Warning: this post is
Too
Long;
Don't
Read it.

Everyone here can acknowledge that when you drop a brick, it will accelerate at 9.8 m/s/s towards your big toe, and we can all call that "gravitation" (even if you don't believe in the Force of Gravity). We can also all agree that the greater the mass of the brick, the greater the force impinged upon aforementioned toe on impact.
There are three possible explanations for this phenomena:
- We are on the inside of a giant wheel (the Halo model) and it is spinning at such an angular velocity to cause we humans who live at its circumference to experience centripetal acceleration, even though technically centrifugal force doesn't exist.
- We live on the surface of a disk which is accelerating upward at 9.8 m/s/s (the magic elevator model). While we experience grativation, gravity doesn't exist.
- We are on the surface of a spheroid planet, with a mass and radius such that at our distance from the center, we experience a gravitation acceleration of 9.8 m/s/s (the round earth model)
If you lived inside a house with no windows there's really no way you could tell the difference between these three models. (If your house were small enough, the "magic elevator" could actually be an accelerating rocketship.)
So how could you find out the source of this mysterious downward force called gravitation? By observation, which means you need to get out of that armchair and out of the house.
1. Halo Model: If you were in the Halo, you'd of course notice this giant thing swooping up into the sky (which would probably look really cool at night). But from a physics standpoint, you could travel the entire length and width of the cylindrical halo, and your gravitational force would remain constant. But there's a limit, for the halo has an edge, and you wouldn't want to fall off.
Plus by observing things
not on the halo such as stars and other celestial bodies, you can clearly see your circular movement. All stars would appear to rotate around two points marking the end points of the axis of rotation of your halo.
2. The magic elevator model: (Better known as Flat Earth Theory) Again, we could travel the length and width of the flat earth, and we'd find gravitation to be constant, but there's the limit of not being able to cross the edge (the Ice Wall).
Again, we can observe the motions of heavenly bodies not on our magic elevator. They'd by definition have to accelerate downward at the rate of 9.8 m/s/s, which would make astronomy a little inconvenient. Since we don't see everything accelerating downward away from us, we have to conclude that the entire universe is with us on this magic elevator, which makes for an interesting physics problem.
If the entire universe is accelerating upwards at 9.8 m/s/s then this means that the force of gravitation is the same everywhere, and is aligned downward everywhere. The motions of our solar system's planets pose an especially intractable problem, because there has to be some force holding them "up" just as the ground beneath our feet holds us "up" since we're accelerating upwards.
3. Round Earth Model: (We'll make things complicated by making this Round Earth rotate on an imaginary axis.) If we lived on a round earth, then several things must occur. For one, explorers should be limitless in circumnavigating the planet. The motions of the stars in the night sky must appear to rotate around two imaginary points marking the end points of our earth's axis.
The force of gravitation, attracting us towards the ground, must be explained somehow, since we're not inside a spinning Halo, nor are we on a magic elevator. Therefore we'd have to perform experiments demonstrating that for some unknown reason, things with mass are attracted to one another.
http://en.wikipedia.org/wiki/Cavendish_experimenthttp://www.fourmilab.ch/gravitation/foobar/ (do this one in your basement!)
What would be really great would be if we came up with some "law of gravity" we could apply that to the rest of the planets and moons of our solar system. If things with mass are attracted together, then the Earth and Moon would have to have some way of not crashing into each other, and would therefore have to swing around each other, so that each planet's centripetal acceleration perfectly counteracts the force of "gravity" at that distance.
You know, it would be really great if
all planets and moons could obey this law. I'm tired of living on this Flat Earth.
