Improve Sky's Scrollable physics

sky/framework/animation/mechanics.dart

Index: sky/framework/animation/mechanics.dart
diff --git a/sky/framework/animation/mechanics.dart b/sky/framework/animation/mechanics.dart
index 7ed6da19f7e58f97abbc842522ec646d1cb729c6..d862d75c8d38a3c08660b20d33ca5e0a16c56438 100644
--- a/sky/framework/animation/mechanics.dart
+++ b/sky/framework/animation/mechanics.dart
@@ -5,7 +5,7 @@
 import 'dart:math' as math;
 
 const double kGravity = -0.980;
-const double _kMinVelocity = 0.1;
+const double _kMinVelocity = 0.01;
 
 abstract class System {
   void update(double deltaT);
@@ -69,6 +69,23 @@ class ParticleInBox extends System {
   }
 }
 
+class ParticleInBoxWithFriction extends ParticleInBox {
+  final double friction;
+  final double _sign;
+
+  ParticleInBoxWithFriction({Particle particle, Box box, this.friction})
+      : super(particle: particle, box: box),
+        _sign = particle.velocity.sign;
+
+  void update(double deltaT) {
+    double force = -_sign * friction;
+    particle.applyImpluse(force * deltaT);
+    if (particle.velocity.sign != _sign)
+      particle.velocity = 0.0;
+    super.update(deltaT);
+  }
+}
+
 class Spring {
   final double k;
   double displacement;