Node not moving - ios

Question

I'm trying to throw a node around the screen, I've taken the below code but nothing actually happens.
When the scene loads the node spawns in the middle of the screen then drops to the bottom.
The code was taken from elsewhere in the site and granted it was a bit old so I'm assuming I'm missing something?
var sprite: SKSpriteNode!
var touchPoint: CGPoint = CGPoint()
var touching: Bool = false
override func didMove(to view: SKView) {
self.physicsBody = SKPhysicsBody(edgeLoopFrom: self.frame)
sprite = SKSpriteNode(color: UIColor.red, size: CGSize(width: 50, height: 50))
sprite.physicsBody = SKPhysicsBody(rectangleOf: sprite.size)
sprite.position = CGPoint(x: 50.0, y: 50.0)
self.addChild(sprite)
}
func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
for touch: AnyObject in touches {
let location = touch.location(in: self)
if sprite.frame.contains(location) {
touchPoint = location
touching = true
}
}
}
func touchesMoved(touches: Set<NSObject>, withEvent event: UIEvent) {
for touch: AnyObject in touches {
let location = touch.location(in: self)
touchPoint = location
}
}
func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {
touching = false
}
func touchesCancelled(touches: Set<NSObject>!, withEvent event: UIEvent!) {
touching = false
}
func update(currentTime: TimeInterval) {
if touching {
let dt:CGFloat = 1.0/60.0
let distance = CGVector(dx: touchPoint.x-sprite.position.x, dy: touchPoint.y-sprite.position.y)
let velocity = CGVector(dx: distance.dx/dt, dy: distance.dy/dt)
sprite.physicsBody!.velocity = velocity
}
}
}