A system rotating

Hi Za Kh. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

You need to be adept at both, and able to convert from one to the other. If you are given most of the quantities in one form you may elect to work in that system.
Good luck with your studies. http://thumbnails112.imagebam.com/37333/0363e9373324851.jpg [Broken]

 

You should choose whichever coordinates make the problem easiest to solve.

For example, you would never use polar coordinates with the origin at the center of the earth to lay out the foundations of a house - Cartesian coordinates with the origin at one corner are much easier. On the other hand, if you were trying to calculate the orbit of a satellite, the polar coordinates would make the job way easier.

As you practice solving more different and more difficult problems, you will start to develop a sense of which coordinate system is likely to work best for any given problem. If you've ever watched an experienced cabinetmaker at work... He'll have a dozen different handsaws, and somehow he always reaches first for the one that will work best for whatever cut he needs to make. That comes from practice.