From 1c2acd3320963884821d52530b24637abd88354f Mon Sep 17 00:00:00 2001
From: Nat Lasseter <Nat Lasseter nathan@bytemark.co.uk>
Date: Wed, 28 Jun 2017 19:55:45 +0100
Subject: Causally air resistance.

---
 particle.js | 6 ++++++
 sketch.js   | 4 +++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/particle.js b/particle.js
index 66f4f6e..c62dd47 100644
--- a/particle.js
+++ b/particle.js
@@ -9,6 +9,12 @@ function Particle(x, y, m, c) {
     this.acc.add(p5.Vector.div(force, this.mass));
   }
 
+  this.applyDrag = function(drag_coeff) {
+    var rev_vel = createVector().sub(this.vel);
+    var drag = rev_vel.mult(drag_coeff);
+    this.applyForce(drag);
+  }
+
   this.update = function() {
     this.vel.add(this.acc);
     this.pos.add(this.vel);
diff --git a/sketch.js b/sketch.js
index 3cce527..2c6b9d3 100644
--- a/sketch.js
+++ b/sketch.js
@@ -1,5 +1,6 @@
 var ps = [];
 var gravity;
+var air_resistance = 0.5;
 var wid = 800;
 var hei = 600;
 
@@ -14,6 +15,7 @@ function draw() {
 
   for(var i = ps.length - 1; i >= 0; i--) {
     ps[i].applyForce(gravity);
+    ps[i].applyDrag(air_resistance);
     ps[i].update();
 
     stroke(ps[i].colour);
@@ -30,7 +32,7 @@ function draw() {
           mouseX, mouseY, random(2, 8),
           color(random(0, 360), random(90, 100), random(50, 100))
         );
-    p.applyForce(createVector(random(-40, 40), random(-150, 0)));
+    p.applyForce(createVector(random(-200, 200), random(-400, 0)));
     ps.push(p);
   }
 }
-- 
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