Quinn+Gilligan

Basically, tops dont fall over because the angular momentum is greater than the force exerted on it by gravity. Think of it like riding a bike, as long as you remain in motion, you are still perfectly balanced. The moment you stop however, your leg (an outside force) is required to keep the bike from falling. This is because there is no longer momentum, or to put simply, there is no longer opposing movement to counteract gravity. Once the angular momentum of a top fades, a top falls over. As long as it remains spinning though, it is able to balance on a single point, and briefly defy gravity.