# The Naked Scientists Forum

### Author Topic: Why is centrifugal force NOT gravity?  (Read 17929 times)

#### Geezer

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##### Re: Why is centrifugal force NOT gravity?
« Reply #25 on: 21/03/2011 03:49:32 »

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

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##### Re: Why is centrifugal force NOT gravity?
« Reply #26 on: 21/03/2011 04:48:32 »

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.

« Last Edit: 21/03/2011 05:09:03 by Geezer »

#### wolfekeeper

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##### Re: Why is centrifugal force NOT gravity?
« Reply #27 on: 21/03/2011 12:07:21 »
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

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##### Re: Why is centrifugal force NOT gravity?
« Reply #28 on: 21/03/2011 15:51:02 »
A thought experiment.

#### Geezer

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##### Re: Why is centrifugal force NOT gravity?
« Reply #29 on: 21/03/2011 17:54:26 »
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

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##### Re: Why is centrifugal force NOT gravity?
« Reply #30 on: 21/03/2011 18:56:04 »
Centrifugal is just another name for describing inertial force in a circle.

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##### Re: Why is centrifugal force NOT gravity?
« Reply #30 on: 21/03/2011 18:56:04 »