I would like to know which of the two verbs is used to tell someone that you're serious about not being touched. If he touches you the second time, something's going to happen to him. Someone told me that anfassen sounds stronger than berühren. Is it true?
Anfassen is to touch with your hands without being overly gentle.
Berühren is to touch (with something) without gaining a hold in a very somewhat gentle manner.
The English phrase
would be translated using "anfassen" unless you have a good reason to use the other verb.
In that situation you would say:
Berühren is very unspecific - you can touch something or someone with every body part. While anfassen you can only something with your hands. Moreover, anfassen presupposes some willful objective, some goal that is to be reached by using the hands. You can't, for instance, say:
is okay. Without "versehentlich" it is at least open, whether it was a wilfull act or not. Even if wilfull, berühren is not appropriate to describe some forceful or violent activity. As @Em1 correctly points out, berühren is good for tender, not very intense, and not long lasting, touches, at least when it is about living beings.
In physics, it's a bit different:
I'd like to point out, that although the direct translation of
this cannot be generalized in the meaning of the OP.
I guess the reason is psychosounding.