# Law of universal Gravitation

1. Nov 18, 2013

### Coco12

1. The problem statement, all variables and given/known data

The radius of the earth is 6400km. A 7200n space craft travels away from the earth. what would be the weight of the space craft at the following distances from the Earths surface: 6400km, 12800km

2. Relevant equations

Fg=mg
F=Gm1m2/r^2

3. The attempt at a solution
Just want to confirm I'm doing this right
So first I want to find the mass of the space craft so I use the fg=mg formula. I know that the mass of the earth is 5.98*10^24kg.

Gravitational constant is 6.67*10^-11

I use the universal gravitation formula

The r will just be the radius of the earth PLUS the distance from the earth's surface. (Converted to m)

Then just plug into the equation. Am I doing this right?

2. Nov 18, 2013

### nasu

It is not wrong but too complicted. They give these nice numbers for a reason.
Look at the numbers. By what factor increases the distance? What does this tell about the change in force?

3. Nov 19, 2013

### Coco12

The factor increases by 2 (6400+6400) then 3 (6400+12800)

So u mean I would just multiply 7200 by 1/4 for the distance 12800 from th earths center
And 1/9 for the distance 19200 ?

4. Nov 19, 2013

### BOAS

You have what is known as 'an inverse square law'

ie

Force due to gravity decreases with 1/r^2

5. Nov 19, 2013

### nasu

Yes. This will do it.