Well, you transform the direction of the velocity in Vector3(0, 15, speed). This is the speed in x, y, z. So to change the speed in which it is thrown in the y-direction change the 15 to something higher. Or, even better, at the top of your script type var height = 15.0; amd where the 15 stands type height. Now you can change the height without going into the script. :)
if i use this code, the physic is not working. if i throw the object to the wall, then the object will go through the wall. i have add rigidbody, but also not working, because i think the object force to go to the position that i have set before.