rotating frame

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Infinitesimal rotations

The components of a vector \mathbf{x}\, in some fixed frame O\, are related to the components \mathbf{x}'\, in a rotating frame O'\, through some rotation:

\mathbf{x} = \mathbf{O} \mathbf{x}'\,.

An infinitesimal change in this vector is found by the product rule:

d\mathbf{x} = \mathbf{O} d\mathbf{x}' + d\mathbf{O}\, \mathbf{x}' \,,

but an infinitesimal rotation takes the form

d\mathbf{O}\,\mathbf{x}' = d\boldsymbol{\Omega} \times \mathbf{x}'\,,

so we may write

d\mathbf{x} = \mathbf{O} d\mathbf{x}' + d\boldsymbol{\Omega}\times \mathbf{x}' \,.

More specifically,

\frac{d\mathbf{x}}{dt} = \mathbf{O} \frac{d\mathbf{x}'}{dt} + \boldsymbol{\omega}\times \mathbf{x}' \,,

where we have defined \boldsymbol{\omega} dt = d\boldsymbol{\Omega}\,. This can be written as

\left(\frac{d}{dt}\right)_O = \left(\frac{d}{dt}\right)_{O'} + \boldsymbol{\omega}\times\,,

where \left(\tfrac{d}{dt}\right)_{O'}\, is the rate of change of a vector quantity due solely to the changes in the rotating frame.

Example: rigid body

For instance, suppose a particle is at rest in the rotating frame O'\,, and let us fix an instant in time so that the separation between the particle and some point O\, on the rotating body is \mathbf{r}=\mathbf{r}'\, in both frames. Then

\frac{d\mathbf{r}}{dt} = \boldsymbol{\omega}\times\mathbf{r}\,,

or

\mathbf{v} = \boldsymbol{\omega}\times\mathbf{r}\, (rigid body).
on to angular velocity

References

Further reading:[1] [2]

  1. Landau, L.D.; Lifshitz, E.M. (1997). Mechanics (3rd ed). Butterworth-Heinemann. ISBN 0-750-62896-0. 
  2. Herbert Goldstein, Charles P. Poole, John L. Safko (1980). Classical Mechanics (3rd ed). Addison Wesley. ISBN 978-0201657029. 
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