I honestly don't think pain is a good motivator. It's actually been proven positive enforcement works better than negative physical reenforcement on dogs. Excessive physical reenforcement can actually cause anger and anxiety issues. Here are just a few websites that explain why:
Why a dog should not be hit - by Sammy Stein - Helium
http://ezinearticles.com/?Never-Hit-a-Dog-When-Dog-Training&id=1033027
Professionals also suggest you should never hit your dog.
See I'd consider that possibly child abuse depending on how often it happened. I was strangled until I nearly blacked out, thrown into walls and doors (I have a dent in my skull because of it), back handed across the face for spilling soda on my sister, had my toys and bed broken, ect. This would happen on a pretty much daily basis until I was like five or six and went to live with my mom. I really think the only thing it taught me was to be angry. As a child the more I was hit the more I acted out and after a while I just became use to it. Like it was something completely normal. I actually started punishing myself as a child by slapping myself in the face or slamming my head into walls. Which I don't think is close to healthy or normal. Though I can say other factors may have played into that. Such as my dad doing the same thing to himself, seeing my dad beat my step mom, and my biological mom. I think when your present physical violence as a way to show your displeasure or unhappiness with something your child is going to pick that up. That's how I started showing my own displeasure with things, by acting out physically. I'm going to sound like one of those creepy serial killers or something but I started hurting other children and animals when I was unhappy with them because that's how I thought it worked. You hit something when you're mad and that's what you do. I think physiological abuse can also be just as bad if not worse in some case.
Physical, Physiological, and borderline sexual abuse can make one really fucked up child. I could go on and on about the consequences of the different forms of abuse but I'll stop here for now.
In any case I feel any kind of abuse is abuse. What defines abuse to me is the excessive use of it. When it comes to sexual abuse any kind sexual interaction with a child whether it be words, suggestions, or physical not matter how small is abuse to me. I also think server neglect is a form of abuse as well.
Anyways these are just my opinions based off my own experiences. Sorry for the long rant there. D: