Using ratios to obtain the weight of a person at a specific distance

[needingtoknow]

Homework Statement

Assuming the person weighs 980 N on the surface of the Earth, how would you use ratios to obtain the weight of a person at 128 000 km above the surface of the Earth.

Homework Equations

Fg is proportional to 1/d²

The Attempt at a Solution

The answer in the key says 0.072 N. I didn't know exactly where to start but by working backwards from the answer I have determined that it is 20 times the distance from the centre of Earth. I know how to solve these questions using ratios if I am given the times the distance from the centre of Earth.

For example for a previous question: Three times the distance from the centre of Earth, here is how I solve it.

Fg is proportional to 1/d^2

1x3 = 3 therefore 980 = 1/3^2

980 / 9 = 109 N

This method works everytime when I am given the times the distance from the centre of Earth. But when I am given a specific distance I do not know how to get the answer. Can someone please help? Thanks!

 

[Sunil Simha]

Since F\propto\frac{1}{d^2}, You can write F = k\frac{1}{d^2} where k is the constant of proportionality.

So if F_1\propto\frac{1}{d_1^2} and F_2\propto\frac{1}{d_2^2}

then \frac{F_1}{F_2}=\frac{\frac{1}{d_1^2}}{\frac{1}{d_2^2}}.

The constants of proportionality cancel.

 

[Chestermiller]

The radial distance from the center of the Earth to the surface is ? What is the ratio of 128000 km to this distance?