Very simple question about calculating weight

  • #1
100
3

Homework Statement


Calculate the weight of each member of the truss structure.

Homework Equations


density = mass/volume

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?
 

Answers and Replies

  • #2
kuruman
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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)
 
  • #3
Ray Vickson
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Homework Statement


Calculate the weight of each member of the truss structure.

Homework Equations


density = mass/volume

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?
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.
 
  • #4
100
3
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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