How Do You Calculate Altitude Changes in Earth's Gravitational Field Strength?

[Jtappan]

Homework Statement

(a) Find the altitude above the Earth's surface where Earth's gravitational field strength would be two-thirds of its value at the surface. Assume re = 6.371 103 km.
km
(b) Find the altitude above the Earth's surface where Earth's gravitational field strength would be one-third of its value at the surface.
km

Homework Equations

m = (GMe/Re^2)

The Attempt at a Solution

Im not entirely sure the equation is right for this type of problem. I do not even know where to start for this problem.

 

[Saladsamurai]

Homework Statement

(a) Find the altitude above the Earth's surface where Earth's gravitational field strength would be two-thirds of its value at the surface. Assume re = 6.371 103 km.
km
(b) Find the altitude above the Earth's surface where Earth's gravitational field strength would be one-third of its value at the surface.
km

Homework Equations

m = (GMe/Re^2)

The Attempt at a Solution

Im not entirely sure the equation is right for this type of problem. I do not even know where to start for this problem.

I think your equation is wrong. $$F_g=\frac{GMm}{r^2}$$

Do you know how to set up a proportion of equations?

 

[Jtappan]

Could you please explain how to do that? :-)