I'm creating a basic "game" in iOS 4.1. The premise is simple, there is a green rectangle ("disk") that moves/bounces around the screen, and red rectangle ("bump") that is stationary. The user can move the red "bump" by touching another coordinate on the screen, but that's irrelevant to this question.

Each rectangle is a UIImageView (I will replace them with some kind of image/icon once I get the mechanics down). I've gotten as far as detecting when the rectangles collide, and I'm able to reverse the direction of the green "disk" on the Y axis if they do. This works well when the green "disk" approaches the red "bump" from top or bottom, it bounces off in the other direction. But when it approaches from the side, the bounce is incorrect; I need to reverse the X direction instead.

Here's the timer I setup:

- (void)viewDidLoad {
    xSpeed =  3;
    ySpeed = -3;
    gameTimer = [NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector(mainGameLoop:) userInfo:nil repeats:YES];    
    [super viewDidLoad];
}

Here's the main game loop:

- (void) mainGameLoop:(NSTimer *)theTimer {
    disk.center = CGPointMake(disk.center.x + xSpeed, disk.center.y + ySpeed);

    // make sure the disk does not travel off the edges of the screen
    // magic number values based on size of disk's frame
    // startAnimating causes the image to "pulse"
    if (disk.center.x < 55 || disk.center.x > 265) {
        xSpeed = xSpeed * -1;
        [disk startAnimating];
    } 
    if (disk.center.y < 55 || disk.center.y > 360) {
        ySpeed = ySpeed * -1;
        [disk startAnimating];
    }


    // check to see if the disk collides with the bump
    if (CGRectIntersectsRect(disk.frame, bump.frame)) {
        NSLog(@"Collision detected...");
        if (! [disk isAnimating]) {
            ySpeed = ySpeed * -1;
            [disk startAnimating];
        }    
    }
}

So my question is: how can I detect whether I need to flip the X speed or the Y speed? ie: how can I calculate which edge of the bump was collided with?