# Gravity earth and moon

1. Oct 30, 2009

### chantelkohl

1. The problem statement, all variables and given/known data
Somewhere between the earth and the moon is a point where the gravitational attraction of the earth is cancelled by the gravitational pull of the moon. The mass of the moon is 1/81 that of the earth. How far away from the center of the earth is this point?

a) 8/9 the way to the moon
b) 9/10 the way to the moon
c) 3/4 the way to the moon
d) 80/81 the way to the moon

2. Relevant equations
F=GMm/r^2

3. The attempt at a solution
GM/r^2=G(1/81)M/r^2-d

2. Oct 30, 2009

### Pengwuino

I don't really get what that 'd' is doing hangen out there all by itself. If you look at your attempt, you need to have the 'r' on the other side denoted as something different, say for example r', where r' is the distance to the moon. What you're looking (in my opinion) is a ratio of the distances from the earth to that certain point where the forces or equal and the distance from the moon. Start by multiplying both sides by $$r'^2$$ where r' is the distance to the moon.