wave5-physics-velocity-force-application

Physics Velocity and Force Application

Intent

Apply forces and impulses to Unity Physics dynamic bodies by writing PhysicsVelocity from systems scheduled in FixedStepSimulationSystemGroup before PhysicsSystemGroup.

Use When

• Continuous forces (gravity well, wind, magnetic attraction) on dynamic rigid bodies
• Setting initial velocity on spawned physics bodies
• Impulses (explosions, projectile hits) on dynamic bodies

Avoid When

• Moving kinematic bodies — write LocalTransform directly; PhysicsVelocity is ignored for kinematics
• Applying forces in SimulationSystemGroup (variable rate) — always use FixedStepSimulationSystemGroup

Senior Pattern

[UpdateInGroup(typeof(FixedStepSimulationSystemGroup))]
[UpdateBefore(typeof(PhysicsSystemGroup))]
[BurstCompile]
public partial struct ForceSystem : ISystem
{
    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        state.Dependency = new ApplyForceJob
        {
            DeltaTime = SystemAPI.Time.fixedDeltaTime,
            WellPosition = SystemAPI.GetSingleton<GravityWell>().Position
        }.ScheduleParallel(state.Dependency);
    }
}

[BurstCompile]
public partial struct ApplyForceJob : IJobEntity
{
    public float DeltaTime;
    public float3 WellPosition;

    [BurstCompile]
    public void Execute(ref PhysicsVelocity velocity, in PhysicsMass mass, in LocalTransform transform)
    {
        // Guard: mass.InverseMass == 0 for static/infinite-mass bodies
        if (mass.InverseMass == 0f) return;

        float3 dir = math.normalize(WellPosition - transform.Position);
        velocity.Linear += dir * (1f / mass.InverseMass) * DeltaTime;
    }
}

// One-shot initial velocity via ISystemStartStop:
public void OnStartRunning(ref SystemState state)
{
    foreach (var (initial, vel) in SystemAPI.Query<RefRO<InitialVelocity>, RefRW<PhysicsVelocity>>())
    {
        vel.ValueRW.Linear = initial.ValueRO.Linear;
        vel.ValueRW.Angular = initial.ValueRO.Angular;
    }
}