Wonderful World of Marisics

While the laws of physics are generally accepted (as they are generally unwavering), there are worlds that possess their own, alternative laws of physics. The worlds of videogames. Named for its discoverer, Mario Mario, hero and resident of the Mushroom Kingdom. Marisics is the study of videogame physics and how they differ from that of our own Earth. While there are many slight variations to be found from game to game, some laws are fairly consistent. Nonetheless, please treat all statements as generalizations and be aware that there are often exceptions.

Gravity
Gravity does exist in videogames, but its effects are significantly different. Gravity in videogame worlds -even most virtual Earths- is much weaker. Characters have been known to jump over twice their height, at least, while Mario -famous for his jumping capability- is able to jump from three to five times his 155cm height, and even higher with the use of springboards. On that note, springboards -which can be anything from a simple, randomly placed duo of platforms separated and connected by two metal bars, to a large leaf or mushroom- often produce a tremendous increase in jump height, despite the lowered gravity.
Another interesting aspect of marisics gravity is that it causes little-to-no acceleration. A leaping character will move upwards at a constant rate, pause at maximum height momentarily, then immediately plummet at terminal velocity back downwards. All objects appear to have the same terminal velocity, as well, though Mario is often slightly faster than them.
Recently, an advance has been made in the study of gravity by Mario himself in the new game Super Mario Galaxy, in which he confirms that it is possible for objects with radii under two metres to not only hold a body to itself in the presence of other identical objects (without being drawn to them), but to do so with the exact same gravitational influence as any given planet. It should be noted, however, that these objects were suspended in a somehow oxygenated space. Paradoxically, when Mario visited the moon in Paper Mario: Thousand Year Door, he was able to jump much higher, which suggests that the Paper world has its own marisics.
Another interesting note is that several games contain gravitational shifts in specific areas, which are often selective about who and what they affect. For example, there are places where it's possible to jump and end up affixed to the ceiling, and other areas where other inhabitants walk freely on a wall (which is built like the floor), but one who enters is bound to the floor.

From these facts, it can be inferred that the marisics gravity's influence is constant in every object large enough to walk upon, provided it is in space. Gravity also instills velocity, not acceleration in an object. It is possible that each object somehow has its own terminal velocity, but it seems more probable that there is a standard falling speed for all objects which can be altered by certain acts. These aspects make calculations for marisics much simpler. Some scientists claim that this is so it would be easier to program into the games, but these are the types of scientists that ruin the fun and never get invited to any of the cool parties.

Momentum
Momentum holds much less staying power in marisics-afflicted worlds. While a simple jump across a gap tends to be pretty straightforward (aside from the character's ability to change direction in midair), once interrupted by a collision, a number of things may happen. In the Mushroom Kingdom, the colliding object will injure the jumper and render him or her intangible to impacts for a brief period of time, passing through him or her without either actually losing momentum. In the Megaman world, any impact will cause the recipient to pause, flash, and move backwards a short distance, even if the collision came from behind! If the struck person isn't on a platform at the end of this process, they will fall at an immense speed, often to their death.
Momentum may also be stopped voluntarily, or in some cases simply altered. Again, Mario is a pioneer in this field with his often-mimicked Ground Pound technique. The move, initiated in the air, consists of a complete stop in motion, a rotation, and a sudden forceful burst of downward momentum. Most notable residents of the Mushroom Kingdom have been able to replicate this, as well as a Bandicoot by the name of Crash, a bounty hunter known as Dante, and a certain genus of Pikmin (plant-creatures), so it's a common attribute. There is also a kick similar to this that hurtles the user downward at an angle. Famous users of this are F-Zero Grand Prix champion/ bounty hunter Captain Falcon (who bursts into flame while using it, but only because of the hyperactive nature of his atomic structure), the aforementioned Dante, and one-time vampire hunter Johnathan Morris.
In the Super Smash Bros. series, momentum tends to override gravity. When a character is struck under the right circumstances, they will rocket out at an angle not necessarily dependent on the angle of the impact, then slow and fall normally (unless they chose to fall rapidly). This is known as a "launch". For whatever reason, only momentum or gravity may exert a force on a launched body at any time.

The intangible effect can be best described as a phase-shift. When a collision takes place, the character receives all of the kinetic energy of the object, but instead of being knocked away by it, is actually knocked into having their atoms vibrate differently, and so begins passing through certain things. Presumably, the matter that can still be touched is dense enough that it is unhindered by this phase shift. The offending object is also allowed to continue its trajectory because, while every action has an equal and opposite reaction, the reaction comes after the phase-shift has taken place and so cannot hit it.
In the Megaman world, it seems that the direction of force from an impact is relative to the individual being struck. If this is due to a property of the objects, that would mean that when a hit from behind pushes a being back, it actually exerts a negative force. This is, of course, impossible, so the answer must lie in the individual. Somehow, the bodies of playable Megaman characters transfer any shock to their bodies to their fronts and expel force from there. This force is always the same, so enough energy from the impact is absorbed to leave the specific amount that pushes the individual back about three pixels. The Ground Pound is probably a controlled from of this process, except instead of the kinetic energy from an impact, it redirects the momentum of a jump. It also seems that small amounts of energy can be released to push a body into other directions, and if this energy is condensed, then jetting in one direction without the ability to change it becomes possible. Basically, instead of an equal and opposite reaction, every action has a set reaction in a variable direction.

Air Density
Again, this is a trait most apparent in the Mushroom Kingdom. The air -which is of unknown composition- can be passed through normally most of the time, but may also be responsible for rock-solid barriers known as "invisible walls". Invisible walls are one of the leading causes of head injuries to videogame protagonists, as the experience of encountering one could be a simple resistance to an attempt to walk forward, or a full-blown wall blocking one's path when fired out of a cannon. Even more dangerous is the fact that sidescroller characters may be "chased" by a moving invisible wall, which can crush them should they be caught between it and a solid object. Nobody knows for sure how such a strange thing can exist, but it could be that certain areas push away air so as to become low pressure areas. In doing so with the incredibly thick air, they essentially become perpetual, solid, silent storms.
Another notable aspect is that air is theoretically dense enough to disperse in water. Even in worlds where characters need to hold their breath, they are able to do so for several minutes at a time. This means that the water must be "breathable" to a degree, thus the theory stated above.
Most interesting, however, is the double-jump. While this maneuver is exceedingly common, its implications are fascinating. In order for someone to kick off of air, it would have to be extremely dense. While this supports the theory for how it works in water, it also raises the question: How is easy movement through it possible? The best theory so far is that air will naturally close in under the feet of anyone moving through it. As the initial jump takes a body up, the air moves around it and congeals beneath it. The second jump kicks this air out of the way, and is often accommodated by a flip, preventing a third jump.

Spikes
Spikes have an interesting characteristic. Besides their ridiculous abundance in videogames, a single touch of a perfect spike often exerts immense physical force; enough to make Megaman -an advanced combat robot- explode from peak condition or to launch somebody out of sight. An interesting exception to this is Sonic the Hedgehog's spikes. They are not instantly fatal and actually repel those who run into them, doing no more than a standard collision. Some experts argue that the energy exerted from these spikes is so great that they cannot actually be touched, and so are less dangerous. Metal spikes are more common, but naturally-forming thorns with the same effect have been known to exist.
Apparently, a spike only possesses this property if it is a perfect cone shape. However, shapes reasonably similar to a spike may have a sharply reduced version of this effect. In the aforementioned Smash Bros. series, one character (actually named Link, but in possession of countless pseudonyms) has been known to simply hold his spike-like sword in front of himself, and instantly launch (see Momentum) anyone who touches it at the same speed per character, regardless of how much momentum it has.
The current accepted theory to explain the properties of spikes is that their shape creates a kind of resonance. The vibrational kinetic energy of atoms within the spike is built up and channelled by the conical shape along the spike's length to its tip. For some reason, spikes cannot give off any type of energy other than kinetic, so at a single touch, as much of this accumulated energy as possible is expelled. In an imperfect spike, much of this energy must be lost, and it is all spread evenly throughout the blade.

Extra Lives
The most seemingly impossible thing in a video game is a character's ability to die (or at least be knocked out), then suddenly reappear at a checkpoint. This can only be done a limited number of times, but that amount can be increased by "1-Ups", which can be anything from a green mushroom to a T-shirt.
As most logical reasoning is unable to explain this phenomenon, the only acceptable conclusion is that using a 1-Up is essentially rewinding time. The 1-Up items must possess a type of energy that allows temporal movement, so when their owner undergoes the shock of passing out, time immediately "rewinds" to the last checkpoint. Following this path of thinking, checkpoints must act as a kind of temporal magnet once activated, so the 1-Up user will be "drawn" to that spot.

In the end, the world of Marisics is a vast and ever-changing study. As time goes on, it seems that all worlds' marisics shuffle closer to our own, but there will always be anomalies, and there will always be questions for the classics.