4 ms·

The rest of the world can't due to the reverse issue, notwithstanding also not knowing intuitiviely whether that's pounds-force or pounds-mass (I am guessing th

by NamTaf 5y ago

The rest of the world can't due to the reverse issue, notwithstanding also not knowing intuitiviely whether that's pounds-force or pounds-mass (I am guessing the former and the acceleration is factored in). These conventions that make it easier for you don't hold so well for an international audience.

jaclaz 5y ago
Yes and no. Before the current conventions (I am old enough to have studied in those times) pressure on soil would have been expressed in (still metric) in kgf/m2 or kgf/cm2 or - simplified - in kg/m2 that way (IMHO) it is easier to "visualize" pressure. The 5 kpa would have been (roughly) 500 kg/m2, i.e. 5-6 people standing very near to each other within 1 square meter (to give you a quick term of comparison slabs in buildings are calculated to bear between 250 and 400 kg/m2, some particular ones up to 600 kg/m2). If you imagine the soil as a liquid, with a density of around 2,000 kg/m3, double that of water, when you remove (dig) 1 meter of soil the surface below can surely carry (as it had done undisturbed for years) some 2,000 kg/m2 (usually much more), if you dig some 3 meters, you have surely 6,000 kg/m2 (that is the typical depth if digging for any "ordinary" building with a basement or underground garage). The 11,000 pounds per square foot correspond to (roughly) 54,000 kg/m2. This is a lot but seemingly not 100 times the "average" house, or - maybe better - the average house has a very high safety factor impolied.
- NamTaf 5y ago
  Fair points all round, but if you specify something as 500 kg/m² I know that you mean 500 kg of mass per m² and so know to multiply by 9.81 to get a force and thus pressure. I know it can’t already be a force, because that’s a newton. I’m not suggesting that a “mass per area” convention is bad, rather that pounds is so interchangeably used as a force and as a mass that it’s not immediately obvious to the layman (especially international and not familiar with imperial units) which is being used here, because both could reasonably fit in this particular case when using area as a denominator.
  - jaclaz 5y ago
    What I was trying to evidence was not metric vs. imperial, only about "visualizing" the whole stuff. A Pascal by itself is not easy to visualize, a number of kg on a square meter (or for those used to it a number of pounds on a square foot) are IMHO easy to imagine, the 500 kg/m2 we were talking about can be visualized as 5-6 people standing one close to another or as a tank 1 m x 1 m filled with 50 cm of water. As a side note, there is no conversion between kg and kgf, they are essentially the same thing, the 9.81 is already included, and it comes into play when you start using the (BTW proper SI) Newton: https://en.wikipedia.org/wiki/Kilogram-force https://en.wikipedia.org/wiki/Kilogram-force

by NamTaf 5y ago

jaclaz 5y ago

Yes and no. Before the current conventions (I am old enough to have studied in those times) pressure on soil would have been expressed in (still metric) in kgf/m2 or kgf/cm2 or - simplified - in kg/m2 that way (IMHO) it is easier to "visualize" pressure. The 5 kpa would have been (roughly) 500 kg/m2, i.e. 5-6 people standing very near to each other within 1 square meter (to give you a quick term of comparison slabs in buildings are calculated to bear between 250 and 400 kg/m2, some particular ones up to 600 kg/m2). If you imagine the soil as a liquid, with a density of around 2,000 kg/m3, double that of water, when you remove (dig) 1 meter of soil the surface below can surely carry (as it had done undisturbed for years) some 2,000 kg/m2 (usually much more), if you dig some 3 meters, you have surely 6,000 kg/m2 (that is the typical depth if digging for any "ordinary" building with a basement or underground garage). The 11,000 pounds per square foot correspond to (roughly) 54,000 kg/m2. This is a lot but seemingly not 100 times the "average" house, or - maybe better - the average house has a very high safety factor impolied.

NamTaf 5y ago
Fair points all round, but if you specify something as 500 kg/m² I know that you mean 500 kg of mass per m² and so know to multiply by 9.81 to get a force and thus pressure. I know it can’t already be a force, because that’s a newton. I’m not suggesting that a “mass per area” convention is bad, rather that pounds is so interchangeably used as a force and as a mass that it’s not immediately obvious to the layman (especially international and not familiar with imperial units) which is being used here, because both could reasonably fit in this particular case when using area as a denominator.
- jaclaz 5y ago
  What I was trying to evidence was not metric vs. imperial, only about "visualizing" the whole stuff. A Pascal by itself is not easy to visualize, a number of kg on a square meter (or for those used to it a number of pounds on a square foot) are IMHO easy to imagine, the 500 kg/m2 we were talking about can be visualized as 5-6 people standing one close to another or as a tank 1 m x 1 m filled with 50 cm of water. As a side note, there is no conversion between kg and kgf, they are essentially the same thing, the 9.81 is already included, and it comes into play when you start using the (BTW proper SI) Newton: https://en.wikipedia.org/wiki/Kilogram-force https://en.wikipedia.org/wiki/Kilogram-force