diff --git a/src/systems/physics.js b/src/systems/physics.js
index c9b6cf7..0da8b9d 100644
--- a/src/systems/physics.js
+++ b/src/systems/physics.js
@@ -2,10 +2,12 @@ game.system.Physics = () => {
   const update = (elapsedTime) => {
     for (let id in game.entities) {
       const entity = game.entities[id];
-      if (entity.hasComponent("momentum")) {
+      if (entity.hasComponent("momentum") && entity.hasComponent("appearance")) {
         const {dx, dy} = entity.components.momentum;
         entity.components.position.x += dx * elapsedTime;
         entity.components.position.y += dy * elapsedTime;
+        entity.components.position.x = Math.max(0, Math.min(game.canvas.width - entity.components.appearance.width, entity.components.position.x));
+        entity.components.position.y = Math.max(0, Math.min(game.canvas.height - entity.components.appearance.height, entity.components.position.y));
       }
     }
   }