title: How to write a bouncing ball from scratch without cocos2d
body: <p>It is so easy to write some code to achieve the bouncing ball based on the open source project cocos2d,<a href="https://stackoverflow.com/questions/9343988/ios-cocos2d-simple-colission-bounce-do-i-need-box2d-chipmunk">here</a> is the question. </p>
  
  <p>So without cocos2d, how to implement this with the simplest code. </p>