A doubt from the book Galactic Dynamics from Binney and Tremaine

  • #1
Hi...

I was reading through the second edition of the book Galactic Dynamics from Binney and Tremaine.

Chapter 3 :eqns 3.8 and 3.9.

3.8 says

[tex]r^2 \frac{d \phi}{dt}=L=constant[/tex].

Then 3.9 writes

[tex]\frac{d}{dt}=\frac{L}{r^2}\frac{d}{d\phi}[/tex]

But is this correct? For example, if I act this on r, the radial coordinate, I get

[tex]\frac{dr}{dt}=0[/tex]

For a general orbit in a central force, r is not a constant.
So is there a mistake in the book, or am I missing something?
 

Answers and Replies

  • #2
George Jones
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Yes, it's correct, but the notation is a little sloppy.

Let

[tex]r \left ( t \right) = \tilde{r} \left( \phi \left ( t \right) \right). [/tex]

These are really different functions (of different vraiables) , but this type of notational abuse, denoting different, but related functions by the same letter, is very common in physics.

Using the chain rule, what is [itex]dr/dt[/itex]? Using [itex]d/dt[/itex] from the book, what is [itex]dr/dt[/itex]?
 
  • #3
Yes..I understand the mistake I am making...

I was assuming [tex]r[/tex] and [tex]\phi[/tex] are independent variables.....

But once an orbit is determined by the initial conditions, they are no longer independent...

Is that correct?
 

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