Calculating an Astronaut's Weight on Planet X

[shimizua]

Homework Statement

An astronaut weighs 672 N on the Earth. What is her weight on planet X, which has a radius Rx = Rearth / 2.00 and a mass Mx = Mearth/9.00?
I know that the radius of Planet X is 3189.05
and that the mass of Planet X is 6.638x10^23

Homework Equations

so i know that i need to find the gravity for planet X found with this equation gx = GM/R^2 and also that the eequation to find her mass is F=Mm/r^2 (at least that is what i think)

The Attempt at a Solution

so i tried using that equation but it was wrong and then i figued out i prob needed the gravity but did not know exactly how to use it cause what do i use G for in the initial equation?

 

[nicksauce]

It's not entirely clear to me what you did, so I'll start from the top. First of all, the formula for force due to gravity is F = GMm/r^2. So you have two equations$$W_{e}=\frac{GM_{e}m}{R_{e}^2}$$, and $$W_{x}=\frac{GM_{x}m}{R_{x}^2}$$. Try taking a ratio of Wx and We and the answer should pop out.

 

[shimizua]

but how do i find out what G is for planet X? that is the part that is confusing me

 

[nicksauce]

G is the same everywhere in the universe. It is a fundamental constant of nature.

 

[shimizua]

ok so G is not gravity then i am guessing. what is G exactly then? like what is its number

 

[nicksauce]

G = 6.67 * 10^-11 m^3/kg/s^2.
http://www.google.com/search?source=ig&hl=en&rlz=&=&q=G&btnG=Google+Search

It is known as the gravitational constant.

It shouldn't matter to your calculation though, as when you take a ratio between Wx and We, the G's cancel out.

 

[shimizua]

YES! got the answer. thanks man. i actually have a few more and if you have time that would be awesome. i will post them now.

 

[nicksauce]

No problemo.