code: function Physics(width, height, ballRadius) {
    this._height = height;
    this._width = width;
    this._ballRadius = ballRadius || 0.2;
    this._world = null;
    this._ballScored = function () {
    };
    this._paddleFixtures = {};
    this._init();
  }
docs: Initialize physics environment
  @param width field width
  @param height field height
  @param ballRadius ball game radius
  @constructor