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zero reference frame for rotation


Neil9327
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They say that there is absolutely no zero referance frame for location or velocity, which is true. I think that there IS a zero frame for acceleration (this would be the acceleration where no external force is required), and that this is defined by the gravitational field where you are located. Where I am sat it is 9.8 m/s2 straight down. So in a sense the presence of mass, which causes gravity, defines the zero frame.

 

However what about rotation? I would define this as the rotational angular velocity where no force is required to hold youself together - ie zero centrifugal force (although I know this is a misnomer).

So what defines what this is? It does seem by coincidence to be the same as the rotation of the rest of the universe. (Indeed as an aside how would we know whether the universe is rotating?)

 

To help answer this question I must ask another:

If you have a small satellite in orbit around the earth once every 90 minutes, it will be in the zero acceleration state as I have defined above - it is weightless. However for it to also be in a zero rotation state, where it feels no centrifugal force, what must be its rotation? Is it a) pointing to a single point in space (such as the sun) or b) always pointing to the earth's horizon as it ploughs it way round the earth.

I think it is a) in which case the earth's gravity has had no influence on the zero rotating reference.

(What do gyroscopes do in orbit? Do they always point to the sun?)

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Rotation implies acceleration! As soon as something is rotating, any other objects are accelerated relative to the rotating object, the acceleration being proportional to the distance between the rotating object and the other object (~wxR, where w is the angular velocity vector and R is the distance vector, x is the exterior product).

 

I do not precisely understand what you mean with reference frame, in the context of rotation. Reference frames are attached to objects. These objects can be some virtual point in space, but also observers or objects under consideration.

 

Suppose you are sitting in a ball, which itself is put in a bigger ball. The smallest ball, in which you are sitting can rotate completely freely inside the bigger ball.

 

Now, a rope is attached to the bigger ball and it is rotated along a fixed point. Now, do you think that there is any form of rotation of the smaller ball inside the bigger ball, such that you do not feel any force? Such a form of rotation does not exist! You can remove the sense of rotation, but then you feel other accelerations. There is a very simple reason for this. You are moving along a curved trajectory, and as soon as an object is moving along a curved trajectory, it is accelerated, and that is what you feel. So, either curvature, or acceleration are felt. They in fact are the result of the same underlying phenomenon. The only difference is that with curvature, there is a component of acceleration in a direction, perpendicular to the direction of motion.

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Hmm I think I see what you mean Woelen. Rotation is acceleration. Of course rotation is NOT acceleration if you are at a point location rotating about youself. Maybe that's where my arguement falls down. If you are at a point location you would feel zero centrifugal force so my definition of this as the zero point of rotational angular momentum has no meaning.

 

I'll have to put my thinking cap on, but for now I'll defer to your greater wisdom on this :)

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