On 1/8/2007 4:31 PM, Michel Jullian wrote:
> Or more likely that braking a null relative motion makes no difference at all,
> but I give up trying to convince you :)
My primary contention is that when the brake is released the wheel is
literally rotating about its own centre.
I will use the turntable example again but instead of starting out with each
wheel's axle parallel to the Earth's axis of rotation let me start out
with each wheel's axle tangent to the Earth's equatorial circle.
The brake is applied on wheel A so it cannot rotate relative to the ground.
The brake on wheel B is released and you set it rotating at 1 rev/day.
Next, consider what happens when you rotate each turntable through 90
degrees so both axles are now parallel to the Earth's axis of rotation.
Superficially wheel B seems to be in the same state as wheel A, since its
relative rotation wrt to the ground has vanished.
However, I contend it is still spinning about its own centre and as evidence
I would look for a difference of tension somewhere in the wheels.
> I have found this thought experiment of yours quite interesting nevertheless,
> it's fascinating that a wheel only looks as if it stays still when you release
> its brake, whereas it is in fact continuing a rotation imparted by the Earth's
> spin, surely not many people realize that!
>
yes, but that is just the entrance to a cave. ;-)
Harry
