The wiki argues that objects fall due to the earth constantly accelerating and gravity is weaker at higher altitudes due to celestial gravity. The closerto the sky the weaker the downward force as the celestial gravity is pulling you up.
But there is less gravity underneath the earth's surface such as in a mine shaft.
How does this explain the downwards force being weaker the further from the sky you are?
Going below the earth's surface is still going further from the sky.
What an astute observation. But it doesn't explain why 'gravity' is less when you go underground, rather than more as is suggested by how celestial gravitation explains lower gravity at greater heights.
This is false. Gravity actually increases as you descend. There are numerous RET excuses for why this is so, but the fact remains that gravity increases below the earth's surface.
Introduction to Physical Oceanography: Third Edition
Physical and Hydrologic Properties of Water-Bearing Deposits in Subsiding Areas in Central California
http://www.exo.net/ti/pinhole/hypermail-00/0064.html
But in the real earth which is denser at the center the
force of gravity actually increases with depth at least down to the depth
of the deepest mines.
Actually, nobody here is right on this one.
In theory - gravity ought to get less as you go deeper into the Earth. In practice it's very complicated and it might well increase.
The simplistic argument for the DECREASE in gravity as you go deeper is this:
As you descend below the surface, more and more of the mass of the Earth is above your head and less and less of it is below your feet. The force of attraction from the mass above you increases and the mass below you decreases. Hence the downward force gets smaller and the upward force gets larger - and the net effect is that the force of gravity decreases as you descend into the Earth.
Another way to imagine this is if you were at the very center of the Earth (yeah, I know, it's kinda hot down there!) - but in that case each little piece of the Earth is pulling on you - but because you're in the center, all of those little forces cancel out - so there is no gravity whatever at the center of the Earth.
HOWEVER: The density of the Earth isn't uniform. The crust is a lot less dense than the deeper materials.
Hence, although there is more volume above you - the mass isn't all that much...so this decreasing gravity effect is countered by the fact that you're getting closer to the denser stuff - and gravity increases the closer you get to things.
But this too varies a lot. In some parts of the world, there is dense, heavy rock near the surface - and in others it's much lighter.
So IN REALITY: In some parts of the world, Gravity does increase for a while as you go deeper - but then it begins to level off - and then actually decrease again. In other parts, it only decreases. However, beyond some insanely deep depth, it ALWAYS eventually starts to decrease until it's zero at the center of the Earth.