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Very simple question about calculating weight

  1. Feb 26, 2017 #1
    1. The problem statement, all variables and given/known data
    Calculate the weight of each member of the truss structure.

    2. Relevant equations
    density = mass/volume

    3. The attempt at a solution
    This is part of a lab that we are doing in my mechanics of materials course. I have designed a structure, and I must calculate the weight of each member. In short, we have been given density, the cross-sectional area of each member, and the diagram of the structure (and thus length of each member). Volume is in in3 and density is in lb/in3 So what I have done so far is:
    1) Calculate the volume of each member by multiplying the length by its cross-sectional area.
    2) Calculate the mass of each member by multiplying the volume by density.

    My question is, when the units cancel I am left with lbs. for the unit of mass. Is this the weight? Or do I need to multiply it by 32.2 ft/s2?
     
  2. jcsd
  3. Feb 26, 2017 #2

    kuruman

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    Homework Helper
    Gold Member

    lbs. is a unit of weight, not mass. When you multiply volume in in3 by density in lb/in3, you get weight in lbs. You do not multiply by 32.2 ft/s2.

    On edit: If you really want mass, check this out
    https://en.wikipedia.org/wiki/Slug_(mass)
     
  4. Feb 26, 2017 #3

    Ray Vickson

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    This issue comes up a lot and has been dealt with quite thoroughly in this Forum; see
    https://www.physicsforums.com/threads/pounds-is-a-unit-of-mass-or-weight.259981/

    As long as the engineering profession continues to stick with the imperial system this will be a continuing headache.
     
  5. Feb 26, 2017 #4
    Okay thanks. It just strikes me as odd that the formula has mass, but yet you end up with weight. It's also crazy since this means the weight of the truss is only 10lbs and yet it holds up a 10,000lb load...But then again this is just a 2D model of what would really be a 3D structure.
     
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