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0 pointsxhkkffbf5mo ago0 comments

It doesn't hold true across the universe? I thought this was one of the more universal things like the speed of light.

0 comments

G, the gravitational constant is (as far as we know) universal. I don't think this is what they meant, but the use of "across the universe" in the parent comment is confusing.

g, the net acceleration from gravity and the Earth's rotation is what is 9.8m/s² at the surface, on average. It varies slightly with location and altitude (less than 1% for anywhere on the surface IIRC), so "it's 9.8 everywhere" is the model that's wrong but good enough a lot of the time.

fragmede5mo ago

It doesn't even hold true on Earth! Nevermind other planets being of different sizes making that number change, that equation doesn't account for the atmosphere and air resistance from that. If we drop a feather that isn't crumpled up, it'll float down gently at anything but 9.8m/s². In sports, air resistance of different balls is enough that how fast something drops is also not exactly 9.8m/s², which is why peak athlete skills often don't transfer between sports. So, as a model, when we ignore air resistance it's good enough, a lot of the time, but sometimes it's not a good model because we do need to care about air resistance.

hackeman3005mo ago

Gravity isn't 9.8m/s/s across the universe. If you're at higher or lower elevations (or outside the Earth's gravitational pull entirely), the acceleration will be different.

Their point was the 9.8 model is good enough for most things on Earth, the model doesn't need to be perfect across the universe to be useful.

JAlexoid5mo ago

g(lower case) is literally gravitational force of Earth at surface level. It's universally true, as there's only one Earth in this universe.

G is the gravitational constant which is also universally true(erm... to the best of our knowledge), g is calculated using gravitational constant.

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