# Homework Help: Period of a planet orbiting the Sun

1. Mar 4, 2014

### Jon.G

1. The problem statement, all variables and given/known data
A planet P orbiting the Sun S is acted upon by a force F= -(mu*r)/r3 (itex won't work here for some reason :S ) per unit mass where $\mu$ is a positive constant and r is the vector $\stackrel{\rightarrow}{SP}$
If the orbit of the planet is a circle of radius a, show that the period of the planet is $\frac{2\pi a^{3/2}}{\mu ^{1/2}}$

The Earth and Mercury are orbiting the Sun.
The Earth is at a mean distance of approximately $1.5 * 10^{8}$ km from the Sun and Mercury at approximately $5.8 * 10^{7}$ km.
Given that the Earth's period is about 365.25 days, find the period of Mercury.
2. Relevant equations

3. The attempt at a solution
Erm.... I really am not sure what to do.
I thought It would have something to do with centripetal force, and using $ω=\frac{2\pi}{t}$ but I can't see how to use that to get the answer :S

Thanks

2. Mar 4, 2014

### tiny-tim

Hi Jon.G!
Yes.

Write out F = ma … what do you get?

3. Mar 4, 2014

### Jon.G

$\sum F=ma$

Forces acting on P are
$F=\frac{-\mu r}{r^{3}}$ and
$F=mrω^{2}$ right?

So would I go to
$ma=mrω^{2} - \frac{\mu r}{r^{3}}$ ?

I can't really see where to go the m confuses me a bit.

Although I just realised that the question states $F= - \frac{\mu r}{r^{3}} per unit mass$ so would I be correct in thinking that I need to multiply this term by the mass (to get the force is N and not N kg-1?

And then the mass could be cancelled out?

4. Mar 4, 2014

yup!

5. Mar 4, 2014

### Jon.G

Alright I think I'm getting somewhere now.

If I have
$mrω^{2} = \frac{- \mu rm}{r^{3}}$
then the m cancels, and an r
$ω^{2} = \frac{- \mu r}{r^{2}}$
so....
$T = \frac{2\pi r}{\mu^{1/2}r^{1/2}}$

What I'm struggling to get my head around now is the r
I just don't know how to work with it

6. Mar 4, 2014

### Jon.G

Also, any hints as to what I'm doing wrong with itex? Then I can actually make this posts legible :S

7. Mar 4, 2014

### tiny-tim

no, you've cancelled r's wrong

(and itex doesn't like your bold tags )

8. Mar 4, 2014

### Jon.G

Of course argh :S

$ω^{2} = \frac{\mu R}{r^{4}}$

Still not sure how to work the vector though.
If the orbit is a circle then wouldn't R be equal to the radius?
But I can't just treat it as a scalar.

9. Mar 4, 2014

### tiny-tim

oh i see … you're worried about r being a vector

it doesn't matter, just use the magnitudes of the vectors

10. Mar 4, 2014

### Jon.G

Well that makes things easier :D
Thanks

For the second part could I not just substitute in the value to find $\mu$ and then find Mercury's time period?

(Even if this is the way, thanks Ronaldo95163. Always good to know of other ways to work through problems )

11. Mar 4, 2014

### tiny-tim

use dimensions (ie what is T proportional to?)

12. Mar 4, 2014

### Staff: Mentor

$$\vec{F}=\frac{-m\mu \vec{r}}{r^{3}}$$ and
$$\vec{F}=m\vec{a}=-m\vec{r}ω^{2}$$
So:$$\frac{-m\mu \vec{r}}{r^{3}}=-m\vec{r}ω^{2}$$ and
$$\frac{\mu }{r^{3}}=ω^{2}$$