I read this somewhere:
When an adult hits an adult it is called assault
When a child hits an adult it is called defiance
When an adult hits a child it is called discipline
Hitting is hitting is hitting - it is wrong, always, to physically, emotional, verbally or otherwise abuse someone else - particular those who are vulnerable and depend on you.
If you honestly cannot think of any other way of parenting your child than assaulting them, then you owe it to them and yourself to seek help, advice and support.
If physical punishments worked, then why is it not ok for your boss to hit you? Or for your child to be hit by their teacher? It is not ok, because it is not ok.
I have never, ever, and will never, ever hit my children. I was smacked, hard, as a child. I remember the pain, hurt and complete bewilderment/feeling of injustice. I work in child protection now and see, hear and read the most awful things about physical punishments - I think it is very hard for there to be a line of what is "acceptable" and what isn't - much better to say none, end of.