3 ms·

why is the sea level lower where the gravity is lower? surely more gravity would create more "pull"? confused by how this works...

rob74 3y ago
Maybe more gravity in other places creates more pull there and pulls the water away from places with less gravity? Similar to how the gravity of the moon pulls at the water, creating high tide? But that's just my guess, I'm not an expert either...
wumms 3y ago
(From the article) "As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average"
netsharc 3y ago
Probably like iron filings and magnets, the stronger magnet would have a bigger pile of iron filings on it. Water isn't compressible, so the distribution would always give the stronger region more water.
- a1o 3y ago
  Water is dense and gravity is proportional to mass, I wonder if there's any chance that water itself is also a contributor to the perceived gravity at that region.
  - kibwen 3y ago
    I don't think the mass of the water would have a very noticeable effect on the perceived gravity. The average depth of the ocean is about 4 km, whereas the radius of the Earth is 6,000 km, and stone is denser than water as well.
pmontra 3y ago
> more gravity would create more "pull"? Exactly. There is more mass below the ocean in the areas around where the hole developed and less mass below the hole. That mass is pulling water from the hole so it became a hole. By the way, less water in the hole and more around it further contribute to making the hole deep because water also have a mass. It doesn't go on forever up to draining the center of the hole dry, there is an equilibrium. Calculating the sea levels could be an unusual physics problem for students.
rcxdude 3y ago
gravity's impact on liquids is much more defined by them being pulled 'sideways' than 'up' or 'down'. For example, the tides are not mostly caused by the water being lighter or heavier due to the tidal forces of the moon and being pulled down by a greater or lesser degree, but by the sideways pull of the water on the 'side' of the earth causing it to bunch up where the sideways pulls converge (which is why you get tides with bodies of water as big as the oceans but not so much with lakes). The same thing happens here: stronger gravity elsewhere pulls the water sideways away from the low-gravity area, causing the depression (in fact, absent dynamic effects and other forces like wind, the surface of a liquid will be an isograv: every point at its surface will have the same gravitational potential).
PopAlongKid 3y ago
This is similar to recent discussions here on HN around the effects of melting ice (Antarctica, Greenland) on sea level. Not only does the meltwater itself cause increase, but the gravitational pull of so much ice pulls the water level around it up higher, resulting in lower sea level everywhere else. When the ice melts, that local gravity lessens, letting the surrounding elevated water spread out to the rest of the ocearn.

rob74 3y ago

Maybe more gravity in other places creates more pull there and pulls the water away from places with less gravity? Similar to how the gravity of the moon pulls at the water, creating high tide? But that's just my guess, I'm not an expert either...

wumms 3y ago

(From the article) "As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average"

netsharc 3y ago

Probably like iron filings and magnets, the stronger magnet would have a bigger pile of iron filings on it. Water isn't compressible, so the distribution would always give the stronger region more water.

a1o 3y ago
Water is dense and gravity is proportional to mass, I wonder if there's any chance that water itself is also a contributor to the perceived gravity at that region.
- kibwen 3y ago
  I don't think the mass of the water would have a very noticeable effect on the perceived gravity. The average depth of the ocean is about 4 km, whereas the radius of the Earth is 6,000 km, and stone is denser than water as well.

pmontra 3y ago

> more gravity would create more "pull"? Exactly. There is more mass below the ocean in the areas around where the hole developed and less mass below the hole. That mass is pulling water from the hole so it became a hole. By the way, less water in the hole and more around it further contribute to making the hole deep because water also have a mass. It doesn't go on forever up to draining the center of the hole dry, there is an equilibrium. Calculating the sea levels could be an unusual physics problem for students.

rcxdude 3y ago

gravity's impact on liquids is much more defined by them being pulled 'sideways' than 'up' or 'down'. For example, the tides are not mostly caused by the water being lighter or heavier due to the tidal forces of the moon and being pulled down by a greater or lesser degree, but by the sideways pull of the water on the 'side' of the earth causing it to bunch up where the sideways pulls converge (which is why you get tides with bodies of water as big as the oceans but not so much with lakes). The same thing happens here: stronger gravity elsewhere pulls the water sideways away from the low-gravity area, causing the depression (in fact, absent dynamic effects and other forces like wind, the surface of a liquid will be an isograv: every point at its surface will have the same gravitational potential).

PopAlongKid 3y ago

This is similar to recent discussions here on HN around the effects of melting ice (Antarctica, Greenland) on sea level. Not only does the meltwater itself cause increase, but the gravitational pull of so much ice pulls the water level around it up higher, resulting in lower sea level everywhere else. When the ice melts, that local gravity lessens, letting the surrounding elevated water spread out to the rest of the ocearn.