# Discussion Forum

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

## Total Mass of a Structure

Hello,

I was wondering how to extract the total mass of system. I have solids model of a beam and a beam model of a beam. I wanted to compare their total masses to each other.

Any help would be greatly appreciated.

Thanks,

Joshua Mendoza

7 Replies Last Post Nov 15, 2012, 11:08 AM EST
Posted: 5 years ago
uh, can't you just compute the volume of the structure and multiply it times its density?

Posted: 5 years ago
Hi

if you compute the volume you are assuming the density is a constant, this might be correct,

but its easier to integrate, over all (or the relevant) Domains, the density "material.rho" this gives you the true total mass, even with several differet materials, as COMSOL will map the different densities of the different domains correctly

--
Good luck
Ivar

Posted: 5 years ago
Thanks,

My next question is how to accomplish this in the Beam module where every component is defined using edge elements. There aren't any domains, how can I find the mass in this situation?

Thanks,

JM

Posted: 5 years ago
Thanks again,

Also if I add a point mass or a boundary mass I don't think integrating over the volume with rho will give me the mass of the entire system. That's is alright because I can add the added mass to the result of the integral to have the entire mass of the system. The question still remains if I only have edge elements how do I find the mass of the system?

Thanks,

JM

Posted: 5 years ago
For beams you can do a line integral to get the area and multiply that manually by density and cross-sectional area or include these quantities in the integral. The area is typically called beam.area and the density is beam.rho.

Nagi Elabbasi
Veryst Engineering

Posted: 5 years ago
Hi

You are right that a point mass is not part of the integration, as you integrate over a specific "Entity" domain or boundary times distance, or edge times area, so you need to add by yourself the extra masses you have attached on points, edges ...

Take care also for the eigenfrequency and the body laod cases, these do neither not include the extra mass. If you have a point mass attached to a domain and you apply a domain load (gravity load), you need also to add the point force corresponding to the added point mass.

This can seem cumbersome, but it is applying the strict way of one physics per domain, or another per boundary, or per edge ... and these are separate and must all be updated.

Just the same way if you add 2 or more physics, you must ensure that the BC are given for EACH physics and completely, symmetry boundaries for "solid" might NOT be also symmetry boundaries for Temperature etc

--
Good luck
Ivar

Posted: 5 years ago
Thank you all very much.

I am able to proceed.

JM

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.