Ragdoll Physics

Ragdoll physics is a feature first implemented in Grand Theft Auto IV. With ragdoll physics, players and NPCs lie on the ground dynamically. The affected model essentially behaves like a "ragdoll", hence the name. Characters no longer lie flat and static on surfaces. Their bodies will conform to any objects realistically and can be moved around by collisions and bullets. However, unlike other games that use ragdoll physics, characters in GTA games do not seem to be affected to the fullest extent and still conform to some preset animations, thus assuming a more "human" posture and not like a dummy being thrown around (eg. when you jump off a tall building, the subject would land straight then topple over, rather than landing on one foot and immediately falling to the ground).

When the ragdoll effects on NPC or protagonist is activated by having them hit by a car, falling, etc., they will not just flap around like a "lifeless" ragdoll if they are still alive; they will brace themselves while getting thrown about. When a protagonist or NPC gets hit by with a car (but doesn't die from the impact), they will brace themselves as they are getting thrown about the road. Up until they are killed, they will continue to brace their face with their arms and their legs in the fetal position. If a protagonist or NPC is killed by getting hit by a car, their body will be thrown about lifelessly without bracing. This is added for realism as real life people will automatically brace themselves whenever they are thrown. Ragdoll physics are a bit inconvenient at certain times, especially when bailing out of a vehicle or ejecting involuntarily out of a vehicle, unlike the previous renditions, as the player may continue to flop around until death. This feature adds realism and a sense of more control in the game.

In Grand Theft Auto V, the ragdoll physics have been reduced in bouncing and duration of the flips and impacts, but leave much more damage to the protagonist. When they bail out a car, they will try to brace themselves, but instead of flipping or bouncing, they will slide on the road until they stop, when they are stopped, before getting up, they will stay on the ground as they were taking a break or been in pain. When falling from a mountain, the smallest touch can be fatal to the player, they will keep rolling until they land on a flat surface or a rock, also when falling from a building, they will instantly roll in the air screaming. During the freefall time, they will be facing the sky (most times) moving their arms and legs without stopping. This is much more realistic, as if a person falls from a great height, they would move their arms and legs in random ways. However, when a NPC or protagonist is dead, the ragdoll still flips and slides on the ground, unlike in GTA IV, where after landing they would only slide for around a meter.

Physics in older games
As ragdoll physics were added in Grand Theft Auto IV, in previous games if a player or NPC falls off a building or hill they would fall completely flat even if the ground below is steep. Also if a player or NPC falls on the ground he will land with both feet on the ground and then fall flat. Even if the player or NPC dies from the impact of the fall they will still hit the ground the same way. In Grand Theft Auto IV, if the player falls from a low height Niko will do a stunt roll, but in previous games the player will fall flat on their feet, able to keep moving, with a slight bend forward (this is very hard to notice, but still there) in Grand Theft Auto: San Andreas. If a player or NPC is hit by a car in a game previous to Grand Theft Auto IV, they will fall on their back and then get back up if not killed, or they will fall the same way but slide over the top of the car; in Grand Theft Auto IV, Niko or an NPC will fly over the top of the car and can hit the ground in many different ways depending on how high they fall, the speed of the car, and the angle of the collision.

Trivia

 * In GTA IV and its episodes, if you buy guns from Little Jacob, Terry Thorpe, or Armando Torres and then kill them, their bodies will not use the ragdoll physics and will not move at all, even with explosives. It is unknown why this occurs, but it could be a glitch.
 * GTA IV, in the beta, didn't have ragdoll physics. It can be seen rarely when killing a pedestrian, falling in an animated way, or completely removing ragdoll (with mods), the beta animations will be used instead.
 * Occasionally, if players are too far away from NPCs who die from explosions, the ragdoll physics may not trigger properly and NPCs will fall flat on their backs all the same way.