# Space Chase - a challenging question

1. May 9, 2007

### vle1

1. The problem statement, all variables and given/known data

http://img518.imageshack.us/img518/2716/question11mediumjj8.jpg [Broken]

This was an extra question for final exam some years ago, i hope that some one can direct me to obtain the right answer. I could not good find equations that I learn to answer this

Last edited by a moderator: May 2, 2017
2. May 9, 2007

### andrevdh

Navigate: Mechanics -> Orbits -> Kepler's laws. Then scroll down for more info on the subject on the page that you land on.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html" [Broken]

Note that Kepler's third law is also applicable to the circular orbit of the space station.

Last edited by a moderator: May 2, 2017
3. May 9, 2007

### nrqed

You know the period you want.

Use Kepler's third law to find the semimajor axis of the orbit you need.

Once you know the semimajor axis, you know the total energy you will have at any point of your orbit (there is a simple formula giving the total energy at any point of an orbit in terms of the semimajor axis and the masses).

Now set that energy equal to $1/2 m v^2 - G m M /r$ where r is the initial distance from the center of the Earth. That will give you the speed you need.

Last edited by a moderator: May 2, 2017