Has anyone ever been able to recreate a body of water bending around a spinning ball in a mass to scale model? If so, please share!
Listen. What you think you know about gravity is wrong. Forget all that, and I'll explain what you need to know.
BasicsGravity is observed as an attractive force between objects with mass. The more mass something has, the greater the gravitational force is that it exerts. Gravity decreases with distance by the inverse square law.
Earth is very massive (literally), and has a gravitational acceleration of between 9.78 and 9.83 meters per second per second, or m/s
2, at the surface. The value is less at the equator than at the poles because the earth bulges slightly and is spinning. However, the centrifugal force at the equator is not enough to fling anyone off, despite the often-quoted statistic that it's spinning at "1,000 miles per hour." In fact, this effect only accounts for a reduction of .03392 m/s
2 at the equator, compared to the poles.
BuoyancyBuoyancy occurs because the fluid medium that is displaced by an object has weight of its own (due to gravity). If an object weighs less than the quantity of fluid it displaces, gravity is acting more strongly on the displaced fluid than the object. Thus, the fluid will be accelerated by gravity toward the center of the earth, and the object will be forced away. If the object is heavier than the displaced fluid, the opposite will occur.
Remember, "down" is not an absolute direction in space, and is instead whichever direction the center of the earth is for a particular location. Hence why people in the southern hemisphere don't "fall off."
TidesGravity is the cause of tides. Because gravity follows the inverse-square law, there is a difference between the strength at the closest and furthest points of a body, and the difference is greater the closer the bodies are. The differential force, relative to the center of the body, is directed outward on the sides closest and furthest from the body. This effect is shown here:
Tides are the reason the moon always faces the earth. The bulge caused by tidal force is rotated as the body spins, and is dragged backward by gravity. This eventually causes the body to rotate at the same rate it revolves, and is known as
tidal locking. This happened to the moon a long time ago, and is currently happening to the earth, causing our days to get slightly longer and the moon to slowly recede.
Tidal locking is also observed throughout the solar system. The Galilean moons of Jupiter are tidally locked to it, as are the moons of Mars.
When a body reaches a point close enough to another body, denoted by the Roche limit, the differential force is enough to rip the body apart. This is the reason why the rings of the gas giants don't coalesce into moons. In fact, it's quite likely that they were once moons themselves, if not asteroids.
Even outside the Roche limit, bodies are still subject to tidal heating. The strong periodic tidal variations of Jupiter's first moon Io cause intense heating, resulting in it being the most volcanically active body in the solar system.
OrbitsGravity is also responsible for orbits. Because gravity follows the inverse-square law, the math works out so that in a two-body system, objects in free fall ideally follow conic sections, which are circles, ellipses, parabolas, and hyperbolas, with the system's center of mass being at the focus.
For this reason, we know that the positions of orbiting bodies can be tracked and predicted using parameters that describe ellipses. These parameters, also known as orbital or Kepler elements, can be somewhat confusing, so I'll just address semi-major axis, eccentricity, and mean or true anomaly. The other elements describe the orientation of the ellipse in space.
The semi-major axis (a) is half of the distance across the widest part of the ellipse. Eccentricity (e) refers to the "flatness" of the ellipse. The true anomaly (Θ or TA) refers to which point along the ellipse the satellite is located. The orbital period, the time it takes to complete an orbit, is directly related to its semi-major axis. That is, if two satellites have the same semi-major axis, they will complete there orbits in the same amount of time, no matter the other parameters.
Because we can describe the orbits of satellites and the moon the same way as the moons of other planets, as well as the planets themselves, we know that the earth is a planet like the others, and will be subject to the same effects.
ShapingGravity causes large enough bodies to collapse into spheres. This state is called hydrostatic equilibrium, and is the result of the same force being applied from all sides. A similar effect can be observed in water droplets, except in that case the force is surface tension, not gravity. Since we have already established that the earth follows the same rules as other planets, the natural conclusion is that the earth itself was collapsed into a sphere by its own gravity.
ConclusionThis should be all you need to know about gravity. Its existence is not up for debate, despite what the people on this forum think, because it can be observed as the governing force of the solar system and the universe at large scales.
You won't need to concern yourself with advanced details like relativity or dark matter or the exact cause of gravity, and you're not qualified to disprove them. Leave the advanced physics to advanced physicists and the advanced astronomy to advanced astronomers, because they dedicate their careers to knowing what they're talking about in these subjects. They know that dark matter and relativity make enough sense, and the best you can do is take their word for it.
If you want to view and experiment with gravity, I suggest you purchase a copy of Universe Sandbox
2.