----- 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_?
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")
Michel
