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This one has been annyoing me.

  1. May 4, 2008 #1
    1. The problem statement, all variables and given/known data
    Suppose you have a very long steel bar, with a regular pentagonal cross-section where each side of the pentagon is 2 cm in length. This bar will be suspended vertically over a very deep chasm. At the bottom end of the steel bar, there is a spherical steel weight with a diameter of 50 cm. Going up the bar, at every 3 meter increment, there is a spherical steel weight with a diameter of 30cm.

    If the density of the steel is exactly 7.8 g/cm3, the steel can withstand a tensile stress of 400 MPa, and gravity is 9.81m/s2, what is the maximum possible length, in metres, that the bar can extend without breaking? Round down to the nearest metre, submit only a number (with no additional or extraneous information), and disregard any deformation of the bar or any other forces acting on the bar.

    2. Relevant equations
    Acircle= Pie (R)2

    I seem to not remember the formula for this, such as the area for the regular pentagon. My son brought this up to me and I told him I will do my best but I hit a stump.
  2. jcsd
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