diff --git a/Ball.cpp b/Ball.cpp
index 64d0bec..0ded1f3 100644
--- a/Ball.cpp
+++ b/Ball.cpp
@@ -1,7 +1,7 @@
 #include "Ball.hpp"
 #include <cmath>
 
-Ball::Ball(Point& _center, double _p, double _q) :
+Ball::Ball(Point& _center, double _v_x, double _p, double _q) :
     Center(_center),
     surface(_center, _p, _q),
     init_h(_center.z),
@@ -10,12 +10,14 @@ Ball::Ball(Point& _center, double _p, double _q) :
     A(_center.z),
     B(0),
     U(sqrt(2 * G_CTE * _center.z)),
-    T(0)
+    T(0),
+    v_x(_v_x)
 {
 }
 
-void Ball::_compute_pos() {
-    Center.x = A + B * crt_time - (G_CTE / 2) * crt_time * crt_time;
+void Ball::_compute_pos(double dt) {
+    Center.z = A + B * crt_time - (G_CTE / 2) * crt_time * crt_time;
+    Center.x += dt * v_x;
     surface.update_center_pos(Center);
 }
 
@@ -35,6 +37,10 @@ void Ball::_compute_U_n() {
     U *= 1/2;
 }
 
+void Ball::_compute_v_x() {
+    v_x *= 1/2;
+}
+
 void Ball::update_pos(double dt) {
     crt_time += dt;
     if (crt_time >= T) {
@@ -42,6 +48,7 @@ void Ball::update_pos(double dt) {
         _compute_U_n();
         _compute_A_n();
         _compute_B_n();
+        _compute_v_x();
     }
-    _compute_pos();
+    _compute_pos(dt);
 }
diff --git a/Ball.hpp b/Ball.hpp
index 5980f5d..383979d 100644
--- a/Ball.hpp
+++ b/Ball.hpp
@@ -3,7 +3,11 @@
  * A ball is the kind of real object, with a physical model, and the implicit
  * surface.
  * The physical model is strongly inspired from
- * http://www.physics-online.info/book1/chapt1/sect1a/problem1a-12/Problem1A-12.htm
+ * http://www.physics-online.info/book1/chapt1/sect1a/problem1a-12/Problem1A-12.htm,
+ * apart for the x-axis drift.
+ * For this one, the assumption done is that the speed is divided by two at
+ * each bounce, which is a reasonable assumption since it is the same for the
+ * vertical bounces.
  **/
 
 #include <cstddef>
@@ -19,12 +23,14 @@ class Ball {
         size_t bounce_number;
         double crt_time;
         double A, B, U, T; // Coefficients for the physical model.
-        void _compute_pos();
+        double v_x;
+        void _compute_pos(double dt);
+        void _compute_v_x();
         void _compute_A_n();
         void _compute_B_n();
         void _compute_U_n();
         void _compute_T_n();
     public:
-        Ball(Point& _center, double p, double q);
+        Ball(Point& _center, double _v_x, double p, double q);
         void update_pos(double dt);
 };