Just need to know, when does Force of Friction = Fnet
This question is far too vague. If you want homework help, you're going to have to post a specific homework problem with details.
That having been said, it's obvious that the force of friction acting on an object is equal to the net force acting on that object IF that friction force is the only force present. It's sort of a case of ask a silly question, get a silly answer.
If I'm understanding your question correctly, if you you have a block on a surface and you apply a force to the side of the block the force due to friction will equal the applied force until what happens?
You can certainly speculate as to what his scenario is, but unless he tells us, we can't help him. I was trying to drive home the very important point that the question that he was asking was ill-posed. At the very least, it was insufficient, because it has only a trivial answer, which I gave him. We can't elaborate because there is no universal way of doing so (the answer is entirely situation-specific).
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