# Homework Help: Help with Gravity

1. Nov 15, 2007

### iamanoob

For a spacecraft going directly from the Earth to the Moon, beyond what point will lunar gravity begin to dominate? That is, where will the lunar gravitational force be equal in magnitude to the Earth's gravitational force?
m from Earth
i know that the earth is 3.8*10^8m from the moon

2. Nov 16, 2007

### katchum

F=G.m1.m2/r^2

m1 = spacecraft = ??? = 1000 kg?
m2 = earth
m3 = luna
G = gravitational constant 6.67 × 10−11 N m2 kg−2

F1= G.m1.m2/r1^2
F2= G.m1.m3/r2^2
r1+r2=3,8.10^8m
F1=F2

4 equations with 4 variables F1,F2,r1,r2. r1 is the answer.

Am I right, I'm not sure.

Last edited: Nov 16, 2007
3. Nov 16, 2007

### Sourabh N

You are right, but you are not supposed to do this thing; you should have better made iamanoob to do this, helping him on his way.

4. Nov 16, 2007

### katchum

Right! But he still has to calculate the answer. :)

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