I'm not 100% sure of this, but I feel that if you place a comma after "it" you're setting off "to overcome it," rather than "and then to overcome it." That is, if I were going to put a comma after "it," I'd put one after "then" as well.
I have a logical problem with "opposite." You have just created this force. It can't define itself. You must first establish the relationship between
Yeah, as mentioned earlier, it is not very clear what "it" refers to in your sentences. I am only guessing that what you meant is something like:
"They create a new force which cancels out the opposite force, so to overcome the opposite force, they have to amplify (or increase) the strength of the new force"