On: Japan maglev train breaks world speed record again

[littlejohnk]

"the wheel rims were close to supersonic"

This doesn't make any sense. The record is around 575 km/h. The speed of sound is 1225km/h. How would the wheel rim be travelling faster than the train? The while rim rotates 1m for every 1m the train travels, surely?

Can you explain what you mean?

[maxerickson → littlejohnk]

The top of the wheel is moving faster than the train. If the contact point between the wheel and rail is not slipping, the relative velocity between the wheel at that point and the rail is 0.

To maintain that condition, the top of the wheel has to travel at twice the speed of the train. Imagine a point on the wheel. Each time the train moves ones circumference forward, that point also makes a circular trip of 1 circumference.

So at the top, when the motion of the point relative to the train is full forward, the net velocity is twice the train. At the bottom, when the motion of the point relative to the train is full backward, the net velocity is 0.

There are diagrams and further explanation here: http://physics.stackexchange.com/questions/48234/why-is-the-...

[HCIdivision17 → maxerickson]

Your link follows to a pretty good set of animations on the topic (linked by the OP of that thread):

http://www.animations.physics.unsw.edu.au//jw/rolling.htm

Good stuff, and even has an interesting note about how train wheel rims acually go backwards as the wheel turns!