Hi guys,
I have a question regarding the Mass Properties tool.
When I use it to calculate the mass properties for my assembly it reads out Mass=60lb.
That would mean the weight (on earth) of my assembly is 60.0lb*32.2ft/s^2=1932lb(force) however the assembly does not weigh anywhere near that in reality.
I made a cube of water 10x10mm and it read mass 1g, which is its weight (on earth).
So when it reads mass does it really mean weight, and if so why?? Little confused.
Thanks!
Hi Dmitry,
The old fashioned way is to use slugs, which is a unit of mass. You have a unit of weight denoted as lbf and a unit of mass denoted as lbm. 1 lbf is equal to 1 lbm if gravity is 32.2 ft/s^2. This can be a little confusing, but let's have the units convince us. Lets now take a look at how the two are defined.
1 lbm is equal to 0.4536 kg is equal to
1 lbf (or weight) is defined as the amount of force required to accelerate 1 slug, 1 ft/s over 1 second's time. (ft/s^2)
so at 32.2 ft/s^2, 1 slug would equal 1 slug * 32.2 ft/s^2 * (1 lbf/(1 slug*ft/s^2)).
The ft/s^2 and the slugs cancel out, leaving 32.2 lbf.
1 lbf = 4.448 Newtons
Therefore, 32.2 lbf = 32.2 lbf * (4.448 N/lbf) = 143.23 N
F = m*a, where a = 9.81 m/s^2 here on the earth, pretty much
1 N = 1 kg *m/s^2
143.23 N at 9.81 m/s^2 = 14.6 kg
14.6 kg * (1 lbm/0.456 kg) gives us 32.2 lbm, which is "equal" to our original 1 slug mass calculated to 32.2 lbf at gravity equal to 32.2 ft/sec^2.
Now, the key is "standard acceleration" which is 32.2 ft/s^2 or 9.81 m/s^2. If we then take that 32.2 lbm and accelerate it to twich the standard acceleration, we can work backwards to get the lbf.
32.2 lbm = 14.6 kg
14.6 kg * 19.62 m/s^2 = 286.45 N
Again, 1 lbf = 4.448 Newtons so 286.45 N = 64.4 lbf
So at 2g, 1 lbm = 2 lbf, 3g, 3 lbf, and so on.
Per Scott's comment, the mass is based upon the density and as density is defined as the mass per volume, the lb's listed by Solidworks are mass, but here on earth can be used as weight also
Hi Dimitry,
You are correct. I have also noticed the confusion between mass and force in both British and SI units.
This should be considered a very old bug that must be fixed.
A mass can never be used as weight, they don't even have the same unit of measurement! Confusing mass and weight in the field of mechanical engineering is a huge mistake.
I noticed the same problem and agree with Hacene
Dmitry,
The mass is all based on the density of the material you are calling out in your material
property file.
I would check the density to see how accurate that is.