UIKit Gravity!

ViewController.h

@property (strong, nonatomic) UIDynamicAnimator *animator;
@property (strong, nonatomic) UIView *boxView;

ViewController.m

- (void)viewDidLoad
{
  [super viewDidLoad];
  
  self.boxView = [[UIView alloc] initWithFrame:CGRectMake(0, self.view.frame.size.width / 2, 50, 50)];
  
  self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
  
  UIGravityBehavior *gravity = [[UIGravityBehavior alloc] initWithItems:@[self.boxView]];
  // Determines which direction the gravity will pull the box!
  gravity.gravityDirection = CGVectorMake(0,1);
  
  [self.animator addBehavior:gravity];
  
  // now to pair it with a collision boundary!  
  UICollisionBehavior *viewBoundaries = [[UICollisionBehavior alloc] initWithItems:@[self.boxView]];
  viewBoundaries.translatesReferenceBoundsIntoBoundary = YES;
  [self.animator addBehavior:viewBoundaries];
  
  UIDynamicItemBehavior *dynamicBehavior = [[UIDynamicItemBehavior alloc] initWithItems:objs];
  dynamicBehavior.elasticity = 0.8;
  [self.animator addBehavior:dynamicBehavior];
}