Change force creator references to force applier. Expand on difference between...

Change force creator references to force applier. Expand on difference between create_force function and force_applier_t.

include/forces.h

@@ -4,9 +4,11 @@
 #include "scene.h"
 
 /**
- * Adds a force creator to a scene that applies gravity between two bodies.
- * The force creator will be called each tick
- * to compute the Newtonian gravitational force between the bodies.
+ * Calling this function will add a specialized force applier to a scene.
+ * The force applier will apply gravity between two bodies in each tick.
+ * 
+ * The force applier will compute the Newtonian gravitational force between the
+ * bodies.
  * See https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation#Vector_form.
  * The force should not be applied when the bodies are very close,
  * because its magnitude blows up as the distance between the bodies goes to 0.
@@ -19,9 +21,11 @@
 void create_newtonian_gravity(scene_t *scene, double G, body_t *body1, body_t *body2);
 
 /**
- * Adds a force creator to a scene that acts like a spring between two bodies.
- * The force creator will be called each tick
- * to compute the Hooke's-Law spring force between the bodies.
+ * Calling this function will add a specialized force applier to a scene.
+ * The force applier will apply the spring force between two bodies in each tick.
+ * 
+ * The force applier will compute the Hooke's-Law spring force between the 
+ * bodies.
  * See https://en.wikipedia.org/wiki/Hooke%27s_law.
  *
  * @param scene the scene containing the bodies
@@ -32,9 +36,11 @@ void create_newtonian_gravity(scene_t *scene, double G, body_t *body1, body_t *b
 void create_spring(scene_t *scene, double k, body_t *body1, body_t *body2);
 
 /**
- * Adds a force creator to a scene that applies a drag force on a body.
- * The force creator will be called each tick
- * to compute the drag force on the body proportional to its velocity.
+ * Calling this function will add a specialized force applier to a scene.
+ * The force applier will apply a drag force to a body in each tick.
+ * 
+ * The force applier will compute the drag force on the body proportional to 
+ * its velocity.
  * The force points opposite the body's velocity.
  *
  * @param scene the scene containing the bodies