Oil and gravity If we put oil in a cup and then we put water in the same cup, the oil will rise to the top. But isn't gravity supposed to stop that. Like if the oil was at the bottom isn't it supposed to stay there since gravity is pushing it down. Or it's just because gravity isn't THAT strong.
edit: I meant the water will rise to the top.
 A: Gravity pulls both the oil and the water down. But water is denser, so it pulls the water down more strongly, past the oil. Thus, you end up with water at the bottom and oil on top. If you drop sand in, gravity will pull that down even more strongly....
A: Archimedes' Principle says that
The buoyant force on a submerged object is equal to the weight of the fluid displaced.
In fact, no word about gravity in that rule. Why?  In Archimedes' rule gravity is implied, it is taken for granted. Buoyancy is considered a force counteracting  gravity, a force on its own.  In water, objects differ in buoyancy, not gravity. And, without gravity there cannot be buoyancy as buoyancy is the upthrust that needs the downthrust as its counterforce.
Gravity which is defined as an accelerating force (F = m x a), does not depend on the object's density, it only depends on the mass of the object. Newton's F = m x a cannot explain buoyancy.
On  scales (Archimedian lever) weights of "objects" are set against each other - their masses are measured using gravity, not their densities. Mass (which gravity refers to) of an object does not depend on the object's density. Thus, the  "object" - the boat   has the m, the mass, in the formula. A drop of oil and a drop of water that have  the same mass (not densities) taken as "objects" on scales will be accelerated and drawn to earth in the same way.  Only with the water surrounding  boats and oil drops will buoyancy come in.
To explain the why's and reasons of buoyancy gravity seems not enough. Wikipedia and other sources elaborate on "pressure". Such reasoning may not lead to the conclusion that buoyancy depends on the depth of immersion, i.e. the pressure of the surrounding water, thus it's depth,  which would be  untrue. Buoyancy as a force does not depend on depth in water, nor on the height or the form of the object immersed, thus cannot be derived or calculated from the pressure a certain level of liquid has. To explain buoyancy refering to pressure  may in my opinion be considered tautological; it "cannot be the height of the can".
Wikipedia on buoyancy:
"Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object."
In any case, those sources consider buoyancy being derived from gravity, as the pressure of a water column is a result of gravity.
To sum up:
"Or it's just because gravity isn't THAT strong."
Gravity as a force only relates to the mass of the object, no difference in strength of gravity.
"edit: I meant the water will rise to the top."
The oil will remain on top because its downthrust derived from gravity will be counteracted by its buoyancy (upthrust) that depends on its density, seen as volume of water displaced.
A: The configuration with oil on top and water on bottom has a lower center of mass than water on top with oil on bottom. Gravity will always tend to cause systems or objects to move their center of mass to the lowest point. Just as an inverted pyramid will tip over.
If gravity were stronger as you say, this would happen even faster, not be less likely to happen.
