Half-Life and Half-Life 2

Story: The events of the first Half-Life game begin on Gordon Freeman’s second day at the Black Mesa Research Facility. That morning, he participated in an experiment using an anti-mass spectrometer (used to analyze exotic crystals from the planet Xen, which have anti-mass). These Xen crystals were being used in an effort to create a safe and reliable method of teleportation, an area of particular interest to Gordon. The experiment went horribly wrong and triggered a Resonance Cascade, which created a dimensional rift and allowed creatures from the Xen homeworld to run amok on Earth.
Gordon Freeman fought his way through the Black Mesa facility as it was over run by aliens like headcrabs and U.S. Marines who were sent to destroy the compound and everyone in the area. After attempts to shut the dimensional rift down failed, he travelled through the rift and killed the Xen overlord, who was maintaining the opening.
Many creatures who came through the rift while it was open remained behind and the event attracted the attention of the Combine, an imperial coalition of different alien species, who invaded and conquered Earth shortly thereafter.

Combine Civil "Protection" Officers

The events of Half-Life 2 chronicle Gordon Freeman’s efforts to lead a resistance force against the Combine, who run the planet in a fashion often compared to Orwell’s 1984. Gordon is awakened by a character with a suit and briefcase known only as “G-Man,” who makes subtle appearances throughout the game.
Gordon has allies in humans and one Xen species, the Vortigaunts, who always refer to him as “The Freeman,” emphasizing his status as a savior figure. He and his allies successfully infiltrate the Combine’s central hub in City 17, the Citadel, and retrieve vital information. Dr. Breen, a traitorous human who serves as liaison between the Combine and the citizens of Earth, attempts to escape through a portal and in doing so threatens to destroy the Citadel. Gordon stops him, only to cause a massive explosion and eruption of portal energy, although Gordon is apparently saved by the mysterious G-Man.
Setting: Physics plays a large part in the setting as well as the plot of Half-Life. A physics experiment gone wrong initiated the main conflict and continues to drive it. The greek letter λ, which stands for the decay constant of a substance undergoing exponential decay, is used as an emblem by the resistance against the Combine.

Gameplay: As first-person shooters, the games of the Half-Life series focus on various weapons, their uses, and opponents like headcrab zombies and Combine soldiers.
One of the signature weapons available in Half-Life 2 is the Gravity Gun, which can be used to solve many puzzles and for self-defense. In the final stage of the game, it is upgraded and is also usable as an offensive weapon.
Physics: Collision, ragdoll, water, and explosion physics play major roles in this series.
Half-Life 2 uses the Source engine, which uses networked physics originating from the Havok 2 physics engine and heavily modified by Valve. The abilities of the Source engine's physics system are summarized by the Valve Developer Community:

Machinery: Construct detailed machinery with functioning mechanisms, gears, belts, and pulleys.
Vehicles: Non-player characters and, in a multiplayer game, other players can come along for a ride in cars that feature realistic suspensions with springs on each wheel and wheels that slip and skid depending on the surface material. Individually tunable parameters include horsepower, gearing, max speed, shift speed, tire material, tire friction, spring tension/dampening, and more. Leaning during acceleration, deceleration, and turning enhances the realism of the driving experience.
Deformable Objects: Physics-based animation simulates flexible hair and clothing and blends ragdoll physics with predefined animations.
Ropes and Cables: can be dynamically shaken or broken by level events.
Constraint System Manager: Realistically define bone movement within a physical system using a series of constraints that enable a nearly limitless range of complex movements.
Inverse Kinematics: ensures that characters’ limbs correctly react to environments, such as moving around obstructions and adjusting for uneven terrain.

Characters: The Half-Life series is unique, especially in the first-person shooter genre, in that the protagonist is a physicist. He has no military training, except for some weapons training he acquired through Black Mesa, but is glorified as a liberator. Other characters in the game consistently comment on Gordon’s knowledge and skill.
Influence: Although much of the physics presented by the Half-Life games is imaginary, Half-Life 2 has sparked interest in physics for many players and presents an overall positive image of the field, thanks in part to its physicist protagonist.
Headcrabs attack

Story: The events of the first Half-Life game begin on Gordon Freeman’s second day at the Black Mesa Research Facility. That morning, he participated in an experiment using an anti-mass spectrometer (used to analyze exotic crystals from the planet Xen, which have anti-mass). These Xen crystals were being used in an effort to create a safe and reliable method of teleportation, an area of particular interest to Gordon. The experiment went horribly wrong and triggered a Resonance Cascade, which created a dimensional rift and allowed creatures from the Xen homeworld to run amok on Earth. Gordon Freeman fought his way through the Black Mesa facility as it was over run by aliens like headcrabs and U.S. Marines who were sent to destroy the compound and everyone in the area. After attempts to shut the dimensional rift down failed, he travelled through the rift and killed the Xen overlord, who was maintaining the opening. Many creatures who came through the rift while it was open remained behind and the event attracted the attention of the Combine, an imperial coalition of different alien species, who invaded and conquered Earth shortly thereafter.
Combine Civil "Protection" Officers	The events of Half-Life 2 chronicle Gordon Freeman’s efforts to lead a resistance force against the Combine, who run the planet in a fashion often compared to Orwell’s 1984. Gordon is awakened by a character with a suit and briefcase known only as “G-Man,” who makes subtle appearances throughout the game. Gordon has allies in humans and one Xen species, the Vortigaunts, who always refer to him as “The Freeman,” emphasizing his status as a savior figure. He and his allies successfully infiltrate the Combine’s central hub in City 17, the Citadel, and retrieve vital information. Dr. Breen, a traitorous human who serves as liaison between the Combine and the citizens of Earth, attempts to escape through a portal and in doing so threatens to destroy the Citadel. Gordon stops him, only to cause a massive explosion and eruption of portal energy, although Gordon is apparently saved by the mysterious G-Man. Setting: Physics plays a large part in the setting as well as the plot of Half-Life. A physics experiment gone wrong initiated the main conflict and continues to drive it. The greek letter λ, which stands for the decay constant of a substance undergoing exponential decay, is used as an emblem by the resistance against the Combine.
Gameplay: As first-person shooters, the games of the Half-Life series focus on various weapons, their uses, and opponents like headcrab zombies and Combine soldiers. One of the signature weapons available in Half-Life 2 is the Gravity Gun, which can be used to solve many puzzles and for self-defense. In the final stage of the game, it is upgraded and is also usable as an offensive weapon. Physics: Collision, ragdoll, water, and explosion physics play major roles in this series. Half-Life 2 uses the Source engine, which uses networked physics originating from the Havok 2 physics engine and heavily modified by Valve. The abilities of the Source engine's physics system are summarized by the Valve Developer Community: Machinery: Construct detailed machinery with functioning mechanisms, gears, belts, and pulleys. Vehicles: Non-player characters and, in a multiplayer game, other players can come along for a ride in cars that feature realistic suspensions with springs on each wheel and wheels that slip and skid depending on the surface material. Individually tunable parameters include horsepower, gearing, max speed, shift speed, tire material, tire friction, spring tension/dampening, and more. Leaning during acceleration, deceleration, and turning enhances the realism of the driving experience. Deformable Objects: Physics-based animation simulates flexible hair and clothing and blends ragdoll physics with predefined animations. Ropes and Cables: can be dynamically shaken or broken by level events. Constraint System Manager: Realistically define bone movement within a physical system using a series of constraints that enable a nearly limitless range of complex movements. Inverse Kinematics: ensures that characters’ limbs correctly react to environments, such as moving around obstructions and adjusting for uneven terrain. Characters: The Half-Life series is unique, especially in the first-person shooter genre, in that the protagonist is a physicist. He has no military training, except for some weapons training he acquired through Black Mesa, but is glorified as a liberator. Other characters in the game consistently comment on Gordon’s knowledge and skill. Influence: Although much of the physics presented by the Half-Life games is imaginary, Half-Life 2 has sparked interest in physics for many players and presents an overall positive image of the field, thanks in part to its physicist protagonist.	Headcrabs attack