Introduction: The Role of Physics in Games

Physics simulation has become a cornerstone of modern game development, enabling everything from realistic vehicle handling to dynamic destruction systems. Well-implemented physics can make game worlds feel alive and responsive, creating emergent gameplay moments that surprise and delight players.

However, implementing physics systems can be challenging. Strike the wrong balance, and you might end up with performance issues, unreliable gameplay, or mechanics that feel unnatural. In this article, we'll explore how to implement physics that enhance your game's experience while avoiding common pitfalls.

"Game physics don't need to be realistic—they need to be believable and serve the gameplay. Players expect things to behave consistently, not necessarily according to real-world physics."

— Erin Catto, Creator of Box2D physics engine

Understanding Game Physics Fundamentals

Real Physics vs. Game Physics

It's important to understand that game physics are not the same as real-world physics. Games use simplified approximations that prioritize:

  • Performance: Calculations need to complete within a tight frame budget
  • Stability: The simulation should never explode or behave erratically
  • Gameplay feel: Physics should serve the game's intended experience
  • Player expectations: Objects should behave in ways players intuitively understand

Physics Time Steps

Most physics engines use fixed time steps for simulation to ensure stability and determinism. This means physics calculations happen at regular intervals (e.g., 60 times per second) regardless of the game's frame rate. Understanding this distinction is crucial for implementing smooth physics that don't break when frame rates fluctuate.

Core Components of Physics Systems

Game physics systems typically include these fundamental components:

Collision Detection

Determining when and where objects intersect

Collision Response

Calculating how objects should react after colliding

Rigid Body Dynamics

Simulating how solid objects move and rotate

Constraints

Limiting how objects can move relative to each other

Choosing Your Approach: Physics Engines vs. Custom Solutions

Using Established Physics Engines

Most game developers use existing physics engines rather than building systems from scratch. Popular options include:

  • PhysX: NVIDIA's physics engine, integrated into Unreal Engine
  • Havok: Powerful commercial physics middleware used in many AAA games
  • Box2D: Open source 2D physics engine popular for mobile and indie games
  • Bullet Physics: Open source 3D physics library used in games and film
  • Unity Physics: Unity's built-in physics system based on PhysX

Pros of Using Established Engines

  • Battle-tested stability and performance
  • Rich feature sets out of the box
  • Community support and documentation
  • Continuous improvements and optimizations

Cons of Using Established Engines

  • Less control over internal algorithms
  • Potential overhead for simple use cases
  • May require licensing for commercial projects
  • Might not be optimized for your specific needs

When to Build Custom Physics

While most projects benefit from using established engines, there are cases where custom physics solutions make sense:

  • When you need a highly specialized behavior that existing engines don't support well
  • For extremely performance-constrained environments
  • When your physics needs are very simple (e.g., basic platformer movement)
  • When you want complete control over the simulation for deterministic gameplay

Developer Tip

Even when using a general-purpose physics engine, consider implementing custom solutions for specific gameplay elements that need tight control or unique behavior. For example, character controllers often use raycasting and specialized movement code rather than relying entirely on the physics engine.

Implementing Physics for Different Game Types

2D Platformers and Action Games

2D games often use simplified physics focused on precise control:

  • Character movement: Usually handled with custom code rather than pure physics simulation
  • Collision detection: Often simplified to axis-aligned bounding boxes (AABBs) or capsules
  • Environmental interaction: Typically uses raycasting to detect ground, walls, and ceilings
// Example of simplified 2D platformer physics in pseudocode
function updatePlayerPhysics(deltaTime) {
    // Apply gravity
    velocity.y += GRAVITY * deltaTime;
    
    // Check if player is on ground
    let isGrounded = checkGroundCollision(position, PLAYER_HEIGHT);
    
    // Handle jump input
    if (isGrounded && jumpPressed) {
        velocity.y = JUMP_FORCE;
    }
    
    // Apply horizontal movement
    velocity.x = horizontalInput * MOVE_SPEED;
    
    // Calculate new position
    let newPosition = position + velocity * deltaTime;
    
    // Check for collisions and adjust position
    newPosition = resolveCollisions(position, newPosition);
    
    // Update position
    position = newPosition;
}

Case Study: Celeste's Movement System

The platformer Celeste uses a custom physics system that prioritizes responsive controls and consistent behavior. Rather than simulating realistic physics, it implements carefully tuned movement equations that make jumping and air control feel precise and satisfying. The game uses grid-based collision detection with corner correction to prevent the player from getting caught on edges, resulting in fluid movement that never feels unfair.

3D Action and Adventure Games

3D games typically need more complex physics handling:

  • Character controllers: Often use capsule colliders with specialized movement code
  • Object interaction: May use full rigid body physics for items in the environment
  • Optimization: Usually employs physics layers and selective collision to manage performance

Common Pitfall

Don't rely on full physics simulation for player characters in most action games. This often leads to unpredictable movement and loss of control. Instead, use a kinematic character controller that can interact with the physics world but has its own movement logic.

Racing and Vehicle Simulation

Vehicle physics are among the most complex systems to implement well:

  • Wheel physics: Simulating suspension, friction, and tire deformation
  • Drivetrain: Modeling engines, transmissions, and differential systems
  • Aerodynamics: Calculating drag and downforce at different speeds

Most racing games use a balance of realistic simulation and game-friendly approximations. Even "simulation" racing games simplify certain aspects of vehicle physics to maintain playability and performance.

Performance Optimization Techniques

Physics simulation can be computationally expensive. Here are strategies to optimize performance:

Collision Optimization

  • Broad-phase collision detection: Using spatial partitioning (quadtrees, BVH, etc.) to quickly eliminate collision pairs
  • Simplified collision shapes: Using primitives like spheres and boxes instead of complex meshes
  • Collision layers and masks: Ensuring objects only check collisions with relevant entities
Collision Optimization

Visualization of broad-phase collision detection using spatial partitioning to reduce the number of potential collision checks.

Physics LOD (Level of Detail)

Just as graphics use LOD to reduce complexity at a distance, you can apply the same concept to physics:

  • Disabling physics for distant objects
  • Using simplified collision shapes for objects far from the player
  • Reducing simulation frequency for less important objects

Sleeping and Activation

Most physics engines support "sleeping" for objects that haven't moved for some time:

  • Objects at rest stop consuming CPU cycles until disturbed
  • Properly configured sleeping thresholds can dramatically improve performance
  • Waking should happen automatically when objects are hit or affected by forces

Performance Tip

In most cases, it's better to use many simple physics objects instead of a few complex ones. For example, a chain might be better simulated as multiple connected rigid bodies rather than a single complex soft body.

Physics-Based Gameplay Features

Beyond basic movement and collision, physics can enable many engaging gameplay features:

Destruction Systems

Dynamic destruction adds immersion and interactivity to game environments:

  • Fractured meshes: Pre-dividing objects into breakable pieces
  • Runtime fragmentation: Calculating break points when damage occurs
  • Performance management: Limiting fragment count and lifetime

Cloth and Soft Body Physics

Simulating deformable objects adds realism to characters and environments:

  • Cloth simulation: For flags, clothing, and fabric
  • Soft bodies: For organic objects that squash and stretch
  • Hair and fur: Often using specialized strand-based physics

Implementation Note

Modern graphics APIs like DirectX 12 and Vulkan, along with compute shaders, have made GPU-accelerated physics more accessible. This allows for more complex simulations of cloth, particles, and fluids without overloading the CPU.

Ragdoll Physics

Ragdoll systems replace pre-animated death sequences with dynamic physical reactions:

  • Joint hierarchies: Connecting rigid bodies with constraints to form a skeleton
  • Muscle simulation: Adding tension to joints for more realistic behavior
  • Hybrid approaches: Blending between animation and physics for more control

Case Study: Half-Life 2's Physics Gameplay

Half-Life 2 revolutionized physics-based gameplay with its Gravity Gun, which allowed players to manipulate objects in the environment as both tools and weapons. The game's physics puzzles and combat scenarios demonstrated how physics could be central to gameplay rather than just visual window dressing. By ensuring physics objects had consistent, predictable behavior and appropriate weight, the developers created intuitive mechanics that players could experiment with creatively.

Testing and Debugging Physics

Physics systems require thorough testing due to their complexity and potential for unexpected behavior:

Visualization Tools

Implement debugging visualizations to understand what's happening in your physics system:

  • Collision shape visualization
  • Contact point and normal displays
  • Force and velocity vector visualization
  • Physics property inspectors
Physics Debugging

Example of physics debugging visualization showing collision shapes, contact points, and force vectors.

Common Issues and Solutions

Be prepared to address these common physics problems:

Tunneling

Problem: Fast-moving objects pass through thin colliders

Solution: Use continuous collision detection (CCD) for fast objects

Jittering

Problem: Objects vibrate when stacked or at rest

Solution: Adjust solver iteration count, enable sleeping, use fixed time steps

Unstable Stacking

Problem: Piles of objects collapse unrealistically

Solution: Tune friction values, increase solver accuracy, consider static colliders

Performance Spikes

Problem: Physics calculations cause frame rate drops

Solution: Profile to identify bottlenecks, limit active physics objects, simplify collision shapes

Conclusion: Physics that Serve Your Game

Implementing effective physics in games is about finding the right balance between realism, performance, and gameplay. The most successful physics implementations are those that:

  • Support and enhance the core gameplay rather than distracting from it
  • Perform well across your target hardware specifications
  • Feel consistent and predictable to players
  • Create opportunities for emergent gameplay and player creativity

Remember that "realistic" physics aren't always the goal—"satisfying" physics that match player expectations and create fun experiences should be your priority. Start with established physics engines and libraries when possible, and don't be afraid to implement custom solutions for specific gameplay needs.

In future articles, we'll explore specific physics implementations for different game genres and dive deeper into advanced topics like fluid simulation, vehicle physics, and GPU-accelerated physics systems.

David Chen

About the Author

David Chen

David is a technical director specializing in game physics and simulation. He has implemented physics systems for racing games, action titles, and VR experiences, with a focus on creating systems that balance realism with gameplay requirements.