How can this be wrong? (Simple circular motion)

[lando45]

"An Earth satellite moves in a circular orbit 730 km above the Earth's surface. The period of the motion is 99.2 min. What is the magnitude of the centripetal acceleration of the satellite?"

OK so to solve this I used the formula centripetal acceleration = v^2/r

To calculate velocity (v) I did the following:

v = distance traveled (d) / time taken (t)
d = 2piR = 2 x pi x 730,000 = 4586725.274m
t = 99.2 x 60 = 5952s
v = d/t = 4586725.274/5952 = 770.619ms^-1

Then we already have r which is 730,000m so v^2/r is:

770.619^2/730,000 = 0.813ms^-2

But this is answer is wrong...HOW?!

 

[George Jones]

From is where is the radius of the orbit measured?

 

[lando45]

Ah! So I need to add the radius of planet Earth to the overall radius?

 

[George Jones]

Ah! So I need to add the radius of planet Earth to the overall radius?

You aren't given an overall radius, you are given the height above the Earth's surface. You need to find the radius of the circle that is the orbit.

It looks like you're basically on the right track.