Test Friction simulation

packages/newton/lib/src/friction.dart

@@ -17,7 +17,7 @@ class Friction extends Simulation {
         _v = velocity;
 
   double x(double time) =>
-      _x + _v + Math.pow(_drag, time) / _dragNaturalLog - _v / _dragNaturalLog;
+      _x + _v * Math.pow(_drag, time) / _dragNaturalLog - _v / _dragNaturalLog;
 
   double dx(double time) => _v * Math.pow(_drag, time);
 

packages/newton/test/newton_test.dart

@@ -8,4 +8,24 @@ import 'package:test/test.dart';
 
 import 'package:newton/newton.dart';
 
-void main() {}
+typedef bool SimulationTestHandler(int millis);
+
+void main() {
+  test('test_friction', () {
+    var friction = new Friction(0.3, 100.0, 400.0);
+
+    expect(friction.isDone(0.0), false);
+    expect(friction.x(0.0), 100);
+    expect(friction.dx(0.0), 400.0);
+
+    expect(friction.x(1.0) > 330 && friction.x(1.0) < 335, true);
+
+    expect(friction.dx(1.0), 120.0);
+    expect(friction.dx(2.0), 36.0);
+    expect(friction.dx(3.0), 10.8);
+    expect(friction.dx(4.0) < 3.5, true);
+
+    expect(friction.isDone(5.0), true);
+    expect(friction.x(5.0) > 431 && friction.x(5.0) < 432, true);
+  });
+}