Unity 2D Physics

Rigidbody2D body types

Body Type	Behaviour
Dynamic	Moves under force/gravity/velocity. Most gameplay objects.
Kinematic	Moves only via `MovePosition`/`MoveRotation`. No automatic forces. Use for player controllers when you handle motion manually.
Static	Doesn't move. Optimised. Use for terrain, tilemaps.

[RequireComponent(typeof(Rigidbody2D))]
public class Push : MonoBehaviour {
    private Rigidbody2D _rb;
    void Awake() {
        _rb = GetComponent<Rigidbody2D>();
        _rb.interpolation = RigidbodyInterpolation2D.Interpolate;
        _rb.collisionDetectionMode = CollisionDetectionMode2D.Continuous;  // critical for fast movers
    }
    public void Push2D(Vector2 dir) => _rb.AddForce(dir * 10f, ForceMode2D.Impulse);
}

Collider2D shapes

BoxCollider2D, CircleCollider2D, CapsuleCollider2D, EdgeCollider2D (open polyline), PolygonCollider2D (closed shape). Stick to primitives where possible — polygons with many vertices are expensive.

CompositeCollider2D merges multiple child colliders into a single optimized shape (standard for tilemap collision).

Joints

Joint	Use
Hinge2D	Doors, windmills, ragdoll limbs
Distance2D	Two bodies at fixed distance — chains, ropes (with multiple links)
Spring2D	Bouncy connections — suspension, jelly
Slider2D	Constrained linear motion — drawers, pistons
Fixed2D	Glue bodies together until break force
Wheel2D	Vehicle wheels (suspension + motor)
Target2D	Move toward a target with damping (mouse drag)
Friction2D	Dampen relative motion (handy on conveyor return paths)
Relative2D	Anchor at a relative offset

Effectors

Auto-applied behaviours on a collider:

Effector	Effect
PlatformEffector2D	One-way platforms, flip-through detection
AreaEffector2D	Constant force inside the area (wind, water current)
PointEffector2D	Attract/repel from a point
SurfaceEffector2D	Conveyor belt — moves objects in contact
BuoyancyEffector2D	Water buoyancy + drag

Tag the collider as Used By Effector and add the effector component on the same GameObject.

Layer collision matrix

Edit > Project Settings > Physics 2D > Layer Collision Matrix. Cull pairs (Player/Player, EnemyProjectile/Enemy) — both correctness and perf.

Contact filters & queries

private static readonly Collider2D[] HitsBuffer = new Collider2D[16];
private ContactFilter2D _filter;

void Awake() {
    _filter = new ContactFilter2D {
        useLayerMask = true,
        layerMask = LayerMask.GetMask("Enemy"),
        useTriggers = false,
    };
}

void Hit() {
    int n = Physics2D.OverlapCircle(transform.position, 0.5f, _filter, HitsBuffer);
    for (int i = 0; i < n; i++) HitsBuffer[i].GetComponent<IDamageable>()?.Damage(10);
}

Allocation-free OverlapCircle/Box/Raycast overloads avoid GC pressure in hot paths.

2D physics gotchas

Z position affects rendering but NOT 2D physics (everything is at z=0 collision-wise).
Mass matters for collision response; tweak via Rigidbody2D Mass (or Auto Mass = ON for density-based).
Continuous Collision Detection — required for high-speed objects (bullets) to avoid tunneling.
Simulation Speed: 50Hz default (FixedUpdate). Higher → smoother + heavier; 30Hz acceptable for casual.
Z-axis Job — physics 2D doesn't run jobs by default; enable in Project Settings → Physics 2D > Use Multithreaded Simulation.

Anti-patterns

Anti-pattern	Fix
`transform.position` for Rigidbody2D objects	`MovePosition` / set `linearVelocity` in FixedUpdate
Polygon collider with hundreds of points	Decompose into primitives or composite
Discrete collision on bullets	Continuous CCD
Many per-frame `OverlapCircle` allocations	Use NonAlloc / ContactFilter2D overloads
Joints with default break force = infinity for breakable links	Set Break Force / Break Torque
Effector without `Used By Effector` flag	Tick `Used By Effector` on the collider

Production checklist

Layer collision matrix culled
Static colliders + composite for tilemap-like geometry
Continuous detection on fast bodies
Allocation-free physics queries in hot paths
Joints have realistic break forces
Physics 2D substep sufficient for target framerate

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