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Comment by symmetricsaurus | Hacker News Reader

Roller coaster design is one of the areas where you can actually use the 3rd time derivate of position, the jerk, to good effect. So the jerk is the change in acceleration over time. A typical motion with high jerk is when you are riding in a car and the driver turns the wheel quickly, not a very comfortable experience.

The same is true for roller coasters; high jerk motions are uncomfortable. So you have to not only take the limits of comfortable acceleration into consideration but also try to minimize the jerk.

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Jerk is also a limit in how fast a subway can move between stations. There is a clear limit to how much jerk average standing passengers can survive before they fall down.

Jerk is also the guy talking loudly on his cellphone while a subway moves between stations. There is a clear limit to how much jerk average standing(or sitting) passengers can survive before they fall down on the guy and knock his teeth out. :)

I always wondered if there were a sociological commentary in the fact that yank is defined as jerk * mass.

aaaah so the bigger the jerk the harder you have to yank him to bring him down?

Actually no I thought the term might have been coined by British physicists after spending too much time around us Americans...

I don't think this is the limiting factor in most subway systems though. I've never been on one that accelerated during the entire first half of the segment and decelerated during the second half at the same rate, which would be the case if they were being limited by jerk.

Might not be in most but it certainly is for some. The subway in the Atlanta Hartsfield Jackson airport accelerates and decelerates that way. It makes it feel like you're probably going way faster than you are. I've seen just a few people almost fall over if they aren't holding on properly.

>I've never been on one that accelerated during the entire first half of the segment and decelerated during the second half at the same rate, which would be the case if they were being limited by jerk.

No, that would be the case if the only limit was jerk. When stations are close to each other, it feels like this is exactly what happens.

The acceleration curve you describe would have infinite jerk at the start, stop and halfway points. A |jerk| minimizing curve for a fixed travel time would gradually (linearly) increase to maximum acceleration during the first 1/4 of the trip, gradually decrease back to no acceleration over the next 1/4, gradually increase to full deceleration over the next 1/4, and finally gradually decrease to no deceleration over the last 1/4.

DC-powered rail systems such as the Chicago "L" can be jerky because the motor controllers are stepped not continuous. It's basically late 19th/early 20th century technology. The AC-powered systems in their newer rolling stock offer smoother operation.

I think the stops would have to be very close together for this to be a factor. If a train trip is jerk limited, then the train would not reach maximum acceleration between stops. It is more likely that a train trip is acceleration limited where the train does not reach full velocity between the stops.

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