# Gravity

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Gravity, in the real world, is defined as the mutual force of attraction between all particles or bodies that have mass. In practical terms, gravity means that things have weight and that, when unsupported, will fall towards the earth. In games, gravity is simulated in many different ways, ranging all the way from simplistic constants to precise complex mathematical models. The implementation of gravity, as a rule, is not necessarily uniform (applying to all entities in the gameworld) or realistic.

In a game such as Super Mario World, gravity is evidenced by noting that unsupported objects (that don't fly) fall towards the bottom of the screen. In the FPS Delta Force, shots fired by the player are affected by gravity and lost altitude as the move further away from the weapon that fired them. In a similar fashion, a player can throw a grenade straight up in the air and have it land in the same spot.

There are also games that implement the pseudo-physical notion of gravity indirectly. Instead of relying on a physical model, different objects are assigned numerical weights and thus limit the ability of certain characters to carry them. This is commonly seen in role-playing games where the player must manage an inventory of items and a weight system is used to limit the amount, and type, of things the characters can carry.

## Strong Examples

### Halo 2

In Halo 2 all of the grenades and player movements are ruled by the laws of gravity. When a player jumps in the air he will fall down after jumping. Furthermore, if a player falls off of the side of a building he will fall with normal gravity type rules. When the player throws a grenade, the grenade will obey the rules of gravity. It will act just like a baseball would in real life. Gravity affects how far and how high you can throw it.

### Prey

Prey literaly added new twists on how the player experiences gravity in a first person shooter. Prey is based on the Doom 3 engine devolped by ID, which uses realistic physics to determine how objects interact in the game environment. Prey uses gravity in 2 new and unique ways.

Firstly, Prey gives the player the ability to change the direction gravity is applied. When a player shoots a "gravity switch" on the ceiling, the player's world is flipped upside-down so that they are now walking on what was previously the ceiling. Anything that is not attached to the floor, which includes the player, crates, weapons, and enemies falls to the ceiling respecting the change in gravity.

Secondly, Prey allows the player or computer controlled enemies to defy gravity by walking on gravity "catwalks," which allow the character to walk on walls as long as they have at least one foot touching it. As soon as the player walks off the catwalk or jumps, they will fall off to the floor and sometimes hurt themselves when falling from great distances. This adds a unique aspect to FPS gameplay because enemies may come appear above or behind you. The player has to be aware of what is above and below him in addition to infront and behind.

### Untold Legends: Brotherhood of the Blade

In this fantasy RPG the player is able to pick up and carry items. Each item has a numerical weight assigned to it and the player controlled character can only carry as much weight as his/her character's strength allows for. The system is inconsistent however in that gold coins collected do not have any weight at all, regardless of the number carried.

## Weak Examples

### Metroid Prime

Samus eventually finds an upgrade called Gravity Boots which enable her to have a second jump. Even though the planet she is on has pretty much the same laws of gravity as we do on Earth, she is able to gain a boost in the middle of the air after doing a first jump.

### Super Mario Bros

Though the majority of the game world is affected by gravity, certain entities are immune. Of the non-sentient entities, the primary example is that of the bricks that hang in mid air. Of the sentient entities, the most prominent example is one of the enemy called 'Lakitu' who dwells in a cloud in the top of the screen. We call this a weak example because gravity does not affect all the entities in the game in the same way.

### Elder Scrolls IV: Oblivion

Oblivion uses the Havok physics engine to simulate many physical laws. Arrows, for example, are affected by gravity very similarly to the way they would be in real world. If the target is missed, they will bounce off the walls or floor. Also, both player and NPC will lose health when falling more then a few feet. Jumping in the water however softens the fall and does not damage the player.

### Super Smash Bros Melee

The game world has gravity that pulls all players and objects toward the ground however players don't accelerate while they fall, and players have double and even triple jumps that can defy this gravity.

### World of Warcraft

Players are under the effects of gravity most of the time. If they jump off of a ledge, they will fall. If they fall far, they will be hurt, and if they fall very far they will be killed. However, non-player characters and monsters are not: they are always on the ground, or sometimes scripted to appear to be flying or floating. There are also ways appropriate to a fantasy game that a player can change the way gravity affects him or her: magical spells to fall slowly or levitate, and flying creatures to ride upon.

### Escape Velocity: Nova

Though set in space, with planets that should have a large gravitational pull, the game engine does not take gravity into effect when the player is navigating there spaceship.

### Super Mario 64

While Mario is limited by gravity for the majority of the game, his winged hat enables him to fly in certain levels. He is therefore able to defy the laws of gravity.

### Pac-Man World 2

In Pac-Man World 2, gravity effects most things such as Pac-Man, enemies, landscape, and most of the objects throughout the level. However, gravity has no effect on pac-dots, power pellets, tokens, fruit, extra lives, power-ups, health wedges, b-doings, and basically everything else that is helpful to Pac-Man throughout his journey. Pac-Man himself is constantly limited by gravity except when he eats the red pac-dot at the begining of a pac-dot chain. When this happens, Pac-Man is lifted into the air, defying all laws of gravity, and gracefully floats his way through the pac-dot chain which takes him on giant loops and swirls in the sky. When the chain ends, Pac-Man is dropped back onto the earth and gravity is once again applied.

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