This is an oft used puzzle where the N of balls changes or you're asked to find the lighter or heavier ball.
In this case, the "easy" answer is 3 weighings. You're guaranteed to find it with 8 balls. 4v4. Then 2v2. Then 1v1.
But you can actually do it in 2 weighings. You do 3v3 and leave 2 balls unweighed.
If side a is heavier, weigh 2 of the balls. If the scale balances the third ball is heavier. If it doesn't balance the heavier side contains the heavier ball.
Same if side b is heavier.
If the scale balances then just weigh the two remaining balls against each other.