Hello,
> I know this will be very hard to calaculate the weight and gravity but i was wondering if you could help. I have a cannon facing up and I have bombs that shot out of it going to where ever the user taps on the screen. I want the bombs to look like this when you tap. The "T" is where the user taps.
>
> http://parrish.homeip.net/public/example.png

Search Google for 'Gaffer On Games' => he has a physics / numerical integration series that will give you all the stuff you need.

Good luck.

From a physics point of view, keep in mind horizontal velocity and vertical velocity are seperate. So, for horizontal velocity, just move the x to the tap, and for vertical, move it to (tapY + powerOfCannon) initially then down until tapY == positionY. That will give you the path, the velocity should not be too difficult.