Calculating Distance from Moon's Centre for Gravitational Field Change

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Dark_Dragon
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An astronaut standing on the surface of the moon experiences a gravitational force of attraction of 160 N. He then moves away from the surface of the moon to an altitude where gravitational force is 40 N.

a) How far away from the centre of the moon is this new location in terms of the radius of the moon?

I know that Fg is proportional to 1/r^2

I don't know how to figure it out, can anyone point me in the right direction?

Thanks.
 

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Well, ask yourself two questions:

(1) How far away from the center of the moon is he when he is standing on the surface of the moon?

(2) How far away from the center of the moon would he need to be to reduce 1/r^2 to 1/4 of what it is on the surface of the moon?
 
aww man i didnt even think of it, thanks very much mate :)