With a bit of luck you can get the ball straight inside the "U" with a single launch. If you can't do it that way just make a platform beneath the "U", bring the ball right under the "U" then launch it up at the right moment.
For an alternative solution see the video walkthrough below.
Physics Drop Level 21 Walkthrough, Solution, Cheats for Android, iPhone, iPad, iPod. Note: This is "Physics Drop" by "IDC Games"