Rename force_creator_t to force_applier_t, expand on explanation of force applier.

include/scene.h

@@ -14,9 +14,12 @@ typedef struct scene scene_t;
 /**
  * A function which adds some forces or impulses to bodies,
  * e.g. from collisions, gravity, or spring forces.
- * Takes in an auxiliary value that can store parameters or state.
+ * The force will be applied by the scene at every tick to associated bodies.
+ * 
+ * Parameter is an auxiliary value that can store parameters or state needed 
+ * in function.
  */
-typedef void (*force_creator_t)(void *aux);
+typedef void (*force_applier_t)(void *aux);
 
 /**
  * Allocates memory for an empty scene.
@@ -71,16 +74,17 @@ void scene_add_body(scene_t *scene, body_t *body);
 void scene_remove_body(scene_t *scene, size_t index);
 
 /**
- * Adds a force creator to a scene,
- * to be invoked every time scene_tick() is called.
- * The auxiliary value is passed to the force creator each time it is called.
+ * Adds a force applier (which is invoked every time scene_tick() is called)
+ * to a scene.
+ * The auxiliary value passed into this function will be used by the force 
+ * applier each time it is called.
  *
  * @param scene a pointer to a scene returned from scene_init()
  * @param forcer a force creator function
  * @param aux an auxiliary value to pass to forcer when it is called
  * @param freer if non-NULL, a function to call in order to free aux
  */
-void scene_add_force_creator(scene_t *scene, force_creator_t forcer, void *aux,
+void scene_add_force_applier(scene_t *scene, force_applier_t forcer, void *aux,
                              free_func_t freer);
 
 /**