April 29, 2009

The physics of revolving doors

I think that it is not at all uncommon for a child to be utterly facinated by the wonder which is the revolving door. I remember, as a young boy, getting to go somewhere with my mom or dad or the whole family, and if the building had a revolving door I would behold that door as if it were a wonder of the modem mechanical world. I don't really know why I was so intrigued by these portals to the inside. Certainly, it was not that we were going anyplace particularly upscale or posh. In my mind, I want to believe that we were going somewhere in downtown Des Moines, like the fancy hotel or department store. Likely, we were going to the mall.

I remember the Mauker Student Union on the UNI campus had revolving doors. This was one of the most heavily trafficed buildings on campus. The doors were there to attempt to keep the energy costs associated with heating and cooling the building down. Flanking the revolving doors were standard doors for handicapped accessability. By the time I started college my mom had mostly been using a wheelchair for mobility. I remember many times passing those doors and being reminded of the trade-offs that must often be considered in pursuit of ecological design.

These days, when my commute is such that I can take the Metra between Evanston an Chicago, I will typically have to pass through two revolving doors. One door is at the entrance to the Ogilvie Transportation Center; the other is between the main hall and the platform. A line of people start going through one of these doors, will each exert a torque about the axle. Once the inertia of the initially stationary door is overcome, the rotation of the door will quickly pick up speed such that I have occaisionally seen a nervous commuter elect to skip her turn to go through the door so that the friction of the door on its bearings can slow down the door enough to safely jump in a slot without clipping ones own ankle.

I've seen purses, umbrellas, boxes and bags become temporarily caught in the door. I can't begin to count the number of people dragging their rolly-bags behind them and realizing too late that the bag is not going to make it through with them.

To be fair, most of the daily commuters are seasoned pros at navigating the spinning gauntlet of the OTC doors. It's rare that I witness a revolver incident in the morning, when the vast majority passing through are regulars. Contrast that with the afternoon ride that I share with surbanites coming to the city for dinner or Chicagoans who live close enough to a Metra station to make it a convenient mode of transportation. Last Friday I was behind 6 teens who thought it was a riot that they went through the door at the platform two-to-a-wedge. In general there is a wider range of people riding in the afternoon. In general then, there are more people riding who are unfamiliar with the general ebb and flow of the train station.

Today, I was getting off the train and was the last off the car, although I had been in the front car of the train. The platform has 2 revolving doors to choose from that exit to the main hall. One of the doors had 6-7 people queing up to pass through it. The other door had only 2 people approaching it, so I headed for that door. The person in front of me was moving slow enough that by the time I could start going through the door, I could have gone through the other door just as quickly. But, I was already committed, so I followed right behind her.

As there was now two people pushing on the door the speed of the rotation picked up, as I explained above. Normally, this is not a problem for people passing through these doors. Except, the woman in front of me was looking at her phone while the door was turning. As soon as she detrmined she was "through" the door she came to a full stop. Of course, I was still coming through the door, and the turning door clipped her heel.

This woman, about 10 years younger and 20 pounds heavier started barking at me. I made a quick determination that she had no business slowing down and stopping in front of a revolving door while people are getting off the train behind her. I had a brief notion to just acknowledge her and let her blow off some steam at me. But, even though I heard the sounds of her rebuke coming at me, I had no idea what she was saying. I was wearing my in-ear headphones which pleasantly block annoying sounds.

Typically I think of annoying sounds as the chug of the locomotive's diesel engine or the rumble of the CTA cars on the elevated tracks. Now I can add another sound of those pleasantly blocked by these headphones. :)

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