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« War Unwinnable In Face Of Renewed German Offensive | Main | Giggle Factor Gone »

Physics Reminder

Alan Boyle has a little piece today about the elevators in the tallest building in the world. But this bit is misleading:

Imagine riding in a car going almost 40 mph (60.6 kilometers per hour). Not that impressive, right? But now imagine going that same 40 mph ... straight up.

That gives you some idea how elevator riders must feel in the world's tallest building, Taipei 101.

Actually, you can't feel speed at all. There is no difference in sensation between a twenty mph elevator and a forty mph elevator, other than perhaps vibrations transmitted through the cables and contact with the shaft. Acceleration is what you feel, so the difference is how long it takes you to get up to speed (and back down from it), not what the top speed is.

Similarly, he writes:

The cars go faster on the way up than on the way down perhaps to counteract that free-fall feeling you could get during a rapid descent.

I'm not sure why they go faster up than down, but it can't be for that reason. Again, the "free-fall feeling" arises from the change in speed, not the steady-state.

Posted by Rand Simberg at December 17, 2004 05:45 AM
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I'm just making this up as I go along, but I'd guess that the velocity difference (up vs down) is a nod to gravity. It's a lot easier to decellerate going uphill than downhill.

Posted by slimedog at December 17, 2004 05:58 AM

Man, I hate that sort of physics ignorance, too. It takes the enjoyment out of a lot of science fiction films for me sometimes.

Posted by Astrosmith at December 17, 2004 07:18 AM

It could also be that the car's counterweights, which are going to always be as heavy or heavier than the car itself, will go down faster than they can be made to go up. An elevator is just a controlled balancing act, after all.

Posted by John Breen III at December 17, 2004 07:20 AM

Well, Alan usually does a pretty good job (and I know that he has studied physics). Just a little brain burp, I think.

Posted by Rand Simberg at December 17, 2004 07:25 AM

I suspect that the slower descent is to allow the passenger's ears to equalize pressure. I can picture a car full of people frantically gulping like goldfish in an unaereated tank.

Also, you DO feel the acceleration to speed more keenly on the bottoms of your feet than from the padded seatback of a car. The initial rush gives a perception of speed that would be reinforced by continued vibration and prolonged popping in ones ears. I haven't been in Taipei 101 but I have taken express elevators in a variety of medium tall buildings. Your mind does integrate the cues into a perception of vertical speed.

Posted by Ed Nutter at December 17, 2004 07:40 AM

It would be cooler if they had em go into free fall for 20 stories or so.

Posted by Andrew at December 17, 2004 08:16 AM

I think John Breen III is probably right. I don't think the ear pressure is going to have much of an effect in an airtight building.

I also think there is some pschology involved in that the feeling of upward accelleration is barely noticable while the feeling of the floor dropping out from beneath you can cause panic.

Posted by rjschwarz at December 17, 2004 08:17 AM

rjschwarz:

There is still gravity and that is what causes the vertical pressure gradient, inside a sealed building or not.

In unpressurised aircraft used in commuter airline service it is common to try to limit descent vertical speeds to 500 feet/min for passenger comfort.(ears)

Posted by Mike Borgelt at December 17, 2004 01:21 PM

Mike Borgelt wrote:
In unpressurised aircraft used in commuter airline service it is common to try to limit descent vertical speeds to 500 feet/min for passenger comfort.(ears)

hmm. I think there are a lot of factors there. 500 fpm down is also enough to take many aircraft from normal cruise speed to Vne at cruise power, or even with a power reduction.

I've done descents from 20,000 ft or so to circuit height in unpressurised aircraft (gliders) at around 8000 fpm without any problems, and I would not be at all surprised if you had as well -- that's only 80 knots straight down (with trailing edge airbrakes, yay!) or 115 knots in a 45 degree descent. That sort of descent rate is entirely managable if you know to equalise pressures.

Posted by Bruce Hoult at December 17, 2004 02:33 PM

That sort of descent rate is entirely managable if you know to equalise pressures.

Yes, it's trivial compared to what a diver does on every descent.

Posted by Rand Simberg at December 17, 2004 02:49 PM

it's trivial compared to what a diver does on every descent

Yes, very good point.

18,000 ft to sea level is half an atmosphere of pressure difference (500 mb).

That's the same pressure difference as 16 feet of water.

Posted by Bruce Hoult at December 17, 2004 03:00 PM


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