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Homework Help: Very basic physics (mechanics) question

  1. Aug 29, 2005 #1
    A structural I beam is made of steel. A view of its cross-section and its dimensions are shown in Figure P1.11. The density of steel is 7.56 * 10^3 kg/m^3. What is the mass of the section 8.5 m long?

    Figure P1.11 (shown below)

    I tried doing this by:
    1) changing the 8.5 m into cm... giving me 850 cm.
    2) getting the volume by parts: 2[15cm(1cm)(1cm)] + [848cm (1cm)(1cm)]
    giving me 878 cm^3.
    3) I converted into m^3 and got 8.78*10^-4
    4) I used the density and then got a mass of 6.38 kg

    But it's a wrong answer... any help would be much appreciated!

    Attached Files:

    Last edited: Aug 29, 2005
  2. jcsd
  3. Aug 29, 2005 #2


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    I don't understand how you worked out your volume.

    Try simply working out the area of the cross-section and multilpying that by the length.
  4. Aug 29, 2005 #3
    What you want to do is find the surface area of the I beam. Then you know the height of the beam is 850m and its area, so you can find the volume by multiplying the two.

    The area is (36*15) - (34*14) if im not mistaken.
  5. Aug 29, 2005 #4


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    how you calculated the volume eludes me. How about 2[15*1*850] + 34*1*850 = 54400 cm^3
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