Final answer:
The force keeping a hula hoop moving around a person's waist during circular motion is a centripetal force, acting towards the center of the circle.
Step-by-step explanation:
When a P.E. teacher asks a student to play a game with a hula hoop, the type of force responsible for keeping the hoop moving around the waist is a centripetal force. This force is necessary for any form of uniform circular motion, and it always acts towards the center of the circle. In the case of a hula hoop, this centripetal force is provided by the small, rhythmic movements and adjustments of the body, particularly the hips, which ensure the hoop keeps spinning around the waist. These movements exert a force on the hoop that continually redirects it towards the center of the spinning circle, thus allowing the hoop to maintain circular motion around the waist.