example of transferring velocity of UIPanGestureRecoginizer to a UIView spring animation.

SPVMainViewController.m

#import "SPVMainViewController.h"

@implementation SPVMainViewController
{
    UIView *_weightView;
    UIPanGestureRecognizer *_recog;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    _weightView = [[UIView alloc] initWithFrame:CGRectMake(100.0f, 200.0f, 50.0f, 50.0f)];
    _weightView.backgroundColor = [UIColor redColor];
    [self.view addSubview:_weightView];

    _recog = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(_panRecogFired:)];
    _recog.minimumNumberOfTouches = 1;
    [_weightView addGestureRecognizer:_recog];
}

- (void)_panRecogFired:(id)sender
{
    UIPanGestureRecognizer *panRecog = (UIPanGestureRecognizer *)sender;
    CGPoint vel = [panRecog velocityInView:self.view];
    NSLog(@"vel: %@", NSStringFromCGPoint(vel));

    NSLog(@"state: %d", panRecog.state);

    UIView *recogView = [panRecog view];

    CGPoint translation = [panRecog translationInView:recogView];
    CGFloat curY = recogView.frame.origin.y;

    if (panRecog.state == UIGestureRecognizerStateChanged) {

        // drag view vertially
        CGRect frame = recogView.frame;
        frame.origin.y = curY + translation.y;
        recogView.frame = frame;
        [panRecog setTranslation:CGPointMake(0.0f, 0.0f) inView:recogView];

    } else if (panRecog.state == UIGestureRecognizerStateEnded) {

        // touch up, animate to top of self.view w/ spring
        NSLog(@"final vel: %@", NSStringFromCGPoint(vel));

        CGFloat finalX = recogView.frame.origin.x;
        CGFloat finalY = 50.0f;
        CGFloat curY = recogView.frame.origin.y;

        CGFloat distance = curY - finalY;

        CGFloat animationDuration = 2.0f;

        // normalize velocity as per docs
        // multiply by -1 in this case b/c final desitination y is less
        // than current y and recog's y velocity is negative when draggin up
        // (therefore also works when released when dragging down)
        CGFloat springVelocity = -1.0f * vel.y / distance;

        CGFloat springDampening = 0.5f;

        NSLog(@"dist: %f - spring vel: %f", distance, springVelocity);

        // for clean velocity transfer, use UIViewAnimationOptionCurveLinear
        [UIView animateWithDuration:animationDuration delay:0.0 usingSpringWithDamping:springDampening initialSpringVelocity:springVelocity options:UIViewAnimationOptionCurveLinear animations:^{

            CGRect frame = recogView.frame;
            frame.origin.x = finalX;
            frame.origin.y = finalY;
            recogView.frame = frame;

        } completion:^(BOOL finished) {
            NSLog(@"done animating");
        }];


    }
}

@end