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Homework Help: Deflection of beam

  1. Mar 29, 2016 #1
    1. The problem statement, all variables and given/known data
    I am trying to find the maximum deflection in a brass beam, however after taking a physical reading the theoretical answer I get is quite off and I am wondering what I am doing wrong.

    2. Relevant equations
    They tell us the max deflection is( Load*Length^3)/(48*E*Second Moment of Area)
    They also give us that E = 105GPA for brass

    3. The attempt at a solution
    The mass was 100g so load = 0.98N
    Distance is 0.4 m
    The problem I have is finding the correct moment of area as I think this is were I am getting it wrong. I have seen that for a rectangle it would be b*h^3/12
    Then I = 0.4*0.00318^/12 = 1.07e-9 m^4
    Putting this all together I get 11.63e-6m = 0.0116mm
    The physical answer I got was 0.29mm.
    Any ideas on what I am getting wrong
  2. jcsd
  3. Mar 29, 2016 #2


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    Well, you provide some data, but not all which is necessary to analyze this problem.

    How was the beam supported?
    What were the dimensions of the beam (LxBxD)?
    How was the cross section oriented with respect to the load?

    Your inertia calculation suggests this beam was 40 cm wide and 3.18 mm thick. That's a very odd set of dimensions for the cross section, almost as if this beam were a strip of brass.
  4. Mar 29, 2016 #3
    The cross section of the beam was 400mm long 3.18mm height and 19.02mm deep
    The set up is shown below

    Attached Files:

  5. Mar 29, 2016 #4


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    You used the wrong dimensions in the inertia calculation. The I is calculated for the cross section of the beam, which is normal to the length axis.


    The moment of inertia for the beam in the figure above is I = ba3 / 12
  6. Mar 29, 2016 #5
    Thanks, this really helped a lot
  7. Mar 30, 2016 #6


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    Isn't b equal to 19.02 mm??
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