Understanding the Ratio of r Motion to Revolution Periods - Clarification Needed

[quasar987]

Hi.

When you say "The ratio of the period of the r motion to the period of revolution", does it mean

\frac{T_r}{T_\theta}

or

\frac{T_\theta}{T_r}

?

Judging by the words, I would say it's the first one, but I have many mathmatical reasons to think it it's the second one. I would like a confirmation that it means the second one. Thanks a lot.

 

[Sirus]

Think of it this way. Take the general statement "the ratio of a to b is x:y". This means a:b=x:y, so \frac{a}{b}=\frac{x}{y}. This makes sense. Say the ratio is 4. You can convert this to ratio form: 4:1. If a:b is 4:1, then if b=1, a=4, and if b=2, a=8, and so on...meaning that in general a=4b, so \frac{a}{b}=\frac{4}{1}. Hope this is clear.

 

[quasar987]

Then there's an error in the book!

 

[quasar987]

Simple question of physics

Given that the above sentence means in fact

\frac{T_r}{T_\theta}

Does the following sentence make sense (in the context of a particle moving under a central (radial) force with angular momentum non nul)

"If the ratio of the period of the r motion to the period of revolution is an integer, the orbit is a simple closed curved."

Consider the following simple counter exemple:

\frac{T_r}{T_\theta}=2 \Leftrightarrow T_r=2T_\theta \ (1:2)

Meaning after the particle has covered 4 pi rad around the center of force, the radial oscillation has completed one period. How can that make for a simple closed curve? There will necessarily be an intersection. Whereas if it means the opposite,

\frac{T_\theta}{T_r}=2

the curve is closed and much simpler has it does not intersect with itself.


So is it me or there's an error in the book?