Is Mass the Same as Weight in Truss Calculations?

AI Thread Summary
In truss calculations, weight and mass are often confused due to unit differences. The weight of each member can be calculated by multiplying the volume (length times cross-sectional area) by the density, resulting in pounds, which is a unit of weight, not mass. There is no need to multiply by 32.2 ft/s², as the calculation already yields weight in pounds. The discussion highlights the complexities of using the imperial system in engineering, where mass and weight terminology can lead to misunderstandings. Ultimately, the weight calculated reflects the structure's ability to support loads, despite seeming counterintuitive in a simplified model.
dlacombe13
Messages
100
Reaction score
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?
 
Physics news on Phys.org
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)
 
dlacombe13 said:

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.
 
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.
 

Thread 'Struggling to understand the concept of this question (ice cube in water optics question)'

TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
View full post »
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
View full post »

Similar threads

How do I calculate forces in a truss cross section?
Replies
2
Views
617
Transverse mechanical waves in a steel wire
Replies
10
Views
923
Calculating Linear Charge Density of a Cylinder
Replies
1
Views
3K
Find the length of a roll of GI steel sheet given its weight
Replies
1
Views
1K
Find the weight of a partially submerged cylinder
Replies
25
Views
4K
Relationship between material mass and weight
Replies
5
Views
2K
Finding the normal force of a block under water, under pressure
Replies
10
Views
2K
I need opinions on a Projectile Motion problem that I made up
Replies
11
Views
2K
Measurements Calculation Problem
Replies
3
Views
887
I How to calculate the "weight" of this piece of metal?
Replies
12
Views
1K

Hot Threads

Recent Insights

Back
Top