# Physics problem involving gravity?

1. Jun 20, 2010

### bkell27

A spaceship of mass m travels from the Earth to the Moon along a line that passes through the center of the Earth and the center of the Moon.

At what distance from the center of the Earth is the force due to the Earth twice the magnitude of the force due to the Moon?

r = ? m

I'm thinking of using the F = Gm1m2/r^2 with G being the gravitational constant and m1 being the mass of the earth, and m2 being the mass of the moon. but i don't know why they say "spaceship of mass m" or if that's even the proper formula to use???

2. Jun 20, 2010

### mo_0820

3. Jun 20, 2010

### bkell27

what does that mean?

4. Jun 20, 2010

### mo_0820

Don't consider the mass of spaceship.

5. Jun 20, 2010

### bkell27

what do i use as the force (F) in the formula I stated in order to isolate radius (r) and solve for it?

6. Jun 20, 2010

### mo_0820

At what distance from the center of the Earth is the force due to the Earth twice the magnitude of the force due to the Moon?

the force due to Earth and the force due to the Moon, what do they act on?

7. Jun 20, 2010

### bkell27

no clue what you are asking

8. Jun 20, 2010

### mo_0820

the force act on spaceship,
$$F_E=Gm1m/r^2$$
and?

9. Jun 21, 2010

### Staff: Mentor

You need to compare the gravitational forces acting on the spaceship, not the the gravitational force between earth and moon.