Does everyone learn the same gravity in school or is it different everywhere?
Does everyone learn the same gravity in school or is it different everywhere?
So, I learned in physics class at school in the UK that the value of acceleration due to gravity is a constant called g and that it was 9.81m/s^2. I knew that this value is not a true constant as it is affected by terrain and location. However I didn't know that it can be so significantly different as to be 9.776 m/s^2 in Kuala Lumpur for example. I'm wondering if a different value is told to children in school that is locally relevant for them? Or do we all use the value I learned?
I just learned 'about 9.8' which is true anywhere in the world.
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.
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.
This is why you have so many Russians being thrown out of windows in high buildings. They're testing the local value of g.
Dimitri, come to the window! I have a stopwatch and questions about the local density of the Earth's crust!
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
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.
Even then, once you figure in a safety factor of 2 or 3 as a minimum, the extra precision really gets lost in the fog anyway.
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.
Nope. Mechanical engineering. So usually we say g=10 and then make the steel a bit thicker and call it a day
Well, g is not a real constant, it depends mostly on altitude. The true constant is G. g=9.8 is usually more than enough for your calculations, to the point we often round it to 10 for simplicity, or you remove it completely is the mass is too low. But actual numbers is only the very last step usually. The calculations will be made with letters. The value you use at the end for g depends on the precision you need, so it depends on the precision of the other parameters.
I learned 9.81 m/s2 and 32.2 ft/s2 with the qualifier being at sea level.
This doesn't change the issue presented by OP. Sea level is not level across the world. In fact there are much larger differences than most people expect. The Earth is not perfectly round. Earth rotation causes the equator to be affected by a centrifugal force, making it wider there ( more distance to earth core means less gravity ) than at the poles. Overall, gravity at Earth surface level varies by 0.7%, ranging from 9.76 in Peru to 9.83 in the Arctic Ocean, but it's absolutely not linear. In addition, the Earth is full of gravity anomalies. These cause localized dips and spikes in gravity. Two of the big dogs lips lie in the Indian ocean and the Caribbean. Because water is fluid, sea level is very much affected by local gravity (as well as other factors such as air pressure, salinity, temperature...). Which is also why the moons gravity can cause tides. The permanently lower gravity on these anomalous spots mean that the average sea level here is lower than it would be on a perfect sphere. This difference can be up to two meters in sea level.
I figured they took the best average at sea level across the planet that they could measure.
In freshman college physics we had a lab to measure gravity then had to use our lab result for the rest of the course.
Just don't make the same mistake as one physics lab did. They made a series of measurements and their results showed that gravity quickly increases in fall, falls slowly over winter, and back to about pre-fall levels very slowly in summer. It took quite a while to figure out the reason of this unexpected result. They turned their equipment inside out to find a mistake to no avail. Then they realized that the university stored coal for the central heating and hot water in the basement under the lab...
I also learned 9.8.
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.
In grade school i learned it was about 32 ft/s2, but by high school on it was all 9.8[1/06] m/s2. Then in engineering school it was sometimes 10. None of that had anything to do with local gravity and everything to do with Americans having to be special at first, followed by the fact that our science classes are actually in metric (statics and dynamics were in both as some fields of engineering haven’t metricated yet here). And the 10 is because you can round to a round number by barely even touching your fudge factor so why not.
Interesting - what part of the US are you from?
I was going to say that even here in the US it was 9.81 m/s^2. I don’t remember ever being taught the number in feet (in NYS) nor seeing it for my kids (in MA). Science was always metric
Ohio, and Catholic schools. It was clearly on its way out. In retrospect it was definitely a strange situation where different teachers had different opinions on metric. Some clearly thought it’s fine for science, and others clearly just wanted to quit our two measurement system that does nothing but prolongs the inevitable.
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.
I've learned it as 9.81 but we usually round up to 10 for calculations. (this is for highschool. I haven't gotten to college yet)
We just use 9.8 at my high school for calculations. Also its cool to see another young person on the fediverse (Assuming you are still in highschool).
Close enough I graduated last year 2023. I couldn't get in to the college I wanted so I decided to try it a second time. There's a countrywide exam that gives you a score. It's called yks. I'm currently studying for that exam.
You round it to 10? Do you also round PI to 3 for simplicity? Kids these days.
Rounding of constants always depends on what you are calculating. Getting a rocket into orbit is a case to use the actual local value of g with a bunch of digits (and the change with height, too). If you build a precision tool, some more digits of PI are no bad idea.
But to calculate the lenght of fence to buy to surround a round pond, I actually used 10/3 for "PI plus safety margin" once.
yeah :/ in physics class we do round pi to 3
Seeing as the British invented gravity, most places just use our gravity rather than making their own.
9.81 in Scotland.
Whoa, thats heavy
See Wikipedia for this.
Wow, I also didn't know it varied so much. I assumed it would be within about 9.81+-0.01 worldwide, since I (in UK) was also taught ~=9.81m/s^2
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.
It's at sea level :)
Which sea? The Indian ocean if I recall correctly has a very low gravity value.
we learned it was about 9.8. We actually measured what it was near our school, and I think it came out to 9.82. We were told it was ok to use either 9.8, 9.82 or 10 in exams.
ITT: People who all apparently remembers what the freaking constant for gravity is down to the decimal.
This is the askscience community, what do you expect? :)
At least the guys in here knows their shit. Impressive.
The first time we used it was 7th or 8th grade and we kept using it from then on. How can you even forget?
You should ask these people how many digits of pi they know. My 100-something isn't even that impressive.
All of them. 0,1,2,3,4,5,6,7,8,9.
9.776 still rounds to 9.81, though. It's a fun fact but it's probably not going to mess up your homework.
Apparently back before electronic calculators they often just said 10 at the secondary school level.