diff --git a/src/lib/fixed_length_list.hpp b/src/lib/fixed_length_list.hpp
new file mode 100644
index 0000000..96b0d52
--- /dev/null
+++ b/src/lib/fixed_length_list.hpp
@@ -0,0 +1,34 @@
+template <typename T, int max_length>
+struct fixed_length_list
+{
+  T values[max_length];
+  int length;
+
+  fixed_length_list()
+    : length(0)
+  {}
+
+  void add(const T& value)
+  {
+    for (int i = 0; i < length; i++) {
+      if (values[i] == value)
+        return;
+    }
+
+    values[length] = value;
+    length += 1;
+  }
+
+  void remove(const T& value)
+  {
+    for (int i = 0; i < length; i++) {
+      if (values[i] == value) {
+        for (int k = i + 1; k < length;) {
+          values[i++] = values[k++];
+        }
+        length -= 1;
+        break;
+      }
+    }
+  }
+};
diff --git a/src/physics/particle.cpp b/src/physics/particle.cpp
index 3b85cbb..0df3b09 100644
--- a/src/physics/particle.cpp
+++ b/src/physics/particle.cpp
@@ -18,6 +18,9 @@ namespace physics {
 
     // drag
     velocity *= damping;
+
+    // clear forces
+    force_accum = vec3(0, 0, 0);
   }
 
 }
diff --git a/src/physics/particle.hpp b/src/physics/particle.hpp
index 7732d8a..9d63ce3 100644
--- a/src/physics/particle.hpp
+++ b/src/physics/particle.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "math/float_types.hpp"
+#include "math/math.hpp"
 
 namespace physics {
 
@@ -14,6 +15,16 @@ namespace physics {
     vec3 force_accum;
 
     void integrate(float duration);
-  };
 
+    inline void add_force(const vec3& force)
+    {
+      force_accum += force;
+    }
+
+    inline float get_mass() const
+    {
+      float sqrt_mass = 1.0f / sqrt<float>(inverse_mass);
+      return sqrt_mass * sqrt_mass;
+    }
+  };
 }
diff --git a/src/physics/particle_force_generator.cpp b/src/physics/particle_force_generator.cpp
new file mode 100644
index 0000000..96b5ee4
--- /dev/null
+++ b/src/physics/particle_force_generator.cpp
@@ -0,0 +1,32 @@
+#include "physics/particle_force_generator.hpp"
+
+namespace physics {
+
+  void particle_force_registry::add(particle * particle, particle_force_generator * force_generator)
+  {
+    registrations.add({particle, force_generator});
+  }
+
+  void particle_force_registry::update_forces(float duration)
+  {
+    for (int i = 0; i < registrations.length; i++) {
+      const particle_force_entry& entry = registrations.values[i];
+      entry.force_generator->update_force(entry.particle, duration);
+    }
+  }
+
+  void particle_gravity::update_force(particle * particle, float duration)
+  {
+    particle->add_force(gravity * particle->get_mass());
+  }
+
+  void particle_drag::update_force(particle * particle, float duration)
+  {
+    vec3 force = particle->velocity;
+
+    float drag_coeff = magnitude(force);
+    drag_coeff = k1 * drag_coeff + k2 * drag_coeff * drag_coeff;
+
+    particle->add_force(normalize(force) * drag_coeff);
+  }
+}
diff --git a/src/physics/particle_force_generator.hpp b/src/physics/particle_force_generator.hpp
new file mode 100644
index 0000000..6f2124d
--- /dev/null
+++ b/src/physics/particle_force_generator.hpp
@@ -0,0 +1,47 @@
+#pragma once
+
+#include "physics/particle.hpp"
+#include "lib/fixed_length_list.hpp"
+
+namespace physics {
+
+  struct particle_force_generator
+  {
+    virtual void update_force(particle * particle, float duration) = 0;
+  };
+
+  struct particle_force_entry
+  {
+    physics::particle * particle;
+    particle_force_generator * force_generator;
+
+    bool operator==(const particle_force_entry&) const = default;
+  };
+
+  struct particle_force_registry
+  {
+    fixed_length_list<particle_force_entry, 64> registrations;
+
+    void add(particle * particle, particle_force_generator * force_generator);
+    void update_forces(float duration);
+  };
+
+  struct particle_gravity : particle_force_generator
+  {
+    vec3 gravity;
+
+    particle_gravity(const vec3& gravity)
+      : gravity(gravity)
+    {}
+
+    void update_force(particle * particle, float duration) override;
+  };
+
+  struct particle_drag : particle_force_generator
+  {
+    float k1;
+    float k2;
+
+    void update_force(particle * particle, float duration) override;
+  };
+}