Homework Help: Gravitational forces equal in between an object

1. Sep 20, 2010

huronjubal

1. The problem statement, all variables and given/known data
from earth to mars, determine at what distance (from earth) the gravitation force of the earth acting on the space probe ( mass 200kg) is equal to the gravitational force of Mars on the space probe, if the planets are aligned.
Mass of earth: 5.976 x 10 ^ 24 kg
Mass of Mars: 6.394 x 10 ^ 23 kg
Mass of probe: 200kg
distance between Earth and Mars: 7.83 x 10 ^7 km
gravitational constant: 6.673 x 10 ^ -11
surface gravitational acceleeration of mars: 3.73 m/s^2
" "of Earth: 9.81 m/s^2
2. Relevant equations

F= Gmm / r^2
F=mg

3. The attempt at a solution
I'm not really sure how to get this problem started. If anyone could help me get going or something I can start with. Thank you

2. Sep 20, 2010

zhermes

Well, draw a line with earth on one end and mars on the other, then the probe some distance (x) from earth. In terms of 'x' and the earth-mars distance, whats the distance from the probe to mars?
In terms of 'x,' what is the force of gravity on the probe from earth? What about from mars? at what distance are they equal?

Does that help?

3. Sep 20, 2010

huronjubal

That's what i needed. Thanks for the help!