@MarkT:

An object sat on the surface of the earth experiences a 9.8m/s^2

_upwards_ acceleration.

True, but it also experiences a 9.8m/s^2 acceleration downwards due to gravity. The two balance out and so the object doesn't move.

Lets be clear here, the accelerometer, in its own frame of reference, experiences

an acceleration upwards - that's all it knows (apart from the tidal forces, but they

are much much smaller).

In our frame of reference we also experience an acceleration upwards - we are in

the same frame, to a first approximation, so see the accelerometer as stationary.

Nothing is or appears to be accelerating downwards from this frame. The force from

the floor is not cancelled out by anything, its a true upwards acceleration we feel

from our feet.

This is the key insight behind general relativity, that inertial and gravitational mass

are identical and apparent forces due to gravity are inertial forces.

The fact it doesn't appear to move due to this acceleration is because we are accelerating

with it.

No. We aren't accelerating and neither is the object. If we were accelerating we'd be moving.

see answer above.

There are two opposing forces on the object. The earth's gravity trying to pull it towards the centre of the earth and the force of the ground pushing it up by exactly the opposite amount. If the two forces aren't balanced, the object will be moving.

There is no force due to gravity, there is only curved space, described by the

gravitational tensor.

The fact that we stay the same distance from the centre of the earth is because

space is curved.

No. The curvature of space around earth is caused by the earth's mass and the result of the curvature is what we perceive as gravity.

Yes the curvature is caused by the mass, and YES that's why we don't appear to move

relative to the earth's centre. We perceive the force from the floor, which is due to

the earth being an elastic body, resisting the deformation that the curvature gives it.

An object in free-fall has zero acceleration

That is a contradiction. An object in free fall is, by definition, accelerating.

An object in free fall is free of all forces, and therefore subject to zero acceleration

in its own frame - this makes it an inertial frame (apart from those pesky tidal forces,

themselves the signature that space is curved).

Pete

[/quote]

I recommend a course on general relativity. Its been validated by several elegant

experiments since Einstein proposed it. Without it you cannot explain why atomic clocks run

apparently faster at altitude - with it you realise you have clocks in different accelerating

frames hence the difference