----- Original Message ----- From: "Harry Veeder" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Monday, July 30, 2007 10:58 PM Subject: Re: [Vo]:Re: The Earth and the Wheel
> 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. But you can assert that the Sun and the Earth have a _relative_ motion can't you? > Similarly, the Earth-wheel drawing does not prove > that the wheel is not rotating wrt to its hub. It is however clear that there is no _relative_ motion of the wheel wrt its hub. If you make pencil marks they will stay aligned. >> 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. Zero is fine by me, it's indeed constant, but then why use a turntable if it's not for rotating it? Besides you have lost me with that turntable digression, let's refocus on the original Earth and wheel system shall we? Do you still contend something (spoke tension?) will change after you release the brake from your original wheel on the Earth's equator? Before you answer, ask yourself, will it change again when you re-apply the brake? And then again when you re-release it? Michel
