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Homework Help: Circular motion

  1. Aug 30, 2011 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    3. The attempt at a solution
    In part a, there is an important step, I don't know how to have the step that written in green colour.
    Does this technque have any name?
    Can you give me more example of it?


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  2. jcsd
  3. Aug 30, 2011 #2
    You just manipulate the differentials.
    [itex]\frac{d \dot{r}}{dt}=\frac{d \dot{r}}{dt} \frac{dr}{dr}=\frac{d \dot{r}}{dr} \frac{dr}{dt}=\dot{r} \frac{d \dot{r}}{dt}[/itex]

    edit. corrected last part
    Last edited: Aug 30, 2011
  4. Aug 30, 2011 #3
    It is quick difficult to think of this step.
    If I don't check the solution, I wouldn't write out this.
  5. Aug 30, 2011 #4
    It appears that I messed up the last equality. I corrected the last post.
    It is clearer if you see that it is the chain rule.
    let [itex]\dot{r}=\dot{r}(r(t))[/itex] then [itex]\frac{d \dot{r} }{dt}=\frac{d \dot{r} }{dr} \frac{d{r} }{dt}[/itex]
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