# Homework Help: Circular motion

1. Aug 30, 2011

### athrun200

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. Aug 30, 2011

### bp_psy

You just manipulate the differentials.
$\frac{d \dot{r}}{dt}=\frac{d \dot{r}}{dt} \frac{dr}{dr}=\frac{d \dot{r}}{dr} \frac{dr}{dt}=\dot{r} \frac{d \dot{r}}{dt}$

edit. corrected last part

Last edited: Aug 30, 2011
3. Aug 30, 2011

### athrun200

It is quick difficult to think of this step.
If I don't check the solution, I wouldn't write out this.

4. Aug 30, 2011

### bp_psy

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 $\dot{r}=\dot{r}(r(t))$ then $\frac{d \dot{r} }{dt}=\frac{d \dot{r} }{dr} \frac{d{r} }{dt}$