# Math Help - Help with physics

1. ## Help with physics

Al the astronaut has a weight of 800 N when he is standing on the surface of Earth. What is the force of the gravity acting on him when he is in a space station orbiting Earth at a distance of three Earth radii above the surface?

Can someone help me set this us?

2. ## Re: Help with physics

you should have learned that the gravitational force (weight) between any two objects is inversely proportional to the square of the distance between their center of masses. translated into an equation ...

$W = \frac{k}{r^2}$

where W = the gravitational force , r = distance between them in earth radii, and k is a constant of proportionality.

now, you are given that when r = 1 earth radii , W = 800N ... find k.

Once you've found k ... when r = 4 earth radii, W = ?

3. ## Re: Help with physics

I get confused with radii. How do I do that?

4. ## Re: Help with physics

r = the number of earth radii

800/4^2
????

6. ## Re: Help with physics

Originally Posted by Louisana1
800/4^2
????
yes ... do you understand why r = 4 ?

7. ## Re: Help with physics

not really...maybe because he is already above the surface and that is counted as one?

8. ## Re: Help with physics

Originally Posted by Louisana1
not really...maybe because he is already above the surface and that is counted as one?
that is correct. standing on the earth's surface, you're already one radii from the center ... add 3 more above the surface, you get a total distance of 4 earth radii from the earth's center.

9. ## Re: Help with physics

so if he was below the earth surface. what will the number be??? -4

10. ## Re: Help with physics

Originally Posted by Louisana1
so if he was below the earth surface. what will the number be??? -4
no .. the distance would be a fraction of r, but other physics principles apply in that case.

11. ## Re: Help with physics

ok got it thanks a bunch