the moment equation would never make a jump.

    1. The bending moment diagram shouldnt have any jumps in it. At point B you should get the same value for M(x) for both your moment equation for segment AB and segment BD. I would double check your signs and equations.

    2. I got a jump as well, and I think this happens whenever a moment is applied at that point on the overall FBD. I found that the change/jump in y value at x=6 will be equal to the given couple of 2M at that point on the beam.

  6. If you are having difficulties with your numbers adding up right in this problem, double-check to make sure you are including moments in your equation! The moments in this equation act in the opposite direction to the moment about the cut, so make sure your signs match accordingly! Hopefully this saves somebody a lot of trouble because it took me a long time to figure out that I was making this minor mistake.

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