Earth's gravitational field strength

[D4b34r5]

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 multiplied by 10^3 km.
wrong check mark 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.
wrong check mark km

[Hint: First find the radius for each situation; then recall that the altitude is the distance from the surface to a point above the surface. Use proportional reasoning.]

Homework Equations

F = GMm/r^2

The Attempt at a Solution

I have tried it and I am either calculating wrong or setting it up wrong... or both. One problem I see with the formula is that it doesn't have the mass of the second object.. so I don't know what to do there.

 

[stakehoagy]

you do not need to know the mass of the second object. F=ma and the masses of the second object cancel out. a=9.8m/s^2 on Earth and at the point above the Earth a=9.8/3 let r=radius of Earth + altitude. then plug in and solve for a.

 

[D4b34r5]

Sorry but that still isn't helping me. Are you just using the F=ma formula?