CAD Forum

Whenever I use the Solidworks mass properties tool, I become confused because it gives the 'mass' in pounds or kg (or other variations of g). Furthermore, the 'mass' originates in the material property's 'mass' density, which is given in kg/m^3 or lb/in^3. Pounds are usually a weight and kg is usually a mass, which contrary to popular belief are not the same thing. It appears to me that SW is playing fast and loose with the units and the metrics are mass and the standards are weight. Am I missing something here?

No, you're not missing anything. SW is playing fast and loose with the definition of weight. They are assuming weight at average sea level on Earth. If you need something more accurate or for a different "locale", I'd work in metric and then calculate actual/local weight later when you need it.

lb = lbm

https://en.wikipedia.org/wiki/Pound_(mass)

If you are actually confused, you can educate yourself. In engineering school I was taught to use both lbm and slugs for calculations involving mass. Both have official definitions that are based on the SI kg. It’s not that complicated. If you're just trying to be funny or pedantic, it's falling a bit flat.

You just got owned by the f-up Imperial unit.
Another rover crashed because of Imperial unit.

Let's start with the good.
Metric is the "proper" or "correct" notation.
g, kg - mass
mass x acceleration = force (weight)
kg x earth gravity = weight on earth
1 kg x 9.807 m/s^2 = 9.807 Newton
We usually round it up to 10N

Now the F-up.
Imperial use Pound for both mass and force.
So 1 LB x 32ft/s^2 = 1 LB

To make it clear we usually use lbmass and lbforce.
1 lbmass x 32ft/s^2 = 1 lbforce

All work ok on earth.
Once you're outside earth, Imperial completely fall apart.
When gravity is not 32ft/s^2, 1 lbforce is not 1 lbforce anymore.

Imperial formulas use lots of "short cut" that ignore units and use a constant (without showing unit) to simplify calculation. So it's easy to make mistake using wrong units.
So use metric unit and formula and convert the result to Imperial.

I, and most people, work on the surface of the earth. Having 1 lbm exert a force of 1 lbf due to earth's gravitational field doesn't strike me as outrageous, it actually seems quite practical.

MattW wrote: ↑Tue Jul 16, 2024 7:57 pm I, and most people, work on the surface of the earth. Having 1 lbm exert a force of 1 lbf due to earth's gravitational field doesn't strike me as outrageous, it actually seems quite practical.

I also work on the surface of the earth, and a couple of miles up in the air. But dealing with objects, fluids and gasses in motion, changing speed or direction, needs proper units to do proper calculations.

josh wrote: ↑Tue Jul 16, 2024 9:21 am If you are actually confused, you can educate yourself. In engineering school I was taught to use both lbm and slugs for calculations involving mass. Both have official definitions that are based on the SI kg. It’s not that complicated. If you're just trying to be funny or pedantic, it's falling a bit flat.

Yes, in engineering school, I was taught the same as you. I was also taught to always put the units next to the values. SW says 'lb', not lbm or lbf, which is why it is unclear.

MattW wrote: ↑Tue Jul 16, 2024 7:57 pm I, and most people, work on the surface of the earth. Having 1 lbm exert a force of 1 lbf due to earth's gravitational field doesn't strike me as outrageous, it actually seems quite practical.

And 1 lbm x 32 ft/s^2 = 1 lbf is easy to understand.
Does cars and airplanes accelerate at 32 ft/s^2?

Also gravity is not the same on every location on earth.

I should add a few slug in this conversation
Blob would like that.

Frederick_Law wrote: ↑Wed Jul 17, 2024 2:30 pm And 1 lbm x 32 ft/s^2 = 1 lbf is easy to understand.
Does cars and airplanes accelerate at 32 ft/s^2?

Also gravity is not the same on every location on earth.

I should add a few slug in this conversation
Blob would like that.

Oh you guys and fun with units!
You're all missing some, what about a lb of steam? How many units can we get out of that one?

Blob Steam Slug?

so basically your saying I need to go to a doctors office high up on a mountain top somewhere to weigh in so I can meet my target weight. :-)

Be careful way up on that mountain top, the air gets thin up there, can be hard to keep Oxygen levels up.

Which makes me wonder, since the air is thinner at high altitudes, does a lb of air weigh less on mountain top?

Or on a plane free falling .....
Or move to ISS.

Frederick_Law wrote: ↑Thu Jul 18, 2024 9:16 am Or on a plane free falling .....
Or move to ISS.

LOL....my weight would be zero but my mass would still be to much!

You might lose a few slugs on a free falling plane .....