[Phractality]

I'm still in negotiations with my brain to get it back on the job; I think it wants less physics and more phun. Back to the subject:

If you want a non-rotating reference frame on Earth, try a west-bound bullet train, on smooth horizontal track, fast enough to match Earth's rotation at its latitude. That will eliminate most centrifugal, centripetal or centrifical force, whatever you choose to call it. Of course, you are still orbiting the sun, but that is 365 times less angular speed than the daily rotation.

If you weigh a kilogram mass on a properly calibrated spring scale in a sidereal reference frame, you can then calculate the correct mass of Earth from Newton's universal law. (Most spring scales are calibrated to show mass, instead of force, in the local gravity.) His formulas are not valid in the rotating reference frame of Earth. Or you can use a gravity pendulum instead of a scale.

Dr. Who has a solution to the question of whether he's on a space station with centrifugal (or centrifical, or centripetal) gravity. He always had a yoyo in his pocket, just in case. If the yoyo exhibits Coriolis effect, he's on a rotating platform. Of course, a 24 hour day makes the yoyo much too crude. You might need a Foucault pendulum.

[Geezer]

From the non rotating point of view (the non rotating reference frame) The 'force' that pushes you into the wall is your momentum; the wall 'slams into you' as your momentum tries to make you go in a straight line.

 [] No, it's not. Momentum is not a force. It's centripetal force that constrains your mass to follow a circular path so that your linear momentum is converted into angular momentum. As soon as the centripetal force is eliminated, your angular momentum becomes linear momentum again.   

[wolfekeeper]

That's right, that's why I put it as 'force' instead of force, in a rotating reference frame ordinary linear momentum appears/requires two accelerations to which you can ascribe two pseudoforces to explain the motion within that frame. I mean in a rotating reference frame you would intuitively expect things to still go in a straight line, but they tend to curve, and that can be represented as these accelerations/forces.

Incidentally, linear momentum doesn't convert into angular momentum, they're different things.

[Geezer]

Incidentally, linear momentum doesn't convert into angular momentum, they're different things.

Yes. In one case mass rotates around an axis whereas in the other, mass travels in a straight line, or did you mean something different?

Anyway, if you can can convince me that momentum is not conserved when the string breaks, you'll get my attention  []

[wolfekeeper]

Normally most people talk about angular momentum in terms of rotation, but if I'm measuring angular momentum around my finger and a car drives past it 10ft away at 30 mph, in a perfectly straight line, then from the definition of angular momentum the car has (a constant) angular momentum around my finger, even though nothing is rotating. And if I pick a different finger, the angular momentum is typically different; but the linear momentum is unchanged.

Conversely, something that is rotating, each bit/atom/molecule that's moving has linear momentum as well as angular momentum, and you calculate the two completely separately- one can't turn into the other; they're just different, and independently conserved.

[Geezer]

Normally most people talk about angular momentum in terms of rotation, but if I'm measuring angular momentum around my finger and a car drives past it 10ft away at 30 mph, in a perfectly straight line, then from the definition of angular momentum the car has (a constant) angular momentum around my finger,

Actually, the approaching car's angular momentum is anything but constant relative to your finger. I think you will discover that the rate of change of angle increases and decreases according to the sine of the distance. On that basis there has to be an enormous change in the angular momentum of the approaching car, which, to me at least, sounds a teeny bit suspect.

[Geezer]

Conversely, something that is rotating, each bit/atom/molecule that's moving has linear momentum as well as angular momentum, and you calculate the two completely separately- one can't turn into the other; they're just different, and independently conserved.

Yes, that's true. But guess what, when you sum all the linear and angular moments, you still end up with the same result that you would have got if you had treated the mass as a solid object.

(That may not be quite true if relativistic effects have to be considered, but in most cases, it's really quite accurate.)

Please indulge me by allowing an old geezer editorial.

As JP will confirm, I know bugger all not much about relativity. However, I hope he will also agree that Einstein did not invalidate Newton's conclusions. Einstein refined Newton's descriptions, but he did not invalidate them.

The phenomena that we observe in most situations were quite accurately described by Newton. I think it is important to get a good grasp of Newton's ideas before we try to embrace relativity, but that is just one geezer's opinion. 

[wolfekeeper]

Actually, the approaching car's angular momentum is anything but constant relative to your finger. I think you will discover that the rate of change of angle increases and decreases according to the sine of the distance. On that basis there has to be an enormous change in the angular momentum of the approaching car, which, to me at least, sounds a teeny bit suspect.

The angular velocity changes drastically, that's true, but unfortunately you've neglected the fact that the change in angular speed is compensated (precisely) by the change in distance, so it turns out that the angular momentum is exactly constant at all times; in fact it's simply distance at closest approach multiplied by speed.

[Ron Hughes]

A thought experiment.

[Geezer]

Actually, the approaching car's angular momentum is anything but constant relative to your finger. I think you will discover that the rate of change of angle increases and decreases according to the sine of the distance. On that basis there has to be an enormous change in the angular momentum of the approaching car, which, to me at least, sounds a teeny bit suspect.

The angular velocity changes drastically, that's true, but unfortunately you've neglected the fact that the change in angular speed is compensated (precisely) by the change in distance, so it turns out that the angular momentum is exactly constant at all times; in fact it's simply distance at closest approach multiplied by speed.

Ah! Sneaky - you're right!

[Ron Hughes]

Centrifugal is just another name for describing inertial force in a circle.