Question:
1) What is the name of the centripetal force in the Gravitron?
2) Why must the Gravitron be spinning at high speed for the ride to work?
Answer:
(1)
When we those kids pinned against the wall, it is tempting to imagine that there are some centrifugal forces pushing them outward against the wall. But there is none. It is their inertia that cause them to want to move tangentially. The wall stops them from doing so by exerting a normal reaction force on them. This normal reaction ends up providing the centripetal force to keep the kids in circular motion.
(2)
While the normal reaction force keeps the kids in circular motion, what is keeping the kids from sliding downwards due to their weight? The answer is the frictional force. Now remember the maximum frictional force between two surfaces depends on how hard the two surfaces are pressed against each other. If the kids do not have a high enough tangential speed, they will not be pressing themselves against the wall hard enough, and the frictional force will not be large enough to hold their weights.
Bonus Video
The same principle is being applied by these stuntmen. The key to pulling off this stunt successfully is to ride at a high speed. The inertia of the bike (tendency to keep moving straight) causes it to be pressed hard against the wall, resulting in a large normal contact force. A large normal contact force is crucial because the amount of friction depends on it. It is the frictional force that holds up the weight of the car.
xmPhysics 