Physics
There are two sets of rules that we work within in any world. The first of these is the "real" set if rules, those that govern the limitations of what we can achieve. It is this set that philosophers and scientists have debated through the ages and will likely continue to do so until the end of us all. No mortal has any true grasp on what these rules are in total. That includes us writers.
You don't need a doctorate in physics to put these rules into action. Many of these rules are things we take for granted on a daily basis. Earth's gravity is such that if we let go of an object that is heavier than the air (most things except for example helium balloons), the object falls to the ground. If your house is on fire you get as low as you can because both smoke and heat rise, so the cleaner, cooler air will be near the floor.
Aren't sure about how particular things work? It's easy enough to do some research and find out. This fact is quite important, actually. Nothing will throw your reader out of a story faster than inconsistent world rules. If your world's physics works exactly as the Earth's does, then a normal person cannot throw a baseball hard enough that it has a chance of going into a low orbit.
Want to give an otherworldly feel? Give the place a stronger or lesser gravity, or make solid objects not so solid or extremely dense. Even the simple factor of changing the color of the sky lets us know that "we're not in Kansas anymore."
In the case of our world, since it's only a step or two off from reality, I'm going to leave physics entirely intact and Earthlike.
Not So Scientific
Of course, not everything that governs our how our world works is known by science. Or should I say that it's not agreed upon by science. There are traditional herbal remedies that only now we are starting to accept, and even scientifically prove why they work. Many experiments revolving around the same issue are inconclusive either from a bad initial set of assumptions or perhaps the inability to properly measure the results.
