Why can't I see the Physics button on the world panel? I'm starting to make physics in blender, but when I was going to edit the parameter "gravity" on the "World => physics" button, I found it nowhere!

Can someone please help me? What can I do? I need it for my animation assignment.

It is like the first image when it should include the buttons shown in the second one. 

Here I have the world panel, but the "physics" subdivision is nowhere to be seen



It should be like this. Sorry for the spanish.
They are in the "Scene" section, not on the world.