How can I make the motion smoother?

I'am programming my first game for Ipad. I have a little problem on an animation. In this game I have a ball bouncing around the screen. I move the ball in this way

CGRect frameRect = ball.frame;  
frameRect.origin.x += ballMovement.x; 
frameRect.origin.y += ballMovement.y;
ball.frame=frameRect;

The ball moves but the animation is sometimes not very smooth... The strange thing is that while I was testing my app on my ipad I discovered that if I close the app and reopen it from the multitasking bar the ball moves way way better! The animation is smoother and fas开发者_如何转开发ter... Can somebody explain why is happening this thing?

Thank you!! Daniel from Italy

If reopening the application makes it smooth, then most likely something has been cached the second time that was not cached the first time. There's nothing in the 4 lines of code you posted that could explain the lack of smoothness. Maybe posting more of the code would help.