The Gravitational Force and the distance from the earth

[Kaze105]

Homework Statement

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.]

Homework Equations

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

The Attempt at a Solution

Well, actually I am at a little loss for this question..

 

[LowlyPion]

Homework Statement

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.]

Homework Equations

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

The Attempt at a Solution

Well, actually I am at a little loss for this question..

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.

 

[Redbelly98]

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!

 

[Kaze105]

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

 

[Redbelly98]

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?