The Gravitational Force and the distance from the earth

1. Sep 11, 2008

Kaze105

1. The problem statement, all variables and given/known data
Find the altitudes above the Earth's surface where Earth's gravitational field strength would be (a) two-thirds and (b) one-third of its value at the surface. [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.]

2. Relevant equations

I do believe i use F=Gm1m2/r^2

3. The attempt at a solution

Well, actually im at a little loss for this question..

2. Sep 11, 2008

LowlyPion

The hint tells it all.

Set 1 equation = F, another to 2/3 F another to 1/3 F.

Solve by dividing one equation by another.

3. Sep 11, 2008

Redbelly98

Staff Emeritus
That's the right equation.

Let's say at the surface we have

Fo=G m1 m2/ro^2

and we want to find the distance r1 where the force F1 is

$$\frac{F1}{Fo} = \mbox{???} = 2/3$$

p.s.
Welcome to Physics Forums!

4. Sep 11, 2008

Kaze105

problem is that, it doenst give me the distance for the altitude...

5. Sep 11, 2008

Redbelly98

Staff Emeritus
Yes, that distance is what we are trying to figure out.

As you said, F=Gm1m2/r^2 . Can you use that equation to express the ratio of two forces,

F1 / F0

where F1 is the force at distance r1, and F0 is the force at distance r0?