Impossible angular momentum

Remember many, many years ago the urban myth that it had been scientifically proven that bumble-bees couldn’t fly? Many people took that to mean that science was clearly bonkers, and good reason to ignore anything a scientist said. Unfortunate, when as a scientist you want to campaign on the fact that science is a useful and productive way of looking at the world.

I have my own version of the bumble-bee problem: physics proves that it is impossible for a child to ride a scooter. I’ve come to this conclusion after playing ‘asteroids’ with oncoming scooter-borne children while walking the last bit of the way to work in the morning.  (Incidentally, I’m amazed that schools allow children to bring scooters, skateboards and the like. Skateboards were most definitely banned at school when I was there – and scooters, thankfully, didn’t exist in great numbers.)

Anyway – the issue. As any cyclist knows, it is not easy to fall off a moving bike. The reason is the angular momentum of the wheels. A spinning wheel carries angular momentum. It is a directional quantity – a wheel that is oriented vertically (as it should be in a bicycle) has a different angular momentum to one spinning the same rate but oriented parallel to the ground (as in a  bike just about to hit the ground). The greater the rate of spinning, the greater the angular momentum of the wheel, so the greater the difference in the two scenarios.  To change the angular momentum requires a torque, or turning moment. This can be done by moving your body off-centre. But if you have a lot of angular momentum to change, you need to apply a huge torque, which means the faster the wheel is spinning the less easy it is to pull it from a vertical position to a horizontal position. In fact, leaning one way will result in the bike changing direction, rather than falling.

A year 12 or 13 physics student should be able to estimate the angular momentum of a bicycle wheel. A similar calculation will give the angular momentum of the scooter wheel.  For the scooter, the wheels are tiny, and much less massive. They carry much less angular momentum (I estimate maybe around a tenth at a similar ground-speed), and that means one should be able to flip them from vertical to horizontal with much less torque. It’s hard to see how anyone could stay upright on one.

However, kids are good at doing impossible things, so why would that bother them?


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