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Physics
Physics is the science dealing with energy, movement and its effects on objects. This area of study explains the forces at work on a hula hoop and the user. You do not have to be a scientist to understand how a hula hoop stands up; you just need to understand that energy can work on people and objects in opposing directions.
Gravity
Gravity is the force that pulls objects towards the earth. When a person is using a hula hoop, gravity constantly pulls it towards the ground. If gravity is acting alone, the hoop would fall to the ground. When a person moves her hips into the hoop, another force is acting on it that prevents it from dropping.
Friction
A person's hips rubbing against the hula hoop causes friction. It is this force that prevents gravity from pulling the hoop to the ground. While gravity pulls the hula hoop down, friction works parallel to the ground and pushes the hoop away from the body. The friction force is perpendicular to the hoop.
Problems
Some people think that a person has to have wide hips in order to use a hula hoop successfully. The size or shape of a person's hips are essentially not a factor. The problem many people have when trying to keep a hula hoop from falling down is that the level of friction from their hips is not equal to the force of gravity. If the friction is less significant than gravity, the hoop will fall. If it is more significant, the hoop will rise upward and the user will not be able to control it with his hips.