# Back on track

1. Jul 14, 2006

### pinestone

A train is moving west at 50 km/hr. In relationship to the tracks, the top point of the trains wheel is also moving east. However, the bottom point of the wheel is moving west. Assuming the wheel rotates at a constant speed, does the train really move at all?

2. Jul 14, 2006

### Tide

If the train is moving west then the top of a wheel is also travelling west but at twice the speed of the train. The bottom point of the wheel is moving neither west nor east unless it is sliding on the track. Of course, the center of the wheel is moving west at exactly the same speed as the train.

I can assure you that the train does move! :)

3. Jul 15, 2006

### LURCH

What Tide said, plus:

If the top of the wheel is moving east and the bottom is moving west, then no, the train does not move; it just sits there spinning its wheels. This of course assumes that the westbound velocity at the bottom and the eastbound velocity at the top are equal. If they're not equal, then the train moves at a rate equal to the difference of the two.