Standard gravity is 9.80665 m/s2. That the number defined by the metric people who set all the world’s units. In schools in the united states of america, we used 9.8. I don’t recal using any more precision than that. Gravity at the surface does vary, but you don’t need more presision than that for most academic purposes.
Is that so? I wonder what the story behind that is. Maybe it’s a surface average?
Most people would probably guess this, but meters and seconds are defined independently of Earth’s gravity, so it doesn’t have a true value, just apparently a standard nominal one.
Standard gravity was adopted as a standard in 1901. That was at the 3rd meeting of the General Conference on Weights and Measures. They redefined a litre as 1 kilogram of water, but the volume of water depends on the pressure, and the pressure depends on the local gravity, so they had to come up with standard values for both standard atmosphere and standard gravity. You also need a standard value for gravity to define a standard for weight measurements which was also done.
Standard gravity is the acceleration at sea level at 45 degrees latitude. The official number was based on measurements made by Gilbert Étienne Defforges in 1888. I can’t find details about his methodology without going to a library or something, and that’s not worth the effort for an internet comment.
No, it’s not worth it. Honestly that’s great all on it’s own. I guess they never had a reason to update it, then, since anybody that needs a more accurate value would just measure it themselves.
It looks like they went back to the original litre definition a few decades later. I’m not sure why they thought defining volume by mass rather than geometry was better in 1901, anyway. Some fun facts about the kilogram itself, since I never get to talk about this stuff:
Since 2019 the kilogram has been based on a “Kibble balance”, which is a contraption that precisely measures the force produced by electromagnetism. The necessary electricity is provided by circuit with a material that has quantised resistance near absolute zero, and a superconducting junction which produces oscillation exactly tied to the current flowing through, which is itself timed by atomic clock. This allows you to measure it out using just the new fixed value of Plank’s constant.
Before 2019 there was just a chunk of metal the was the kilogram, which is hilariously low-tech.
We learned 9.82 m/^2. But in the classes I have as an engineering student we use 10 m/s^2. And I wish I was kidding when I say it’s because it easier to do the math in your head. Well obviously for safety critical stuff we use the current value for wherever the math problem is located at
Going to guess civil. I work on space systems and we don’t have one number. We have the g0 value, which is standard gravity out to some precision, but gravity matters enough we don’t even use point mass gravity, we use one of the nonspherical earth gravity models. It matters because orbits.
Interesting that I learned 32.2 ft/s, but only 9.8 m/s - one less significant figure, but only a factor of two in precision (32.2 vs 32 = .6%; 9.81 vs 9.8 is only 0.1%). Here's the fun part - as a practicing engineer for three decades, both in aerospace and in industry, it's exceedingly rare that precision of 0.1% will lead to a better result. Now, people doing physics and high-accuracy detection based on physical parameters really do use that kind of precision and it matters. But for almost every physical object and mechanism in ordinary life, refining to better than 1% is almost always wasted effort.
Being off by 10/9.81x is usually less than the amount that non-modeled conditions will affect the design of a component. Thermal changes, bolt tensions, humidity, temperature, material imperfections, and input variance all conspire to invalidate my careful calculations. Finding the answer to 4 decimal places is nice, but being about to get an answer within 5% or so in your head, quickly, and on site where a solution is needed quickly makes you look like a genius.
I gotta say, that explanations sounds way better than shrugging and saying “close enough”. But then again our teachers usually say “fanden være med det” meaning “devil be with that” actually meaning “Fu*k it” when it comes to those small deviations
This reminds me of the story of magnetic detonators for torpedos they tried to use in the early days of WW2. They detect the slight disturbance in the Earth’s magnetic field caused by a gigantic hunk of floating metal, and that triggers the detonation.
However, they did not yet know that the Earth’s magnetic field is not consistent over the whole planet, so while they calibrated it to the local field, it functioned very badly in other regions with different field strengths. Torpedo would either detonate far too early, doing minimal damage, or not detonate at all, just hitting the target ship with a loud thunk.
This was largely responsible for the ineffectiveness of American submarines in the early days of our WW2 involvement. Took us a couple years to sort too.
It was called the Mk 42 in case anyone wanted to read a little more. It’s an amusing story. They never wanted to actually properly test them, because they were so damn expensive. So they just didn’t. lol It wasn’t until enough sailors complained and got a high ranking admiral on their side that it got sorted.
People learned different values for g for a number of reasons, but as far as I understand local variability is not one of them. The primary root cause seems to be accuracy of the measurement over time and the age of textbooks/course material.
Over time we have gotten better at measuring the true value of g through advances in technology and this has caused the taught value to shift a little. The value when initially measured had fairly large error margins, meaning that we were sure it was near a specific value but not sure of the exact value. As the tools improved we have reduced the uncertainty, getting to a more accurate and also more precise value, meaning more digits after the decimal as well as higher confidence in each digit. We have also changed what we mean by g over time, bringing it in line with the metric system and basing it on fundamental values and constants. From my understanding the most recent method relies on how much the repulsive force of an electromagnet with a specific number of culombs passing through is overcome by gravity at a specific distance from the center of the mass of Earth, so a little more removed from backyard science than measuring if things drop at the same speed at the top of a mountain and sea level.
Part 2 is the differences in how recent your material is. In my primary school in a relatively affluent area of an affluent country we had textbooks from the last 10 years. My partner went to a school in the same country but a worse area about 5km away from mine. Their school had textbooks literally 20 years old. In that time the measurements had changed, understandings had changed, and they were therefore taught things that were untrue. These sorts of differences based on geography reproduce the impacts of racism and inequality from the past into the future.
While I don’t know the answer and that for simplicity it should probably be a global average, it is probably some “constant“ measured from some location in either Europe or North America before they were able to measure globally using satellites.
Yeah. 9.8 is what I learned. I was generally aware that locality made a difference, but I had no idea that there was that much of a spread. For anything not involving millions of dollars of rocketry and actual satellites a simplified number is likely good enough. Much like Pi, where a couple digits is good enough for most everything and calculating out past 6 digits or so is infinitesimally small.
Volume of a cylinder is πr^2Height
Assuming the height of the tree stays the same, let's say 100'.
Radius is 2' and then we have a 500 year old with a radius of 5'
2' x 100' tree has a volume of 1256'
5' x 100' tree has a volume of 7852'
Trees are made of carbon. Older trees sequester more carbon
Young trees of many species also grow faster, though, and if the old tree dies and decays all that carbon returns to circulation. Forestry, done right, actually is carbon negative. However, it’s also incompatible with the critters that need old-growth forests (and old growth itself soaks up carbon fairly slowly). Environmentalism needs to get better at appreciating tradeoffs IMO.
In the US PNW coast area Douglas Fir trees are harvested for lumber within about 30 years, plus or minus. Maybe the person you were talking to was considering the harvest of the tree to be the moment when the CO2 is "reclaimed"?
Wrt to when the tree pays off the carbon footprint generated by raising and planting the seedling, I guess it's less than three years.
Fun fact: Douglas Fir reach peak carbon fixation rate at about 120 years.
I highly question this. A Dougie at 30 is about a foot across. I just took 7 Dougie’s down on my lot, the largest was 24in at chest height. I can see Puget sound from my place. In fact, I actually counted the rings on one of them and it was 101 years old. Shit. Now I’m gonna go look and measure the 30. I dyed every fifth ring when I counted it initially.
K, so at 30y/o the only stump I left in the ground was only 8.5 inches across and 20in in diameter at 101, so that’s an easy 24in with the bark. The tree was 120ft tall when I felled it in July. A real shame too, I wanted to keep all of them but fire damage. The next day beetles had already hit all of them. I dropped the trees a week after the fire and debarked them to help protect the wood before i could mill them, and there were hundreds of beetle tracks under the burned bark. Pine beetles live under the bark, in the cambium, no bark=no beetle. But the California wood wasps showed up the day I dropped the bark. Those things are terrifying, jet black, 2.5 inches long with an inch long stinger on top of that, so about the width of your palm. Adult pine beetles are about 3inches long when they emerge too. Wicked little fuckers, the both of them
I suppose it’s more of a "that’s when they start binding the meat of the lifetime-CO2-stored. Remember, trees also burn quite a bit of their previously fixated CO2 for energy. Perhaps the amount of CO2 fixed in the first 30 years pales in comparison to that of the next 30?
Could be sort of a "break-even" point? Assuming it's even true, which is a pretty big assumption. You could ask them for a source next time if you hear it often, because I've heard it precisely 0 times before.
Yeah, last time I heard it, it was in this German video: piped.video/watch?v=ThqfNX8EMe4
(I did not note down the timestamp, sorry.)
As I understand, the guy has a PhD in forensics. Obviously, not quite his field of expertise, but I’d expect a biologist to know how a tree works at a basic level.
I have watched other, similar videos of the guy before and since people here seem to not have heard this number before, I’m now consider that it was maybe always this guy who said it. I’m sure, he has some source for it, but it was an offhand, somewhat cynical comment, so maybe he oversimplified…
Not your fault, but that is the most annoying calculator I've ever encountered, as someone who uses the metric system.
I mean, what kind of maniac describes the amount of oxygen produced in pounds?
Also are those US gallons or UK gallons?
The increments used for the circumference of the tree is also incredibly weird, 7 and 3/4 inches? Really? Clearly converted metric to imperial. Why not include a slider to switch to metric, if that's what you've based your numbers on?
Probably because the writer is not reporting her own original research. She is reporting work done by others, they often used metric, and any metric units were converted to common US units because the article was intended for a general American audience.
And why isn’t there a button to restore the original metric units? Same reason why when a newspaper reports a translated quote from Macron or Putin or Xi, there is usually no button to restore the original French or Russian or Chinese: the editor decided that it wasn’t necessary for the intended audience.
The mass of a tree is composed of carbon fixed from CO2, so it doesn’t make any physical sense for a tree to grow at all without absorbing CO2. This is nonsense, trees begin fixing CO2 the moment they start growing.
I coudn’t find a source for your statement, but I did find that it takes a tree 30-40 years to store a ton of CO2, so maybe that’s what they mean? A tree will store carbon as it grows, because it builds itself with carbon from the air. ecotree.green/en/how-much-co2-does-a-tree-absorb
I think there has to be a certain balance. We can’t just cover a massive field even in trees, that creates an unhealthy ecosystem.
Sometimes, as we try to fix things quickly, we miss or ignore the long-term consequences.
It is probably a statement related to the average tree. Also, I believe hemp and bamboo are not trees (but I’m also not a plant scientist) so not really relevant in a statement about trees.
Ehh, cannabis is a woody annual. At least that’s what I’d call it. It dies every season. In some places a stand can reseed itself or a mother plant or two may overwinter for a maximum of one season by being buried under it’s daughter plants after they collapse from senescence, essentially cellular death from old age, which varies by species.
Hi there! Can you please remove the word “retarded” in your first sentence? This word is now generally considered a slur, which runs afoul of rule 6 “Use appropriate language and tone. Communicate using suitable language and maintain a professional and respectful tone.”
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