Difference between density and unit weight with same units

[Typhon4ever]

In my lectures and notes my professor keeps writing out density as lbs/ft³ and unit weight as lbs/ft³. For a material such as concrete it could have a density of 167 lbs/ft³ but a unit weight of 103 lbs/ft³. I am confused as to how these two values are different even though they have the exact same units.

 

[PhanthomJay]

I don't understand why there should be a difference. In the United States, density is typically defined as weight per unit volume, and this is the same as unit weight (about 150 pounds per cubic foot for concrete). In the SI system of measure, density is expressed in mass per unit volume, whereas unit weight is expressed in weight per unit volume,so there is a difference on that system of measure. In any case, the 103 pounds per cu ft value for the unit weight of standard concrete is not correct. However , if the concrete is submerged under water, then the buoyant or apparent density is about 100 pounds per cubic ft or in that ballpark , so maybe that's what prof was talking about?

 

[Typhon4ever]

I don't understand why there should be a difference. In the United States, density is typically defined as weight per unit volume, and this is the same as unit weight (about 150 pounds per cubic foot for concrete). In the SI system of measure, density is expressed in mass per unit volume, whereas unit weight is expressed in weight per unit volume,so there is a difference on that system of measure. In any case, the 103 pounds per cu ft value for the unit weight of standard concrete is not correct. However , if the concrete is submerged under water, then the buoyant or apparent density is about 100 pounds per cubic ft or in that ballpark , so maybe that's what prof was talking about?

Ah sorry, those values refer to a coarse aggregate not conrete. The aggregate has a density of 167 and a unit weight of 103 with the same exact units. I guess the question is the same though, in my notes, the lectures, and my laboratory hands outs the density and unit weights are always different despite having the same units.

 

[mathman]

Could density be referring to the material itself, while unit weight includes the empty space?

 

[jbriggs444]

https://en.wikipedia.org/wiki/Specific_weight indicates that "dry unit weight" refers to the density of a material with voids (which count as part of the volume).