Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 26/04/2016 15:23:11

Title: QotW - 16.04.26 - Is there is no restriction for an elevator ascend?
Post by: thedoc on 26/04/2016 15:23:11

I recall once watching a program about elevators which inferred that there was almost no restriction in the speed an elevator could ascend (for the human body) but there was a limiting speed for the descent. Or was it vice versa?  Not sure but is there any truth in this statement
Asked by Paul Sleath


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Title: Re: How fast can an elevator go without harming the people inside?
Post by: agyejy on 23/03/2016 23:09:23

The first thing to remember is that the speed of the elevator doesn't actually matter. What matters is the magnitude of the acceleration the elevator goes through to get to its top speed. For ascent the acceleration phase pushes you against the floor making you feel slightly heavier. This is usually not too uncomfortable for the passengers. For the descent the acceleration phase actually reduces your weight (you push against the floor less) and if the magnitude of the acceleration is too high you'll become weightless or even be pushed against the top of the elevator. Reductions in weight tend to make people feel sick to their stomach and clearly you can't make people feel weightless or hit the ceiling without causing significant discomfort.

Basically when going up you can usually tolerate a greater acceleration than when going down because the acceleration going down feels an awfully lot like falling to the human body and people generally don't enjoy that when they aren't expecting it.

Title: Re: How fast can an elevator go without harming the people inside?
Post by: evan_au on 25/03/2016 05:20:09

Quote from: Paul Sleath

there was a limiting speed for the descent

I read many years ago that a "parachute ride" in a theme park was designed to give the patrons 90% of their normal weight on the way down. ie a downwards acceleration of 0.1g, or about 1m/s². (Although this ride did seem a little tame by modern standards - you didn't need to be strapped in.)

Since you wouldn't want your trip to or from work to be remotely in the "thrill ride" category (or require a 5 point harness), I suggest that they would try to keep the acceleration in commercial elevators/lifts well under 0.1g.

There is another restriction - the autonomous emergency brakes on an elevator car are designed to detect a lift approaching free fall, and immediately apply the brakes. If the lift routinely reached these accelerations, the emergency brakes would get a regular workout - much to the consternation of the passengers!