# How to solve planet orbital question

1. Oct 8, 2008

### HelloMotto

Ok I remeber a question on my physics test. where theres a planet orbiting a star in some other solar system.

heres what was given to me.
the radius,and the time it took for it to finish 1 revolution. I was asked to find the mass of the sun, that the planet was orbiting around

so heres what i did.

I calculated the distance, which was 2 pi r.

I divided that value by time and got the speed.

then i used Fc=FG formula.
mv^2/r = Gm1m2/r^2

with some cancellations, im left with
v^2=Gm(star)/r

so i just isolated for m and solved it. I remember i got like some number x 10^30kg.

but did i do this question right? was this the right method of solving it?

2. Oct 8, 2008

### mgb_phys

Sounds reasonable, the sun's mass is around 2^30 kg

3. Oct 8, 2008

### HelloMotto

i asked my friend how he had solved it, and he said he used
Fc=FG
4pi^2mr/T^2 = Gm1m2/r^2

does my method and his method work in this question?

4. Oct 8, 2008

### mgb_phys

It's the same thing, you can also use T = 2 $$\pi \sqrt{ ( r^3 / GM )}$$