Scratch

Here are some example scripts that make it look like the cannonball is affected by gravity.  Try this: 

when gf clicked // Put this script in the cannon sprite.
repeat until (key [space v] pressed?)
  if (key [right v] pressed?)
    turn cw [5] degrees
  end
  if (key [left v] pressed?)
    turn ccw [5] degrees
  end
end
broadcast [shoot v]

when I receive [shoot v] // Put this script in the cannonball sprite.
go to [CannonSprite v]
set [xVelocity v] to ([10] * ([cos v] of ([90] - ([direction v] of [CannonSprite v]))))
set [yVelocity v] to ([10] * ([sin v] of ([90] - ([direction v] of [CannonSprite v]))))
repeat until <touching [edge v]?>
  change x by (xVelocity)
  change y by (yVelocity)
  change [yVelocity v] by [-0.5]
end

@iTweakOr

Trigonometry is a mathematical concept that describes the relationship between angles and lengths.  For example, trig allows you to calculate the angles in a triangle when you know the lengths of its sides. 

The post I shared above converts the direction the cannon points into a right triangle with leg lengths xVelocity and yVelocity (legs are the two sides in a right triangle that meet at a right angle).  Imagine a right triangle with a center at the origin (0,0), and one leg going right, one going up.  The cosine function takes in the angle of the triangle and returns the ratio of the horizontal side over the hypotenuse (diagonal).  Likewise, the sine function takes in the same angle and returns the ratio of the vertical side over the hypotenuse.  In these scripts, the trig functions are used to determine the values of xVelocity and yVelocity.

Every frame (repeat until touching edge), the cannonball changes its position by xVelocity and yVelocity, giving the appearance that it is moving.  Additionally, every frame, yVelocity changes by -0.5, giving the effect that gravity is acting on the ball.  In each frame, the ball will change its x position by a constant value, and change its y position by a value that is decreasing, giving the image of falling.

Try

point towards [cannon v]
repeat until <touching [edge v]?>
  move (-5) steps
end

@jsboygenius:
If you want the cannon to follow the mouse, and the cannonball to have no gravity, change the scripts to be this:

when gf clicked // Put this script in the cannon sprite.
repeat until (mouse down?)
  point towards [mouse pointer v]
end
broadcast [shoot v]

when I receive [shoot v] // Put this script in the cannonball sprite.
go to [CannonSprite v]
set [xVelocity v] to ([10] * ([cos v] of ([90] - ([direction v] of [CannonSprite v]))))
set [yVelocity v] to ([10] * ([sin v] of ([90] - ([direction v] of [CannonSprite v]))))
repeat until <touching [edge v]?>
  change x by (xVelocity)
  change y by (yVelocity)
end