----- 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". 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? Michel
