+ model->objects[i].force.y = gravity * model->objects[i].mass;
+
+ /* Friction */
+ v.x = model->objects[i].position.x - model->objects[i].previous_position.x;
+ v.y = model->objects[i].position.y - model->objects[i].previous_position.y;
+ model->objects[i].force.x -= v.x * friction;
+ model->objects[i].force.y -= v.y * friction;