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Elongation of bar under load

  1. Feb 1, 2012 #1


    I may be doing it correctly, but the number seems way to small for the initial calculation. Plus I am really not sure how to factor in those other forces. Modulus of elasticity E = 114 GPa (if unclear)


    ps. answer is 0.804 mm
  2. jcsd
  3. Feb 1, 2012 #2
    Use consistent units. A Pascal is a N/m^2, so don't use mm, you'll just trip yourself up.

    So you've got:
    Ao=.0009 m^2
    AB=BC=CD=.25 m
    E=114 x 10^9 Pa

    As for the other forces, they work in basically the same way, but each force will work on progressively more of the beam. The 80kN will work on the first .25m (as you have worked out), the backwards-acting 40kN will then have .5m to work on, and the 110kN works on the whole thing.
  4. Feb 1, 2012 #3


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    Use consistent units. Using mm is a good choice here. I would use N, mm, MPa. Notice, only E and length in post 1 are inconsistent. Below is an example of consistent units, using mm.

    80 000 N
    Ao = 900 mm^2
    AB = BC = CD = 250 mm
    E = 114 000 MPa​

    (1) By the way, always leave a space between a numeric value and its following unit symbol. E.g., 80 kN, not 80kN. See the international standard for writing units (ISO 31-0). Or see the first image in post 1 for the correct form.

    (2) Numbers less than 1 must always have a zero before the decimal point. E.g., 0.25, not .25. See the above links, or any credible text book.

    (3) Also, Pascal is a man, whereas pascal (Pa) is a unit of pressure or stress. Always use correct capitalization and spelling of units.
    Last edited: Feb 1, 2012
  5. Feb 5, 2012 #4
    I would start by drawing the N-diagram. That is, a graph of how the axial force varies from one end to the other.
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