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Homework Help: Rotational motion

  1. Oct 30, 2011 #1
    Okay so my mechanics teacher is taking a rather mathematical approach to CM and its really confusing me. Can someone explain to me what this actually means?

    d hat{r}/dt=dot{theta} hat{theta} and

    d hat{theta}/dt=-dot{theta} hat{r}

    I mean I know that anything with a hat on top is a unit vector and i also know that theta dot represents angular speed. Thankyou.

  2. jcsd
  3. Oct 30, 2011 #2

    I like Serena

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    Homework Helper

    Hi Jadenag! :smile:

    Yes, [itex]\hat r[/itex] is the local unit vector in the r direction at some point (r, theta).
    We could also write [itex]\hat r(r, \theta)[/itex], since it is a function of r and theta.
    However, if theta increases a little bit (by [itex]d\theta[/itex]), that unit vector changes.
    To be precise its angle changes by [itex]d\theta[/itex].

    Perhaps you can make a drawing of it and consider which vector represent the change in [itex]\hat r[/itex]?
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