|from a scene in City of Ember|
No. The fascinating thing about fiction literature is that it doesn't have to be realistic in order to make sense. Strange but true.
There are two things required in making a fantasy or sci-fi story "make sense."
This is easy. If the world is to be destroyed by an exaggerated natural disaster, have a panel of experts use important sounding words to convince everyone that this is a possibility. If your hero has just landed in a world where everybody flies, have some child explain this in a matter-of-fact tone (while looking at your hero like he is crazy for not knowing this already). Your type of "expert" will vary depending on your story. You only have to make it sound convincing.1. Have a panel of experts explain it.
This may require you to make a list on a separate sheet of paper, but, if you have decided that the world you have created is to have different laws than the world we know, you should keep those laws in mind.2. Stick with the laws of your world.
In our world, we have a law called "gravity". What goes up must come down. We can overcome gravity temporarily with force (for example, the force of jumping will propel you up into the air temporarily, and then gravity brings you back down) or with another law like the law of aerodynamics (this is how we fly a plane in spite of gravity). No matter where our story takes us, we are subject to those laws.
If you have different laws in your world, define them to yourself and then make sure your plot follows those laws.