code: function(impulse, pos){
              pos = pos||{x:0,y:0};
              this.body.applyImpulse(cp.v(impulse.x, impulse.y), cp.v(pos.x, pos.y));
          }
docs: 施加冲量
  @memberOf PhysicsViewMixin
  @param  {Object} impulse 冲量，格式：{x:0, y:0}
  @param  {Object} pos 施力位置离重心相对偏移量，默认0，格式：{x:0, y:0}