5 ms·

The acceleration due to gravity isn’t a constant of nature —- it’s dependent on the mass and mutual distance of both objects. EDIT: distance, not size

moron4hire 5y ago
Perhaps they meant the gravitational constant, which is not known to a very high degree of precision.
- saurik 5y ago
  But we've got to have it down within two standard deviations, right? ...No? Do I just not know what a sigma is? :(
  - penteract 5y ago
    I think you have a misunderstanding about sigma. When describing the measurement of a particular physical constant, the "standard deviation" is something that changes as we get better at making measurements. It basically means "If all our assumptions (e.g. assumptions about how good our equipment is, uncertainties about other physical constants) are correct, then it is unlikely that we would have made the measurements we did if the true value is not within 2 standard deviations of the result we got". When a more accurate measurement is made, then "1 standard deviation" gets smaller, so we know the value better, but it's always true to say "we know the value to within a few standard deviations (given some assumptions made by experimenters)" . If it turns out the measurement was wrong by several standard deviations, then it's very likely that some assumptions were wrong.
    - davrosthedalek 5y ago
      You are absolutely right. For deviations beyond, say 4 or 5 sigma, it's much more probable that it's not a statistical fluke, but a systematic error. Assumption wrong, experiment wrong, theory wrong or something like that. We expect that measurements land within +-1 sigma of the true value with ~68% probability (and 95% for +-2sigma). Implicitly, we assume a Gaussian distribution (often the case to good approximation, at least for small deviations), and also that we can invert the sentence: the true value is within the error band around the measurement with that probability.
SkittyDog 5y ago
Your correction is wrong... The size of the two objects is irrelevant. The only variables are their respective masses and the DISTANCE between them. If it's worth correcting people, it's worth correcting them correctly... Don't you agree? Also... The previous poster was pretty obviously talking about the constant of Gravitation (https://en.m.wikipedia.org/wiki/Gravitational_constant https://en.m.wikipedia.org/wiki/Gravitational_constant). His way of phrasing it is a common English shorthand that I see frequently enough that it's a well understood usage. They may not have used precise language, but their phrasing was definitely less misleading than your (incorrect) correction.
- jacksnipe 5y ago
  I assumed the gp was thinking about 9.8m/s^2 — because on the English speaking non-physics-expert world, this is called the acceleration due to gravity. You’re right that it’s distance, of course, but I was coming from a place of trying to be helpful, and thought that if they were talking about terrestrial physics, it’d be easiest to imply it depended on the size of the earth (which determines our distance from it) and your height.
  - ummonk 5y ago
    That’s not something that could change based on experiment though. We define standard gravity as 9.80665 m/s^2 but the actual value will vary considerably based on location on the earth.
  - SkittyDog 5y ago
    I don't buy that you were coming to this from a place of trying to be helpful. I know that here on HN, we're supposed to assume good motives in other posters, but I just can't do it, here. I think you were trying to score points, and get an ego fix by correcting someone else. Not so much fun when someone else dunks on you, though, is it?
- ummonk 5y ago
  It’s only dependent on the mass of the other object, not the mass of the object whose acceleration we’re measuring.
uoaei 5y ago
I'm sure they meant the gravitational constant. Jeez.

moron4hire 5y ago

Perhaps they meant the gravitational constant, which is not known to a very high degree of precision.

saurik 5y ago
But we've got to have it down within two standard deviations, right? ...No? Do I just not know what a sigma is? :(
- penteract 5y ago
  I think you have a misunderstanding about sigma. When describing the measurement of a particular physical constant, the "standard deviation" is something that changes as we get better at making measurements. It basically means "If all our assumptions (e.g. assumptions about how good our equipment is, uncertainties about other physical constants) are correct, then it is unlikely that we would have made the measurements we did if the true value is not within 2 standard deviations of the result we got". When a more accurate measurement is made, then "1 standard deviation" gets smaller, so we know the value better, but it's always true to say "we know the value to within a few standard deviations (given some assumptions made by experimenters)" . If it turns out the measurement was wrong by several standard deviations, then it's very likely that some assumptions were wrong.
  - davrosthedalek 5y ago
    You are absolutely right. For deviations beyond, say 4 or 5 sigma, it's much more probable that it's not a statistical fluke, but a systematic error. Assumption wrong, experiment wrong, theory wrong or something like that. We expect that measurements land within +-1 sigma of the true value with ~68% probability (and 95% for +-2sigma). Implicitly, we assume a Gaussian distribution (often the case to good approximation, at least for small deviations), and also that we can invert the sentence: the true value is within the error band around the measurement with that probability.

SkittyDog 5y ago

Your correction is wrong... The size of the two objects is irrelevant. The only variables are their respective masses and the DISTANCE between them. If it's worth correcting people, it's worth correcting them correctly... Don't you agree? Also... The previous poster was pretty obviously talking about the constant of Gravitation (https://en.m.wikipedia.org/wiki/Gravitational_constant https://en.m.wikipedia.org/wiki/Gravitational_constant). His way of phrasing it is a common English shorthand that I see frequently enough that it's a well understood usage. They may not have used precise language, but their phrasing was definitely less misleading than your (incorrect) correction.

jacksnipe 5y ago
I assumed the gp was thinking about 9.8m/s^2 — because on the English speaking non-physics-expert world, this is called the acceleration due to gravity. You’re right that it’s distance, of course, but I was coming from a place of trying to be helpful, and thought that if they were talking about terrestrial physics, it’d be easiest to imply it depended on the size of the earth (which determines our distance from it) and your height.
- ummonk 5y ago
  That’s not something that could change based on experiment though. We define standard gravity as 9.80665 m/s^2 but the actual value will vary considerably based on location on the earth.
- SkittyDog 5y ago
  I don't buy that you were coming to this from a place of trying to be helpful. I know that here on HN, we're supposed to assume good motives in other posters, but I just can't do it, here. I think you were trying to score points, and get an ego fix by correcting someone else. Not so much fun when someone else dunks on you, though, is it?
ummonk 5y ago
It’s only dependent on the mass of the other object, not the mass of the object whose acceleration we’re measuring.

uoaei 5y ago

I'm sure they meant the gravitational constant. Jeez.