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?

  • hissing meerkat
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    137 months ago

    In freshman college physics we had a lab to measure gravity then had to use our lab result for the rest of the course.

    • @[email protected]
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      57 months ago

      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…

      • Zoot
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        27 months ago

        Could you explain to me why that last part matters?

        • @[email protected]
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          7 months ago

          I’m assuming they’re indicating that the mass below the apparatus increased in fall (when storage was filled) and decreased slowly through the winter, leading them to measure a changed graviational constant. A back of the napkin calculation shows that in order to change the measured gravitational constant by 1 %, by placing a point mass 1 m below the apparatus, that point mass would need to be about 15 000 tons. That’s not a huge number, and it’s not unlikely that their measuring equipment could measure the gravitational acceleration to much better precision than 1 %, I still think it sounds a bit unlikely.

          Remember: If we place the point mass (or equivalently, centre of mass of the coal heap) 2 m below the apparatus instead of 1 m, we need 60 000 tons to get the same effect (because gravitational force scales as inverse distance squared). To me this sounds like a fun “wandering story”, that without being impossible definitely sounds unlikely.

          For reference: The coal consumption of Luxembourg in 2016 was roughly 90 000 tons. Coal has a density of roughly 1500 kg / m3, so 15 000 tons of coal is about 10 000 m3, or a 21.5 m x 21.5 m x 21.5 m cube, or about four olympic swimming pools.

          Edit: The above density calculations use the density of coal, not the (significantly lower) density of a coal heap, which contains a lot of air in-between the coal lumps. My guess on the density of a coal heap is in the range of ≈ 1000 kg / m3 (equivalent to guessing that a coal heap has a void fraction of ≈ 1 / 3.)

          • Zoot
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            37 months ago

            Thank you for the very well detailed explanation, as well as the visual. Much appreciated!

          • AlexisFR
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            17 months ago

            À better question is why is a university still using coal heating in the modern age?

            • @[email protected]
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              27 months ago

              This observation further compounds the hypothesis of “fun wandering story that has been told from person to person for a long time”

              • @[email protected]
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                37 months ago

                Fits in with the sinking library and Native American graveyard (though i believe that the exact second one may be regionally locked)

        • @[email protected]
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          17 months ago

          Can’t be that big, as the difference in mass close to the instrument only varied in the several tons category, but obviously enough to puzzle the scientists.

          • @[email protected]
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            17 months ago

            Well yeah. I was just curious if the difference was on the order of millimeters or microns /m².