Why is this (my script) going astray?

i have a bowling game and the balls that i used wont stay in the lane and bounce akwardly to the sides of the aisle

Tool = script.Parent
VELOCITY = 100 -- constant

local Pellet = Instance.new("Part")
Pellet.Name = "Roll"
Pellet.Locked = true
Pellet.BackSurface = 0
Pellet.BottomSurface = 0
Pellet.FrontSurface = 0
Pellet.LeftSurface = 0
Pellet.RightSurface = 0
Pellet.TopSurface = 0
Pellet.Shape = 0
Pellet.Reflectance = 0.1
Pellet.Friction = 0
Pellet.Size = Vector3.new(2,2,2)
Pellet.BrickColor = BrickColor.new("Navy blue")
script.Parent.PelletScript:clone().Parent = Pellet
script.Parent.BodyForce:clone().Parent = Pellet
function fire(mouse_pos)


    Tool.Handle.SlingshotSound:play()

-- find player's head pos

    local vCharacter = Tool.Parent
    local vPlayer = game.Players:playerFromCharacter(vCharacter)

    local head = vCharacter:findFirstChild("Head")
    if head == nil then return end

    local dir = mouse_pos - head.Position
    dir = computeDirection(dir)

    local launch = head.Position + 2 * dir

    local delta = mouse_pos - launch

    local dy = delta.y

    local new_delta = Vector3.new(delta.x, 0, delta.z)
    delta = new_delta

    local dx = delta.magnitude
    local unit_delta = delta.unit

    -- acceleration due to gravity in RBX units
    local g = (-9.81 * 20)

    local theta = computeLaunchAngle( dx, dy, g)

    local vy = math.sin(theta)
    local xz = math.cos(theta)
    local vx = unit_delta.x * xz
    local vz = unit_delta.z * xz


    local missile = Pellet:clone()




    missile.Position = launch
    missile.Velocity = Vector3.new(vx,vy,vz) * VELOCITY
    missile.PelletScript.Disabled = false

    local creator_tag = Instance.new("ObjectValue")
    creator_tag.Value = vPlayer
    creator_tag.Name = "creator"
    creator_tag.Parent = missile

    missile.Parent = game.Workspace

end


function computeLaunchAngle(dx,dy,grav)
    -- arcane
    -- http://en.wikipedia.org/wiki/Trajectory_of_a_projectile

    local g = math.abs(grav)
    local inRoot = (VELOCITY*VELOCITY*VELOCITY*VELOCITY) - (g * ((g*dx*dx) + (2*dy*VELOCITY*VELOCITY)))
    if inRoot <= 0 then
        return .25 * math.pi
    end
    local root = math.sqrt(inRoot)
    local inATan1 = ((VELOCITY*VELOCITY) + root) / (g*dx)

    local inATan2 = ((VELOCITY*VELOCITY) - root) / (g*dx)
    local answer1 = math.atan(inATan1)
    local answer2 = math.atan(inATan2)
    if answer1 < answer2 then return answer1 end
    return answer2
end

function computeDirection(vec)
    local lenSquared = vec.magnitude * vec.magnitude
    local invSqrt = 1 / math.sqrt(lenSquared)
    return Vector3.new(vec.x * invSqrt, vec.y * invSqrt, vec.z * invSqrt)
end




Tool.Enabled = true
function onActivated()

    if not Tool.Enabled then
        return
    end

    Tool.Enabled = false

    local character = Tool.Parent;
    local humanoid = character.Humanoid
    if humanoid == nil then
        print("Humanoid not found")
        return 
    end


    local targetPos = humanoid.TargetPoint

    fire(targetPos)
    wait(5)
    Tool.Enabled = true
end

script.Parent.Activated:connect(onActivated)