On 10/20/2015 10:45 PM, clipka wrote:
> 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.
>

No, I don't. I selected an observational frame of reference where I 
could observe both objects, a few posts back.

> 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.
>

I am having difficulty with that. My chosen frame of reference is 
inertial and both the apple and Earth are accelerating. Albeit a very 
small acceleration in the case of the Earth.

> 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.
>

I agree with that.

>>> 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.
>
No. I read radially as meaning "Radiating from or converging to a common 
centre." Not along a radius.

A bit of a slap in the face with a wet fish. *



Misunderstanding understood.

> 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.
>

With a pendulum you could but who am I to teach my grandmother to suck 
eggs when it comes to nitpicking. ;-)

>> 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?
>

Because I asked nicely.

> - 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.
>

Great! a mass of round about 100 grammes distorts the Earth's surface.

> - 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.
>
Rough and ready:
http://hypertextbook.com/facts/2004/WaiWingLeung.shtml


> 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.
>

Personally I prefer to pick my frames to make sense in the real world.
If you can't hit it with a hammer or build a bridge out of it. Then it 
is just an interesting thought.

*
I found this Wiki entry
https://en.wikipedia.org/wiki/Wikipedia:Whacking_with_a_wet_trout

-- 

Regards
     Stephen

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