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 Originally Posted by Renton
Anyway more gravity questions. I'm not sure you really answered in the thread why the "curved spacetime" theory of gravity explains why an object at rest will still be accelerated toward the dominant mass by gravity. If "falling" bodies are moving in straight lines through curved spacetime at with their existing momentum, why does something with no momentum fall to the ground?
:/
Tough one. I've thought about this one a bit and I just don't know. I'm going to do some research and get back to you.
I can do some hand waving and say that nothing is truly at rest in any non-accelerating reference frame, so any 2 bodies will have (or develop) very slight relative motions which are amplified over time.
Once the bodies are moving relative to each other, then things fit the picture.
It's a weak, and probably incorrect, assessment.
Originally Posted by Renton
Somewhat unrelated question. As I understand it, if you're moving through space at constant speed (acceleration = 0) from your reference frame you appear not to be moving and do not feel as if you are moving. This is why we aren't pushed into our seats when on a moving train except when it accelerates from rest. And from a stationary reference point in space, the Sun (and hence its satellites) is moving 370 km/s. But since the earth orbits the Sun at about 30 km/s, that would mean my actual speed is 370 km/s +/- 30 depending on where the earth is in it's revolution. Does this mean that we are in a constant sine-wave like state of acceleration and deceleration that we would be able to feel if our sense of inertia were finely tuned?
Some heavy math and physics by a group of people trying to measure exactly that.
Short answer: Yes. A bit.
The Earth is in free-fall orbit around the sun and moon, so we don't feel their gravitational forces on the Earth's surface.
If you're in a free falling elevator (god forbid), you would not feel Earth's gravity. You would appear weightless in an elevator (for however brief a time).
Things are heavier at sunrise/susnet than at midnight and noon. This is due to tidal forces of the sun.
Which brings up the tidal forces.
The moon's tidal forces are much more prevalent than the sun's, due to the proximity of the moon.
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Things are heavier near the poles than near the equator. Since the Earth is spinning, objects within the spinning (non-Newtonian) reference frame will feel centrifugal force. This force is expressed in opposition to gravity.
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The Earth's mass in not evenly distributed, so the gravitational force varies across the surface.
Things are heavier on mountains, because there's a mountain of extra mass underneath them, pulling them down.
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