# Help with physics

• Oct 15th 2012, 10:16 AM
Louisana1
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?
• Oct 15th 2012, 10:25 AM
skeeter
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 ...

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

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 = ?
• Oct 16th 2012, 01:06 PM
Louisana1
Re: Help with physics
I get confused with radii. How do I do that?
• Oct 16th 2012, 02:03 PM
skeeter
Re: Help with physics
r = the number of earth radii
• Oct 16th 2012, 02:34 PM
Louisana1
Re: Help with physics
800/4^2
????
• Oct 16th 2012, 02:41 PM
skeeter
Re: Help with physics
Quote:

Originally Posted by Louisana1
800/4^2
????

yes ... do you understand why r = 4 ?
• Oct 16th 2012, 02:43 PM
Louisana1
Re: Help with physics
not really...maybe because he is already above the surface and that is counted as one?
• Oct 16th 2012, 02:46 PM
skeeter
Re: Help with physics
Quote:

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.
• Oct 16th 2012, 02:47 PM
Louisana1
Re: Help with physics
so if he was below the earth surface. what will the number be??? -4
• Oct 16th 2012, 03:07 PM
skeeter
Re: Help with physics
Quote:

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.
• Oct 16th 2012, 03:08 PM
Louisana1
Re: Help with physics
ok got it thanks a bunch