Laws of Motion

I see the D&D universe as a pre-Newtonian world. Very much controlled by something similar to Aristotle’s Laws of Motion. At least, that is the way that the most intelligent thinkers of the time believe that it works. All spells that affect the target’s speed or location, such as fly, levitate, teleport, etc. cancel all current forces acting on the target and replaces them with the effects of the spell.
So first I will present the laws of motion as known by most magic users in the D&D universe.
This is how it might be explained to you by the smartest man in the kingdom. [Please understand that the ideas below may represent a view of the world similar to that held in 300 BC, but was later replaced by Isaac Newton’s much more accurate laws of motion.]

The Celestial Sphere

“Objects in the heavens (the celestial sphere) move in circular motion, without any external force compelling them to do so. Objects on Earth (the terrestrial sphere) move in straight lines unless forced to move in a curve.
First, although most commoners think that the Earth is flat, it is indeed spherical. You know. Round like a ball. It only appears to be flat because it is so large. The Earth is in the center of the universe. It is surrounded by the celestial sphere, where lie the sun, the moons, all of the planets and the stars. All of these celestial bodies circle the Earth. The apparent motions of the fixed stars and planets are accounted for by the fact that they are embedded in rotating spheres made of an aetherial, transparent fifth element (quintessence), like jewels set in orbs. The fixed stars do not change their positions relative to one another because they are on the surface of this single starry sphere.
The stars, Sun, Moon, and planets are all made of fire. But whilst the stars are fastened on a revolving crystal sphere like nails or studs, the Sun, Moon, and planets, and also the Earth, all just ride on air like leaves because of their breadth. And whilst the fixed stars are carried around in a complete circle by the stellar sphere, the Sun, Moon, and planets do not revolve under the Earth between setting and rising again like the stars do, but rather on setting they go laterally around the Earth like a cap turning halfway around the head until they rise again.”

The Terrestrial Sphere

“To the motion of non-living things, such as a stone dropped from the hand, is explained by two principles; Natural Motion and Violent Motion.”

Natural Motion

“The 4 elements [earth, air, fire and water] tend to seek their natural place in the order of things. So earth moves downwards most strongly, water flows downwards too, but not so strongly, since a stone will fall through water. In contrast, air moves up (bubbles in water), and fire goes upwards most strongly of all since it shoots upward through air. Most materials that you see around you are mixtures of elements. For example, wood has both earth and air in it, since it does not sink in water.
Natural motion causes undisturbed inanimate objects to travel in a straight line either toward the center of the Earth or away from it. Left undisturbed, a pure Earth would consist of an inner ball of earth surrounded by a shell of water over which would be a layer of air and above all would be an outer layer of fire.”

Violent Motion

“Things also move because they are pushed. A stone’s natural tendency, if left alone and unsupported, is to fall, but we can lift it, or even throw it through the air. We call such forced motion “violent” motion as opposed to “natural” motion. The term “violent” just means that some external force is applied to it.
Heavier things fall faster, the speed being proportional to the weight. The speed of fall of a given object depends inversely on the density of the medium it is falling through. So, for example, the same body will fall twice as fast through a medium of half the density.
For violent motion, the speed of the moving object is in direct proportion to the applied force. This means that if you stop pushing, the object will soon stop moving.”

How this Effects Magical Spells

So the magic caster thinks that a body in motion only stays in motion as long as the force causing it to move continues to push it. Otherwise, it will eventually slow to a stop. So for teleportation – when the subject of the spell arrives at its new destination all external forces stop acting on it and it arrives at its destination as intended. External forces, in this case, would include what we refer to as inertia.
This also makes answering questions like this much easier:
“What if the PC walks through a teleportation gate and arrives at another location thousands of miles away?” Think of the actual, physical conditions. The Earth is spinning about 24 thousand miles per hour from West to East. Depending on where on Earth the other portal is located, inertia could be a big problem. Not to mention orientation.
Even something a simple as a feather fall ring. “What if the wearer was shot out of a cannon?” At the top of the arc, he would begin to fall. So feather fall kicks in and he begins to fall slowly. If inertia is still in effect he will travel much farther and still hit the ground at the speed that he was shot out of the cannon. This is obviously not the intention of the feather fall spell. If on the other hand, inertia and gravity are no longer pushing on the PC and are replaced by the magical feather fall rules, he floats gently down from the point where he begins to fall.
“If you are flying through space on a sailing ship that has a magical gravity bubble surrounding it, what happens if you fall overboard?” I would say that you fall as if you were on earth until you reached the edge of the gravity bubble and then slowly stop when the force of the magical gravity stops pulling you down.