Expert: Philip Stahl - 8/15/2008

Question
QUESTION: Why does one sattlite obit the earth while another is in a stationary orbit?

ANSWER: This is a common misconception. In fact, BOTH satellites orbit, it's just that one orbits at the same rate that the Earth rotates on its axis. So appears "fixed" from that point when one observes it.

Such an orbit is called "geo-stationary" and occurs when a satellite is at a specific altitude of 22,400 miles.

---------- FOLLOW-UP ----------

QUESTION: What causes a sattlite (or anyn other celestial object) to orbit?

Answer
A satellite attains orbit when it is launched at escape velocity - which is about 18,000 mph. If the height above the Earth is then large enough, one will find:

mv^2/ r   =  GMm / r^2

that is, the centripetal force provides the basis for it to mainain orbit, and the key player is the centripetal acceleration:

a = v^2/ r

Thus, so long as the circular velocity (v) is large enough, the satellite will remain in orbit.

You can learn more about the associated force, acceleration below:


http://www.regentsprep.org/Regents/physics/phys06/bcentrif/default.htm


(Note:the term  "centrifugal force" is not used at all. Centripetal force is the only real force acting.


Other aspects of your question as pertains to other bodies in orbit  can be found in the (past) question-answer bank.