1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    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
  4. Feb 26, 2017 #3

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    This issue comes up a lot and has been dealt with quite thoroughly in this Forum; see

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Threads - Very simple question Date
Very simple question law of ohm Apr 30, 2016
Very simple question (basic kinematics) Dec 18, 2014
Very simple entropy question Mar 23, 2014
Very simple Lines of Force question Dec 31, 2013