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paradox1 with vector w
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Short working note by Phil, dated 2.12.17, from an appendix on Euler angles and the computation of ω. It argues that the paradox disappears: the statement that S appears to rotate at -ω from S' is valid, but (-ω,0,0) are still Frame S components, while the Frame S' components are messy. It then applies this to Goldstein's rigid body equation (dL/dt)S' + ω x L = N, where the ω components are Frame S' components.
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Paradox with the vector ω PhL 2.12.17
I think this paradox gets resolved within this doc, so no paradox.
On the One Hand:
Consider this picture
where Frame S is inertial. Suppose it happened that e1 is coming out of the plane of paper on the left. Then you would write ω = ωe1 and then ω = (ω,0,0) and these are the components of ω in Frame S.
If you treat ω like any other vector, you could ask:
"What are the components of ω as measured in Frame S' "?
Since the axes of Frame S' are at some weird orientation, the answer will be:
"the components of vector ω as seen from Frame S' are complicated".
More formally, suppose ei = R(Φ)e'i and suppose ω = ωe1 . Then
(ω)'i = ω e'i = ω [R(Φ)-1ei] = [R(Φ)ω] ei = [R(Φ)ω]i = [R(Φ)]ij ωj
= [R(Φ)]ij ω δj,1 = ω [R(Φ)]i1
and in general this is some complicated expression for each of the (ω)'i components.
On the Other Hand:
Start again with the above picture
Here you would say: The axes of Frame S' are rotating at ω = (ω,0,0) as seen from Frame S.
Now hop onto a camera platform which is rotating CCW at ω about the same axis and this picture becomes
Now Frame S' is at rest and Frame S is rotating in effect at -ω . So you would say
The axes of Frame S as seen from Frame S' are rotating at -ω = (-ω,0,0)
Resolution:
The second item I think is valid, and you end up with the statement as claimed,
The axes of Frame S as seen from Frame S' are rotating at -ω = (-ω,0,0).
However, the components shown of -ω = (-ω,0,0) are still Frame S components, not Frame S' components. The Frame S' components are still something messy.
I jump to Goldstein rigid body here because he is going to use ω components in Frame S' !!
I work further with this stuff in some other doc.
What is the meaning of the ωi in the GPS presentation of rotating objects?
So what is the meaning of ω in this equation (5.37) ? The equation says
(dL/dt)S' + ω x L = N
Since this is a vector equation, we have not yet talked about "components", so we keep going. Now GPS is going to take the components of the above equation!!! This is a tricky operation. If we do this within Frame S' (the rotating frame) then my Commutation Theorem (1.11.1) shows that things commute, so you can say
(∂S'L)'i = ∂S'[ (L)'i] = ∂t(L)'i // using below 1.11.1
Then we really should write rigid body doc
∂t(L)'i + [ ω x L]'i = N'i
or
d(L)'i/dt + εijk(ω)'j(L)'k = (N)'i
and indeed it is the Frame S' components that you want!! This last equation appears as
and you see how they are using a very compact notation. But I agree, their ωi really are the Frame S' components!!!