Recently, I was asked, "Well, if they're the same, as you say, then why do they have different definitions?" Um, for one, there are a great many synonyms in our language. Those are an example of that crazy phenomenon. Secondly, the definitions aren't actually different.
Merriam-Webster (the English teacher in me will not allow me to use Dictionary.com or anything else newfangled when it comes to my reference books) defines the following words as such:
smack - to strike so as to produce a smack; a sharp slap or blow
spank - to strike especially on the buttocks with the open hand
(Hmmm...I'm seeing a pattern emerge already...)
pop - to strike or knock sharply : hit
(I can't make this stuff up, folks.)
tap - to strike a light audible blow; to strike lightly especially with a slight sound; to give a light blow with
hit - to strike a blow
I mean, I didn't have to make anything up here. This is not my opinion or bias. It is what it is. Hitting is hitting is hitting. Give it whatever fanciful name you want, but you're still just putting lipstick on a pig...