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# Centrifugal force

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May I ask a teaser question about centrifugal force. I know what this force is and how it works but can I ask a question. Imagine for the moment that we have a spinning object in deep space. It can be anything but lets say its a frisbee. The frisbee is spinning happily away in a space where there are no stars, galaxies or indeed a universe. Its totally on its own and absolutely nothing exists around it. No gravity waves, no electromagnetic radiation, absolutely nothing exists except for this lonely frisbee in a never ending darkness. My question is: How does it know its spinning. It cannot relate to anything as there is nothing to relate to so where does the centrifugal force come from? Peter

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There wouldn't be any. No space-time, no gravity. No gravity, no centrifugal force.

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This is a sort of Mach's Principle or the bucket argument. I would say, the 'spinning' of the frisbee makes no sense, as there is no mass relative to the frisbee to measure its spin.

Another question is, if you stand with your arms at your side and look up, the stars are fixed in the sky. When you start spinning around, your arms will rise, and the stars will appear rotating around you.

The question is, why do the stars rotating around you make your arms rise?

It seems inertia and momentum are determined by all the matter in the universe. If there were less matter in the universe, your arms would rise at a slower rate, or you would have to spin faster for your arms to rise.

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3 minutes ago, StarMountainKid said:

This is a sort of Mach's Principle or the bucket argument. I would say, the 'spinning' of the frisbee makes no sense, as there is no mass relative to the frisbee to measure its spin.

Another question is, if you stand with your arms at your side and look up, the stars are fixed in the sky. When you start spinning around, your arms will rise, and the stars will appear rotating around you.

The question is, why do the stars rotating around you make your arms rise?

It seems inertia and momentum are determined by all the matter in the universe. If there were less matter in the universe, your arms would rise at a slower rate, or you would have to spin faster for your arms to rise.

The stars are causing your arms to rise? Really?

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5 minutes ago, Rlyeh said:

The stars are causing your arms to rise? Really?

Reading the OP, I remembered Mach's Bucket thought experiment.

"Mach's principle says that this is not a coincidence—that there is a physical law that relates the motion of the distant stars to the local inertial frame. If you see all the stars whirling around you, Mach suggests that there is some physical law which would make it so you would feel a centrifugal force. There are a number of rival formulations of the principle. It is often stated in vague ways, like "mass out there influences inertia here". A very general statement of Mach's principle is "local physical laws are determined by the large-scale structure of the universe".[3]

This concept was a guiding factor in Einstein's development of the general theory of relativity."  etc.

I think in other words, motion is only motion when it is relative to something else. The speed of a spinning frisbee is different if you measure its spin from another spinning frisbee. In this case, the measured spin of the first frisbee is determined relative to the spin of the other frisbee.

It's the same with motion in a straight line. Your relative motion determines your measurement of something else moving.

Motion is meaningless unless it is relative to something else. So, in this sense, in a general way of saying, the matter of the universe determines motion.

My understanding.

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1 hour ago, Peter Mann said:

The frisbee is spinning happily away in a space where there are no stars, galaxies or indeed a universe. Its totally on its own and absolutely nothing exists around it. No gravity waves, no electromagnetic radiation, absolutely nothing exists except for this lonely frisbee in a never ending darkness.

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57 minutes ago, StarMountainKid said:

I think in other words, motion is only motion when it is relative to something else. The speed of a spinning frisbee is different if you measure its spin from another spinning frisbee. In this case, the measured spin of the first frisbee is determined relative to the spin of the other frisbee.

It's the same with motion in a straight line. Your relative motion determines your measurement of something else moving.

Motion is meaningless unless it is relative to something else. So, in this sense, in a general way of saying, the matter of the universe determines motion.

My understanding.

However the centrifugal force isn't simply velocity it's also acceleration as the direction is also changing. You don't need an outside reference to measure acceleration.

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There would be a force acting from the spin axis towards the edge equally around the plate. This force would be higher than the 'at rest' value. The change in this value would be equal to the spin.

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Thanks everyone. The frisbee has a centrifugal force only because it is being influenced by surrounding matter (stars and dust) Am I right so far?  Whether the fresbee is spinning in a static universe or the universe is spinning around a static fresbee  is something that comes to mind but perhaps that's messing my brain a bit too much. So I must assume that if our universe was less dense then the centrifugal force would be less for the same rate of spin. That's right ? Peter

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1 hour ago, Peter Mann said:

Thanks everyone. The frisbee has a centrifugal force only because it is being influenced by surrounding matter (stars and dust) Am I right so far?  Whether the fresbee is spinning in a static universe or the universe is spinning around a static fresbee  is something that comes to mind but perhaps that's messing my brain a bit too much. So I must assume that if our universe was less dense then the centrifugal force would be less for the same rate of spin. That's right ? Peter

If you go by Mach's principle. But I've never seen or heard anyone use it as an explanation.

If I had to guess I'd say the frisbee has centrifugal force because different areas of the frisbee is moving at different velocities.

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1 hour ago, Peter Mann said:

Thanks everyone. The frisbee has a centrifugal force only because it is being influenced by surrounding matter (stars and dust) Am I right so far?  Whether the fresbee is spinning in a static universe or the universe is spinning around a static fresbee  is something that comes to mind but perhaps that's messing my brain a bit too much. So I must assume that if our universe was less dense then the centrifugal force would be less for the same rate of spin. That's right ? Peter

I'm not sure anyone here is a physicist to answer your questions properly (I could be mistaken). The is a member, Sepulchrave, who is a professor of Physics, but he doesn't often visit UM often.

Another example of this question I've read is, if you tied two bricks together with a rope in your empty universe, and spun them around, would the rope become taught that ties them together? In other words, would the bricks experience centrifugal force?

I don't know. I think that before Einstein's theory of relativity, it was thought there was absolute space, absolute motion and absolute time (Newton), and Mach's Bucket thought experiment gave Einstein help in understanding that all these effects were not absolute but only could be measured relative to other events.

It seems to me we can't measure the properties of something without that measurement being in comparison to the properties of some other thing. To define a word we need to use other words in our definition. To define the mass of a proton is assigning it some property relative to some method of definition.

So I think, in a completely empty universe, no property of the frisbee can be defined.

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18 hours ago, L.A.T.1961 said:

There would be a force acting from the spin axis towards the edge equally around the plate. This force would be higher than the 'at rest' value. The change in this value would be equal to the spin.

There is no such thing as centrifugal force. The force you mention in this post actually operates in the opposite direction, towards the center.

1 hour ago, Peter Mann said:

Thanks everyone. The frisbee has a centrifugal force only because it is being influenced by surrounding matter (stars and dust) Am I right so far?  Whether the fresbee is spinning in a static universe or the universe is spinning around a static fresbee  is something that comes to mind but perhaps that's messing my brain a bit too much. So I must assume that if our universe was less dense then the centrifugal force would be less for the same rate of spin. That's right ? Peter

No.

The outer edge of the frisbee is spinning faster than any part of the frisbee as you move toward the center (which, itself, is at rest with respect to the rest of the frisbee.) Different velocities relative to each other, so measurable velocity.

Harte

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4 minutes ago, Harte said:

There is no such thing as centrifugal force. The force you mention in this post actually operates in the opposite direction, towards the center.

No.

The outer edge of the frisbee is spinning faster than any part of the frisbee as you move toward the center (which, itself, is at rest with respect to the rest of the frisbee.) Different velocities relative to each other, so measurable velocity.

Harte

Yes, but I think the question is supposed to be, from where do these properties originate? We don't really know the properties of the spinning frisbee because we can't measure these properties. We can only infer that in an empty universe the frisbee will behave in the same way it would in universe containing other masses.

It's a sort of measurement problem. If we can't measure something, we can say nothing about it.

I agree that the different velocities of the frisbee's surface are measurements relative to the surface itself. So in this sense, the frisbee would experience centrifugal force. On the other hand, if we can't actually measure this property, we can't determine if this is true or not.

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9 minutes ago, StarMountainKid said:

Yes, but I think the question is supposed to be, from where do these properties originate? We don't really know the properties of the spinning frisbee because we can't measure these properties. We can only infer that in an empty universe the frisbee will behave in the same way it would in universe containing other masses.

It's a sort of measurement problem. If we can't measure something, we can say nothing about it.

I agree that the different velocities of the frisbee's surface are measurements relative to the surface itself. So in this sense, the frisbee would experience centrifugal force. On the other hand, if we can't actually measure this property, we can't determine if this is true or not.

We can measure it if we're there.

But we have mass.

We can't measure anything if we aren't there to measure it. It's a pointless exercise in that case.

Harte

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23 hours ago, Peter Mann said:

May I ask a teaser question about centrifugal force. I know what this force is and how it works but can I ask a question. Imagine for the moment that we have a spinning object in deep space. It can be anything but lets say its a frisbee. The frisbee is spinning happily away in a space where there are no stars, galaxies or indeed a universe. Its totally on its own and absolutely nothing exists around it. No gravity waves, no electromagnetic radiation, absolutely nothing exists except for this lonely frisbee in a never ending darkness. My question is: How does it know its spinning. It cannot relate to anything as there is nothing to relate to so where does the centrifugal force come from? Peter

Hey Peter,

That's an interesting question, considering centrifugal force is made up of angular velocity, and acceleration due to mass, momentum and gravity you need to make alot of assumptions to provide a good answer. I assume the frisbee has mass and is three dimensional and a few other assumptions like local reference planes.

If you assume Machs principle is true it would be impossible to determine if the frisbee is spinning due to the lack of references. But you'd still be subjected to differences in centrifugal forces depending of your distance from the centre that is assuming the frisbee has mass.

Centrifugal force is mv2/r. V is basically a chance in velocity, thus if the frisbee is moving uniformly it would be not possible to determine the centrifugal force. So like others have said without reference planes it would be near impossible to determine. But you might feel the effects of it.

All that's assuming mass thus gravitional effects.

Edited by danydandan
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This question started a debate here at L.I.T towers  one contributor, a bit better informed than I, suggested this explanation.

The idea that a vacuum can be rotated without changing its physical properties is flawed.

Virtual particles and anti-particles are continuously created and would not want to follow a rotating path. This also applies to all the virtual particles which are the force propagators (virtual photons and gluons) which hold everything together. This argument therefore negates the original hypothesis that a rotating object in a non-rotating universe is the same as a non-rotating object in a rotating universe.

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2 hours ago, L.A.T.1961 said:

This question started a debate here at L.I.T towers  one contributor, a bit better informed than I, suggested this explanation.

The idea that a vacuum can be rotated without changing its physical properties is flawed.

Virtual particles and anti-particles are continuously created and would not want to follow a rotating path. This also applies to all the virtual particles which are the force propagators (virtual photons and gluons) which hold everything together. This argument therefore negates the original hypothesis that a rotating object in a non-rotating universe is the same as a non-rotating object in a rotating universe.

Which leads me to the question, in the OP's original post where nothing exists except the frisbee (I assume no virtual particles, gluons, etc.), how could the frisbee exist at all? The atoms that make up the frisbee, the forces that maintain the atomic structure would be isolated within the frisbee.

Would these elementary particles and fundamental forces continue to form the frisbee in this isolation where no laws of physics exist external to the frisbee?

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1 hour ago, aztek said:

I remember watching Laithwait host the Royal institute Christmas lectures one year on TV and he had various demonstrations set up. Highly entertaining and thought provoking, especially when you are 13.

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I've watched some similar videos, and I'm not sure I understand the gyroscopic effect, etc.

Something similar in High School physics:  put an electric motor spinning a heavy disk in a suitcase. Ask someone to pick up the suitcase and move it to another place.

When you try to turn the heavy suitcase it will rise up in the air by itself and seem lighter. The same effect as the above video.

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6 minutes ago, StarMountainKid said:

I've watched some similar videos, and I'm not sure I understand the gyroscopic effect, etc.

Something similar in High School physics:  put an electric motor spinning a heavy disk in a suitcase. Ask someone to pick up the suitcase and move it to another place.

When you try to turn the heavy suitcase it will rise up in the air by itself and seem lighter. The same effect as the above video.

I think it is an effect of Gyroscopic Precession.

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A better question is Who is going to catch it.

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On 6/15/2018 at 9:56 AM, Peter Mann said:

May I ask a teaser question about centrifugal force. I know what this force is and how it works but can I ask a question. Imagine for the moment that we have a spinning object in deep space. It can be anything but lets say its a frisbee. The frisbee is spinning happily away in a space where there are no stars, galaxies or indeed a universe. Its totally on its own and absolutely nothing exists around it. No gravity waves, no electromagnetic radiation, absolutely nothing exists except for this lonely frisbee in a never ending darkness. My question is: How does it know its spinning. It cannot relate to anything as there is nothing to relate to so where does the centrifugal force come from? Peter

Nice question.  Einstein would have been proud.  I think this is more about angular momentum than centrifugal force.    With a body out there in space away from everything, and in your question, nothing else exists, it would be at rest in its own reference frame.  If it accelerates or decelerates then the frisbee does  affect the space time around it.  Angular momentum, spinning, is an acceleration and change in direction.  We need the physics jocks to pop in now.  Acceleration and gravity are hard to distinguish so maybe the spinning frisbee warps the space time around it.  Lets see if the experts are looking in.

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Never mind.  I just read through all of the posts.  Some are already here.

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On 6/15/2018 at 12:56 PM, Peter Mann said:

May I ask a teaser question about centrifugal force. I know what this force is and how it works but can I ask a question. Imagine for the moment that we have a spinning object in deep space. It can be anything but lets say its a frisbee. The frisbee is spinning happily away in a space where there are no stars, galaxies or indeed a universe. Its totally on its own and absolutely nothing exists around it. No gravity waves, no electromagnetic radiation, absolutely nothing exists except for this lonely frisbee in a never ending darkness. My question is: How does it know its spinning. It cannot relate to anything as there is nothing to relate to so where does the centrifugal force come from? Peter

if it's spinning there is angular velocity, there is centrifugal force in space as well,