I think everyone of you knows games where you can define a angle and a power and try to hit your opponent that way (see the game Castles on this page). But it is really not easy to solve this problem in a program that needs to simulate the flight of a cannonball. Here you can find a applet which implements one method to get such a cannonball movement and you can also find the sourcecode to it (comments are in German!). Maybe you won't be able to understand every single step and all the Physic stuff I used but I think you'll be able to reproduce this method if you need to. OK, here comes the applet!
Use cursor key to change angle (left, right) and power (up, down). Hit Spacebar to shot a cannonball. Before you can use the keyboard you have to click on the applet once. And here comes the source code: