instantiating object from camera position onto touch position

im stuck with this for like a MONTH! i want to shoot a prefab of a sphere from the center of the camera onto the touched position. i made it by making the ball itself transform forward when i shoot it, but it wont stop if it will collide with something. i want it to be more like an add force power that shoots it with force at start and then it will slow down... pls help !

touch. and 3D