  What is the force that keeps a hula hoop up? Let's find out! You might think that having hips would help, in which case girls ought to be better at this than boys. In fact, your hips do help ... but aren't really necessary at all. Anyone can make it work if they spin it just the right way. So what keeps it up!? To answer that question, let's look first at the force which is trying to make the hoop fall to the ground: gravity.  The force of gravity acts downwards on the hoop at all times; this force should cause the hoop to fall, but of course it doesn't.If the hoop stays at a constant height and does not move downwards, then there must be another force acting in the opposite direction that balances the force of gravity. What is this force? 
It's friction, of course. The force of friction between your body and the hoop exactly equals the pull of gravity, so the hoop doesn't fall. Here's how it works:As the hoop spins, your body exerts a force against the hoop, parallel to the ground, and perpendicular to the surface of the hoop. This is the normal force.  The normal force is what causes friction; frictional force can be calculated using this normal force and the formula: The normal force causes a frictional force perpendicular to FN. This frictional force is opposite to the direction of motion (down) and must exactly equal the force of gravity;  A skilled hula hoop user will be able to maintain a normal force on the hoop during the entire time it circles the body, so the force of gravity is always balanced. A less skilled user will allow the hoop to jump away from the body; every time that happens, the normal force drops, and gravity takes over, pulling the hoop down. And that's all there is to it! |