diff --git a/pointer.js b/pointer.js
index d1779fc..ac1e72f 100644
--- a/pointer.js
+++ b/pointer.js
@@ -46,6 +46,7 @@ export class Pointer {
     });
 
     el.addEventListener('pointerleave', e => {
+      console.log('pointerleave', {x: e.clientX, y: e.clientY});
       this.handlePointerUp({x: e.clientX, y: e.clientY});
     });
 
@@ -55,6 +56,13 @@ export class Pointer {
       const {x, y} = this.sim.screenToSim(e.clientX, e.clientY);
       this.sim.scheduleZoom({x, y}, factor);
     });
+
+    el.addEventListener('focus', () => {
+      console.log('window focus');
+    });
+    el.addEventListener('blur', () => {
+      console.log('window blur');
+    });
   }
 
   handlePointerDown({x: clientX, y: clientY}) {
diff --git a/system.js b/system.js
index df5794b..446381c 100644
--- a/system.js
+++ b/system.js
@@ -1,5 +1,10 @@
 import {OBJECT_HISTORY_SIZE} from './config.js';
 import {MassObject} from './object.js';
+import {
+  add, cross, direction, div, magnitude, mult,
+  square,
+  sub, weightedAvg, zero
+} from './vector.js';
 
 export class System {
   objects = [];
@@ -31,41 +36,31 @@ export class System {
     this.doneCreatingObject();
     this.deselect();
     // Convert pointer velocity to simulation scale
-    const pointer = {...this.sim.pointer.latestVelocity};
-    obj.velocity.x = pointer.x / this.sim.display.scale;
-    obj.velocity.y = pointer.y / this.sim.display.scale;
+    obj.velocity = div(this.sim.pointer.latestVelocity, this.sim.display.scale);
 
     // Including time scale - if time is slow, our motion is relatively faster
     if (this.sim.getOption('compensate.timeScale')) {
-      obj.velocity.x /= this.sim.timeScale;
-      obj.velocity.y /= this.sim.timeScale;
+      obj.velocity = div(obj.velocity, this.sim.timeScale);
     }
 
     if (this.sim.panning?.velocity) {
-      obj.velocity.x += this.sim.panning.velocity.x;
-      obj.velocity.y += this.sim.panning.velocity.y;
+      obj.velocity = add(obj.velocity, this.sim.panning.velocity);
     }
   }
 
-  handlePointerMove({x, y}) {
+  handlePointerMove(r) {
     // If the cursor moves while creating an object, or while an object is selected,
     // update the position using the pointer motion but the velocity using the pointer velocity
     const obj = this.getSelectedOrCreating();
     if (obj === undefined) return;
     const start = this.selectedObjectStart;
-    obj.position.x = start.x + (x - start.pointer.x);
-    obj.position.y = start.y + (y - start.pointer.y);
-    obj.velocity.x = 0;
-    obj.velocity.y = 0;
+    obj.position = add(start, sub(r, start.pointer));
+    obj.velocity = zero;
   }
 
-  // elapsedTime is given in milliseconds
   frame(elapsedTime) {
     // If we're creating an object, increment its mass
     // with the mass creation rate accelerating over time
-
-    // Scaling this parameter because of millisecond conversion
-
     if (this.creatingObject !== undefined) {
       const obj = this.objects[this.creatingObject];
       let massCreationRate = this.sim.getOption('param.massCreationRate');
@@ -90,10 +85,13 @@ export class System {
           return;
         }
 
-        obj.position.x += elapsedTime *
-          (obj.velocity.x + 1 / 2 * obj.currentAcceleration.x * elapsedTime);
-        obj.position.y += elapsedTime *
-          (obj.velocity.y + 1 / 2 * obj.currentAcceleration.y * elapsedTime);
+        obj.position = add(obj.position, mult(
+          elapsedTime,
+          add(
+            obj.velocity,
+            mult(obj.currentAcceleration, elapsedTime / 2)
+          ),
+        ));
       });
 
       // Collisions
@@ -125,12 +123,10 @@ export class System {
             // Set position = center of mass 
             // Set velocity = total momentum / total mass
             // Combine forces
-            // Sdd masses
-            // Sverage color
-            S.position.x = (S.position.x * S.mass + T.position.x * T.mass) / (S.mass + T.mass);
-            S.position.y = (S.position.y * S.mass + T.position.y * T.mass) / (S.mass + T.mass);
-            S.velocity.x = (S.velocity.x * S.mass + T.velocity.x * T.mass) / (S.mass + T.mass);
-            S.velocity.y = (S.velocity.y * S.mass + T.velocity.y * T.mass) / (S.mass + T.mass);
+            // Add masses
+            // Average color
+            S.position = weightedAvg(S.position, S.mass, T.position, T.mass);
+            S.velocity = weightedAvg(S.velocity, S.mass, T.velocity, T.mass);
             S.forces.push(...T.forces);
             S.mass += T.mass;
             S.color = {
@@ -150,12 +146,8 @@ export class System {
       // Predict velocities
       this.forEachObject(obj => {
         const acceleration = {...obj.acceleration};
-        obj.acceleration = {
-          x: (obj.currentAcceleration.x + acceleration.x) / 2,
-          y: (obj.currentAcceleration.y + acceleration.y) / 2,
-        };
-        obj.velocity.x += obj.acceleration.x * elapsedTime;
-        obj.velocity.y += obj.acceleration.y * elapsedTime;
+        obj.acceleration = div(add(obj.currentAcceleration, acceleration), 2);
+        obj.velocity = add(obj.velocity, mult(obj.acceleration, elapsedTime));
 
         // Append to object history
         obj.history.push({position: {...obj.position}});
@@ -177,18 +169,18 @@ export class System {
     if (this.sim.getOption('debug.objectsInfo')) {
       const aliveOnly = this.sim.getOption('debug.aliveObjects');
       this.forEachObject((obj, i) => {
-        const speed = Math.pow(obj.velocity.x ** 2 + obj.velocity.y ** 2, 1 / 2);
-        const accel = Math.pow(obj.acceleration.x ** 2 + obj.acceleration.y ** 2, 1 / 2);
+        const speed = magnitude(obj.velocity);
+        const accel = magnitude(obj.acceleration);
         // Invert y so that the angle is counterclockwise from x-axis
-        const direction = Math.atan2(-obj.velocity.y, obj.velocity.x) * 180 / Math.PI;
-        const accelDir = Math.atan2(-obj.acceleration.y, obj.acceleration.x) * 180 / Math.PI;
+        const velocityDir = direction(obj.velocity);
+        const accelDir = direction(obj.acceleration);
         const {r, g, b} = obj.color;
         this.sim.info[`Object ${i}`] = [
           `<span style="background-color: rgb(${r},${g},${b});">&nbsp;&nbsp;</span>`,
           `${obj.position.x.toPrecision(4)}, `,
           `${obj.position.y.toPrecision(4)}, `,
           `${obj.mass.toPrecision(4)} kg, `,
-          `${speed.toPrecision(2)} m/s, ${direction.toPrecision(2)}°`,
+          `${speed.toPrecision(2)} m/s, ${velocityDir.toPrecision(2)}°`,
           `${accel.toPrecision(2)} m/s<sup>2</sup>, ${accelDir.toPrecision(2)}°`,
           `Alive: ${obj.alive}`,
         ];
@@ -307,7 +299,7 @@ export class System {
     this.selectedObjectStart = undefined;
     this.forEachObject((obj, i) => {
       // If distance to object is less than object's radius, we are touching the object
-      const dist = Math.pow((obj.position.x - x) ** 2 + (obj.position.y - y) ** 2, 1 / 2);
+      const dist = magnitude(sub(obj.position, {x, y}));
       if (dist <= obj.radius) {
         idx = i;
         return null;
@@ -352,13 +344,13 @@ export class System {
     });
     this.forEachObject((A, i) => {
       this.forEachObject(B => {
-        const dx = (B.position.x - A.position.x);
-        const dy = (B.position.y - A.position.y);
-        const dSquared = dx ** 2 + dy ** 2;
+        const r = sub(B.position, A.position);
+        const dSquared = square(r);
         const d = Math.sqrt(dSquared);
         const F = gravity * A.mass * B.mass / dSquared;
-        const Fx = F * dx / d;
-        const Fy = F * dy / d;
+        const Fx = F * r.x / d;
+        const Fy = F * r.y / d;
+        // Equal and opposite forces
         A.forces.push({x: Fx, y: Fy});
         B.forces.push({x: -Fx, y: -Fy});
       }, {alive: true, startWith: i + 1});
@@ -375,26 +367,19 @@ export class System {
       this.reduce((acc, obj) => ({
         count: acc.count + 1,
         totalMass: acc.totalMass + obj.mass,
-        totalMassLocation: {
-          x: acc.totalMassLocation.x + obj.position.x * obj.mass,
-          y: acc.totalMassLocation.y + obj.position.y * obj.mass,
-        },
+        totalMassLocation: add(acc.totalMassLocation, 
+          mult(obj.position, obj.mass)),
       }), {
         totalMassLocation: {x: 0, y: 0},
         totalMass: 0,
         count: 0,
       });
 
-    const centerOfMass = count ? {
-      x: totalMassLocation.x / totalMass,
-      y: totalMassLocation.y / totalMass,
-    } : {x: 0, y: 0};
+    const centerOfMass = count ? div(totalMassLocation, totalMass) : zero;
 
     // Determine average momentum
-    const netMomentum = this.reduce((acc, obj) => ({
-      x: acc.x + obj.mass * obj.velocity.x,
-      y: acc.y + obj.mass * obj.velocity.y,
-    }), {x: 0, y: 0});
+    const netMomentum = this.reduce((acc, obj) => 
+      add(acc, mult(obj.velocity, obj.mass)), zero);
 
     return {totalMass, count, totalMassLocation, centerOfMass, netMomentum};
   }
@@ -408,12 +393,8 @@ export class System {
       // Angular momentum for each object is m * s / d
       // where d is the distance of the object from the global center of mass
       // and s is the magnitude of the cross product of v and r
-      const r = {
-        x: obj.position.x - centerOfMass.x,
-        y: obj.position.y - centerOfMass.y,
-      };
-      const v = obj.velocity;
-      const s = v.x * r.y - v.y * r.x;
+      const r = sub(obj.position, centerOfMass);
+      const s = cross(obj.velocity, r);
       const d = Math.sqrt(r.x ** 2 + r.y ** 2);
       return acc + obj.mass * s / d;
     }, 0);
diff --git a/vector.js b/vector.js
new file mode 100644
index 0000000..1ed2934
--- /dev/null
+++ b/vector.js
@@ -0,0 +1,46 @@
+export const dot = (a, b) => a.x * b.x + a.y * b.y;
+export const cross = (a, b) => a.x * b.y - a.y * b.x;
+export const add = (a, b) => ({x: a.x + b.x, y: a.y + b.y});
+export const sub = (a, b) => ({x: a.x - b.x, y: a.y - b.y});
+export const square = ({x, y}) => x ** 2 + y ** 2;
+export const magnitude = ({x, y}) => Math.sqrt(square({x, y}));
+export const mult = (v, m) => {
+  if (v.x !== undefined) return {x: v.x * m, y: v.y * m};
+  else return {x: m.x * v, y: m.y * v};
+};
+export const div = (v, m) => ({x: v.x / m, y: v.y / m});
+export const zero = {x: 0, y: 0};
+export const weightedAvg = (items) => {
+  let res = zero;
+  let W = 0;
+  for (const [v, w] of items) {
+    res = add(res, mult(v, w));
+    W += w;
+  }
+  return div(res, W);
+};
+
+// Angle is given counterclockwise, assuming screen coordinates
+export const direction = ({x, y}) => Math.atan2(-y, x) * 180 / Math.PI;
+
+export class Vector {
+  x = undefined;
+  y = undefined;
+
+  constructor({x, y}) {
+    this.x = x;
+    this.y = y;
+  }
+
+  dot(v) {return dot(this, v);}
+  cross(v) {return cross(this, v);}
+  add(v) {return new Vector(add(this, v));}
+  sub(v) {return new Vector(sub(this, v));}
+  magnitude() {return magnitude(this);}
+  mult(m) {return new Vector(mult(this, m));}
+  div(m) {return new Vector(div(this, m));}
+
+  toJSON() {
+    return {x: this.x, y: this.y};
+  }
+}