Planet X

1. Oct 13, 2008

shimizua

1. The problem statement, all variables and given/known data
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

2. Relevant 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)

3. 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?

2. Oct 13, 2008

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.

3. Oct 13, 2008

shimizua

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

4. Oct 13, 2008

nicksauce

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

5. Oct 13, 2008

shimizua

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

6. Oct 13, 2008

nicksauce

7. Oct 13, 2008

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.

8. Oct 13, 2008

nicksauce

No problemo.

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