# Very simple question about calculating weight

1. Feb 26, 2017

### dlacombe13

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. Feb 26, 2017

### kuruman

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. Feb 26, 2017

### Ray Vickson

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