Quick question on orbital speed of satellite

[dezhiz]

Homework Statement

The polar orbiting satellite NOAA-N was launched in May 2005. The satellite is now moving in a circular orbit above the Earth's surface at an altidude of 870 km. THe mass of the Earth is 5.97 x10²⁴kg and its mean radius is 6.38 x10⁶m.

Show that the orbital speed of the satellite is 7.41 x10³ms^-1

Homework Equations

i used v=√GM/r
where G = 6.67 x10^-11

The Attempt at a Solution

What I got was..
v = √(6.67 x10^-11)(5.97 x10²⁴)/6.38 x10⁶
v = 7.86 x10³ms^-1

What am I doing wrong? D:

 

[D H]

You are using 6.38x10⁶ meters as the satellite's orbital radius. That value is the mean radius of the Earth, not the orbit.

 

[dezhiz]

ah yea... Thanks!