You've more or less given us the answer in the question.

The usual question is "You have 8 balls. 7 of equal weight, and 1 that is slightly heavier. You also have a pair of scales. What is the minimum number of times you need to use the scales to work out which ball is counterfeit?"

But it works either way.

And yes, I know the answer, but I'll wait for others first. In fact, I'll answer it when I wake up in the morning - even if no-one has answered they'll have had enough time to... and I won't wake up until gone midday.