On 30/7/2007 1:25 PM, Michel Jullian wrote:
>
> ----- Original Message -----
> From: "Harry Veeder" <[EMAIL PROTECTED]>
> To: <[email protected]>
> Sent: Monday, July 30, 2007 8:34 PM
> Subject: Re: [Vo]:Re: The Earth and the Wheel
>
>
>> On 30/7/2007 9:00 AM, Michel Jullian wrote:
>>
>>>
>>> ----- Original Message -----
>>> From: "Harry Veeder" <[EMAIL PROTECTED]>
>>> Sent: Sunday, July 29, 2007 8:55 PM
>>>
>>>>>> If you could stop the Earth rotating, the wheel that has the brake on
>>>>>> will
>>>>>> continue to remain at rest wrt to the ground but the other wheel will
>>>>>> continue to turn once per day wrt to the ground.
>>>>>>
>>>>>> The two motions differ, but the difference is masked by the rotation of
>>>>>> the
>>>>>> Earth.
>>>>>>
>>>>>> Do you agree?
>>>>>
>>>>> I would say on the contrary that they do not differ, unless the Earth
>>>>> stops
>>>>> rotating. Releasing a brake between parts which have no tendency to move
>>>>> wrt
>>>>> each other will make no difference whatsoever, agreed?
>>>>>
>>>>> Michel
>>>>>
>>>>
>>>> No, and here is an argument without stopping the Earth's rotation.
>>>> Mount each supporting frame on its own turntable. A slight turn of each
>>>> turntable will show that only the wheel with the braked released is
>>>> rotating
>>>> about its own centre.
>>>
>>> 1/ It's about as unlikely that the Earth will be given a slight turn (by the
>>> hand of God?) as it is that it might stop rotating, but you're right, to be
>>> complete I should have said "they do not differ, unless the Earth's rate of
>>> rotation changes one way or another".
>>
>> I should have emphasised that the hand of god is not required in this
>> example. The hub rests in a frame and the frame sits on a turntable and the
>> turntable sits on the ground. If I swivel the turntable 90 (or by any
>> amount) after the brake is released a person standing next to the wheel will
>> be able to discover that the wheel is rotating (1 rev./day) about its hub.
>>
>>> The reasons have been helpfully given by
>>> Horace and you seemed to agree with them (with unnecessary defensiveness
>>> though if I may give an independent observer's impression)
>>>
>>> 2/ Actually my "agreed?" question referred to my second sentence above:
>>> "Releasing a brake between parts which have no tendency to move wrt each
>>> other
>>> will make no difference whatsoever" [...implied: in their relative motion,
>>> which will remain null]
>>
>>> 3/ The initially braked wheel and the Earth in your scenario are in the case
>>> of point 2 above. The rotation of the Earth "shakes" the wheel around
>>> admittedly, but the wheel being frictionless this shaking around only
>>> produces
>>> a radial force which as Horace pointed out produces no torque so no change
>>> in
>>> angular momentum.
>>>
>>
>>> 4/ There is no rotation of the wheel wrt its hub after the brake is
>>> released,
>>> agreed?
>>
>>
>> Unfortunately no.
> ...
>
> Then look at your own drawing, which is correct, do you observe a rotation of
> the wheel _wrt its hub_?
I observe the Sun going around the Earth and can represent this observation
with a drawing. However the drawing does not prove that the Earth is not
revolving about the Sun. Similarly, the Earth-wheel drawing does not prove
that the wheel is not rotating wrt to its hub.
> About your other experiment with a turntable if you want to draw conclusions
> wrt the Earth-wheel system don't forget the turntable should be rotating at
> _constant angular velocity_ both BEFORE and AFTER the brake is released, like
> the Earth in the Earth-wheel system. This is not what you said you did ("if I
> swivel the turntable 90 (or by any amount) _after_ the brake is released")
A non-rotating turntable wrt to the ground has a constant angular
velocity of zero so I don't see why your particular conditions
are essential.
Harry
