# Question of the Week Podcast

 Sun, 24th Apr 2016

## How fast can an elevator go?

Listener Paul got in touch to ask if it's true that there's almost no restriction in the speed an elevator could ascend when it comes to the human body, but that there was a limiting speed for the descent. Graihagh Jackson was perplexed and intrigued by the question and so went to meet engineer, Dr Philip Garsed from Cambridge University to get up to speed...

#### Make a comment

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. agyejy, Wed, 23rd Mar 2016

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! evan_au, Fri, 25th Mar 2016

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