diff --git a/debug/tick.ts b/debug/tick.ts
index d0ddd09..3e0c44b 100644
--- a/debug/tick.ts
+++ b/debug/tick.ts
@@ -43,6 +43,8 @@ export function OrbitDemo() {
   cx.scale(1 / 2, 1 / 2);
   cx.translate(512, 512);
 
+  const G = 8000;
+  const PHYSICS_TICKS = 30;
   const masses: {
     x: number;
     y: number;
@@ -51,22 +53,47 @@ export function OrbitDemo() {
     r: number;
     m: number;
     c: string;
-  }[] = [
-    { x: 0, y: 0, vx: 0, vy: 0, r: 10, m: 10000, c: "yellow" },
-    { x: -200, y: 200, vx: 100, vy: 100, r: 4, m: 4, c: "red" },
-    { x: -450, y: 0, vx: 0, vy: 100, r: 5, m: 5, c: "blue" },
-  ];
+  }[] = [];
 
-  const tickSource = tick(16);
+  const tickSource = tick(PHYSICS_TICKS);
   let cancel: Cancel;
 
   const begin = () => {
     // reset / init
     cancel?.();
+    masses.length = 0;
+    masses.push(
+      { x: 0, y: 0, vx: -1, vy: -2, r: 10, m: 1000, c: "yellow" },
+      { x: -150, y: 150, vx: 130, vy: 130, r: 9, m: 10, c: "red" },
+      { x: -450, y: 0, vx: 0, vy: 120, r: 4, m: 5, c: "#0f8" }
+    );
     // subscribe to source, stash cancellation func
     cancel = tickSource((tick) => {
       switch (tick[0]) {
         case "physics":
+          // apply velocities
+          masses.forEach((mass) => {
+            mass.x += mass.vx * (1 / PHYSICS_TICKS);
+            mass.y += mass.vy * (1 / PHYSICS_TICKS);
+          });
+          // apply accelerations
+          masses.forEach((mass) => {
+            masses.forEach((other) => {
+              if (mass != other) {
+                const dx = other.x - mass.x;
+                const dy = other.y - mass.y;
+
+                const rSquared = dx ** 2 + dy ** 2;
+                const f = (G * other.m) / rSquared;
+                const d = Math.sqrt(rSquared);
+                const fx = (f * dx) / d;
+                const fy = (f * dy) / d;
+
+                mass.vx += fx / PHYSICS_TICKS;
+                mass.vy += fy / PHYSICS_TICKS;
+              }
+            });
+          });
           break;
         case "render":
           const [, dt] = tick;