Uniform Circular Motion of a satellite

[DramatikaRocks]

Problem:
I have these exercises that I need help with. They look easy and I have an idea on what to do, but the answer I get is not right. I know I'm doing something wrong and I need any help I can get.

Homework Statement

1.) An Earth satellite moves in a circular orbit 640 Km above Earth's surface witha period of 98.0 min. What are the a.) Speed and b.) the magnitude of centripetal aceleration of the satellite?

Homework Equations

I want to know what is the radius that I use and if I have to change 98 min for seconds?
If I have the speed I can calculate the aceleration.

The Attempt at a Solution

The problem is I'm assuming that the radius is 640km and I divide that with 98.0 min but that is not the right answer.

 

[DramatikaRocks]

the other exercise is :

1.) A boy whorles a stone in an horizontal circle of radius 1.5m and at a height of 2.0m above level ground. The string breaks,the stone flies off horizontally and strikes the ground after traveling a horizontal distance of 10m. What is the magnitude off the
centripetal aceleration of the stone while in circular motion?

2.) The question is that I need the speed and I have no Idea how to get it. And I don't know how to solve this really.

 

[Doc Al]

Homework Equations

I want to know what is the radius that I use and if I have to change 98 min for seconds?
If I have the speed I can calculate the aceleration.

The Attempt at a Solution

The problem is I'm assuming that the radius is 640km and I divide that with 98.0 min but that is not the right answer.

You need the radius of the satellite's circular orbit. 640 km is just the distance above the earth--be sure to include the radius of the earth. Convert distance to meters and time to seconds. Realize that the satellite travels a complete circumference in every period.

the other exercise is :

1.) A boy whorles a stone in an horizontal circle of radius 1.5m and at a height of 2.0m above level ground. The string breaks,the stone flies off horizontally and strikes the ground after traveling a horizontal distance of 10m. What is the magnitude off the
centripetal aceleration of the stone while in circular motion?

2.) The question is that I need the speed and I have no Idea how to get it. And I don't know how to solve this really.

You'll need to use your knowledge of projectile motion. How long does it take for the stone to fall? Use that to figure out what its speed must have been.