Am 20.10.2015 um 17:24 schrieb Stephen:

>> Whatever frame of reference you choose - if it is truly inertial, you'll
>> always find the apple (while airborne) moving at constant velocity (or
>> remaining at rest, which is a special case thereof),
> 
> Assuming that you are not trying to wind me up.
> What are your reasons for saying that? If your frame of reference was
> the apple I would agree with you. But I was talking about a frame of
> reference separate from both of the objects.

Okay, so you agree that the dropping apple's frame of reference is an
inertial one.

Then by definition of what constitutes an inertial frame of reference,
all (sufficiently local) frames of reference in which the apple appears
to be moving at constant speed are inertial, while all in which the
apple appears to be accelerating are non-inertial.

Now no matter what frame of reference you choose, you'll find that
Earth's surface and the apple appear to be accelerating towards each
other; thus any frame of reference in which the apple isn't
accelerating, must have Earth's surface accelerating towards it instead.

>> and Earth's surface
>> accelerating radially from its center at 9.81 m/s^2.
> 
> To me that reads that the Earth's surface (and volume) is expanding.
> Something I personally haven't noticed.

That's because it is in equilibrium with the collapse of space.

What you should be able to notice is you being subject to acceleration:
If someone locked you in an elevator cabin, you couldn't tell the
difference between being "stationary" on Earth's surface, and being
subject to an acceleration of 9.81 m/s^2 in deep space.

Because there is no difference.

> Since we are talking about speeds of metres per second I assume we can
> dismiss any relativistic effects.

Strictly speaking we can't, as the whole thing /is/ a relativistic effect.

> Can you explain why a force of one Newton acting on a mass of 5.97E+24
> Kg can produce an acceleration of 9.81 m/s^2?

No, but why should I?

- Not all of Earth's mass is accelerating at 9.81 m/s^2; the deeper you
go, the smaller the acceration, until it drops to zero at Earth's center.

- I have no idea where you got the one Newton from. The actual force
pushing Earth's surface outward is about _ten_ Newton _per kg of surface
mass_. More precisely it is 9.81 N/kg. Or 9.81 m/s^2.

Yup, that's a mind-bogglingly huge total force acting on Earth's mass.
But electromagnetic forces between atoms under pressure /are/
mind-bogglingly huge.

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