diff --git a/class_materials/exercises/particle_update_position.ipynb b/class_materials/exercises/particle_update_position.ipynb
index b3d829a..6baba13 100644
--- a/class_materials/exercises/particle_update_position.ipynb
+++ b/class_materials/exercises/particle_update_position.ipynb
@@ -35,51 +35,65 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 2,
+   "execution_count": 6,
    "metadata": {
     "pycharm": {
      "name": "#%%\n"
     }
    },
-   "outputs": [
-    {
-     "name": "stdout",
-     "output_type": "stream",
-     "text": [
-      "1.6800000000000002\n"
-     ]
-    }
-   ],
+   "outputs": [],
    "source": [
     "class Particle:\n",
-    "    def __init__(self, mass, velocity):\n",
+    "    def __init__(self, mass=1, velocity=0., position=0.):\n",
     "        self.mass = mass\n",
     "        self.velocity = velocity\n",
+    "        self.position = position\n",
     "\n",
     "    def momentum(self):\n",
     "        return self.mass * self.velocity\n",
     "\n",
-    "particle = Particle(mass=2.1, velocity=0.8)\n",
-    "print(particle.momentum())"
+    "    def update_position(self, dt):\n",
+    "        new_position = self.position + dt * self.velocity\n",
+    "        self.position = new_position\n",
+    "        return"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 3,
+   "execution_count": 7,
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "8.28\n"
+      "1.6800000000000002\n",
+      "0.0\n"
      ]
     }
    ],
    "source": [
-    "position = 8.2\n",
-    "new_position = update_position(particle.velocity, position, dt=0.1)\n",
-    "print(new_position)"
+    "particle = Particle(mass=2.1, velocity=0.8, position=0.)\n",
+    "print(particle.momentum())\n",
+    "print(particle.position)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 8,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "0.08000000000000002\n"
+     ]
+    }
+   ],
+   "source": [
+    "particle.update_position(dt=0.1)\n",
+    "print(particle.position)"
    ]
   },
   {
@@ -118,7 +132,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.10.11"
+   "version": "3.13.6"
   },
   "toc": {
    "nav_menu": {
@@ -142,5 +156,5 @@
   }
  },
  "nbformat": 4,
- "nbformat_minor": 2
+ "nbformat_minor": 4
 }