UIGravityBehavior无效

我正在努力学习UIKit Dynamics.
我创建了一个盒子视图并在其上添加了UIGravityBehavior,但是当我运行Project时没有任何反应!

Viewcontroller.m

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    UIView* box = [[UIView alloc]initWithFrame:CGRectMake(100, 50, 100, 100)];
    box.backgroundColor = [UIColor blackColor];
    [self.view addSubview:box];
    UIGravityBehavior* gravityBehaviour = [[UIGravityBehavior alloc]init];
    [gravityBehaviour addItem:box];
    UIDynamicAnimator* myAnimator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];
    [myAnimator addBehavior:gravityBehaviour];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

这是我的视图controller.m文件.
程序有什么问题？