Monday, 12 August 2013

Physics on the Metro

Have you ever spotted how the air moves on metro trains?

On ordinary trains with short carriages there is often a flow of air through the carriage as the train moves along.  It brings blessed relief on those hot summer days!

But something different happens on those trains that are open from one end to the other, and the effect depends strongly on the number of people on the train.

One of those long un-divided trains on Paris metro line 1
One of those long un-divided trains on Paris metro line 1

If the train is fairly empty, you get a strong rush of air from the front to the back as it accelerates.  Quite often the train accelerates in steps, and you get repeated breezes of different strengths, correlating well with the driver's actions.  Then as the train slows down, the air rushes to the front of the train.

This shows that the mass of air inside the train has to be pushed to get it to speed up and slow down.  The push comes from a pressure gradient which is set up as the train changes velocity.  That gradient is established by filling one end of the train with more air, increasing the local pressure.

I had spotted this happening on a journey on an empty train.  The following day on the same metro line I was expecting to observe the same effect.   However, it happened to a much less noticeable extent.  What was different?  The train was full of people standing.  That meant that the air flow was significantly better damped.

The effect is also much clearer if you sit near the middle of the train than it is at the ends - obviously.  After a week of observations I think this is good(ish) physics.

On the other hand . . . on another journey I watched a roller skater walking down stairs (very carefully) and then rolling along the platform.  As he slowed himself down he spread his feet wide apart, lowering his centre of gravity.  To stop himself he brought them sharply back together, lifting his whole body a few centimetres.  It appeared to me at first sight that he had converted his kinetic energy into potential energy in doing this - but on reflection that is not consistent with conservation of momentum.  This wasn't good physics after all.

Perhaps it was just a flourish in his performance after all.

Isn't physics fun?  It gets everywhere!


Derby Sceptic said...

Having never travelled on the Metro my only experience is with our good old London Underground.

Oh for trains empty enough to experience this effect!

Jamb 2014 said...

The metro, i have never travel on it before..

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