Centripetal force satellite orbiting problem

[pokeefer]

Homework Statement

Question 1: What speed must a satellite have to orbit the Earth at a distance 7.8 x 10^7 m/s from the Earth's centre, where the acceleration due to gravity is 0.065 m/s^2?

Question 2: An object orbits the Earth at a constant speed in a circle of radius 6.38 x 10^6 m, very close to but not touching the Earth's surface. What is its centripetal acceleration?

Homework Equations

Ac = v^2 / r
Fc = mv^2 / r

V = 2(pie)r / T

The Attempt at a Solution

For the first question I assumed that the centripetal acceleration was the force of gravity and so I got:

Ac = v^2 / r
0.065 = v^2 / 7.8 x 10^7
V = 2252 m/s

I don't even know if that is right.

And then the next question is really touch and making me confused. There are too many satellite questions in my homework assignment and its frustrating me. What do I do! :/

 

[gneill]

You could say that the centripetal acceleration was due to the force of gravity, or equal to the acceleration due to gravity. But not equal to the force of gravity. Force is not acceleration.

Your approach on the first question is good. So, keeping that in mind, what is the acceleration due to gravity close to the surface of the Earth?

 

[pokeefer]

That did not help me much at all.

I feel like i don't have enough given information to work with on these questions.

 

[gneill]

What assumption did you make for the first question?