34 ms·

I know, this is an old paper, but I don't follow the this assumption: > The terms jerk and snap mean very little to most people, including physicists and engin

I know, this is an old paper, but I don't follow the this assumption: > The terms jerk and snap mean very little to most people, including physicists and engineers. Almost 20 years ago we defined jerk into our standards for lift applications. I know jerk is an important parameter for any modern rotating machine that includes gears. While in lift applications it is known as the roller coaster effect, people in different parts of the world have a different taste on when they want to use a lift. I know I over simplify when I say, that American people want to have the gut feeling when riding a lift, especially an express lift in those high buildings. In difference in Asian countries the lift ride must be smooth as possible. They don't like to have the feeling of riding a lift at all. In Europe it is something in between. Lift manufacturers have to respect those (end) costumers otherwise the are not chosen. The same in any rotating machine with some sort of gears. Because jerk and those higher orders contribute to the wear and tear of gears. As you want to have longer lasting gears many modern machine manufacturers limit those parameters to reduce wear and tear. So, with a little software change I can demand a higher price because service and maintenance can be reduced.

account42 2y ago
> American people want to have the gut feeling when riding a lift, especially an express lift in those high buildings. In difference in Asian countries the lift ride must be smooth as possible. They don't like to have the feeling of riding a lift at all. In Europe it is something in between How representative are these stated preferences actually of the population. I'd imagine that the individual preferences vary greatly from person to person and also change with age.
- xxpor 2y ago
  They're the preferences of the buying managers.
  - amelius 2y ago
    If I were buying, I'd ask for a sensation of horizontal force when going up/down and see what they'd come up with.
    - peddling-brink 2y ago
      Corkscrew lift.
      - MaxBarraclough 2y ago
        I imagine a spinning lift would be easier.
        function_seven 2y ago
        Finally an elevator that will force other occupants to respect my personal space. We’re all up against the wall during this ascent!
  - deleted 2y ago
    [deleted]
- matsemann 2y ago
  It might not be a strict "preference", more of an expectation how things should be based on previous experience. Like, if you're used to an elevator with a bit of a jerk, an elevator taking you there just as fast but smoother might feel not as fast.
  - fbdab103 2y ago
    Yet it is a captive market. If I am in a building, I only have access to a singular type of elevator. Why not always give the smoothest ride possible unless it is $0.12 cheaper for the installer, so everyone has to suffer forever.
    - failbuffer 2y ago
      You could just as well ask "why not give the fastest ride possible so everyone saves time?"
    - majormajor 2y ago
      An elevator that feels significantly different than the other elevators in the city probably results in more complaints to building management/maintenance about either "the elevator is too jerky!" or "the elevator is too slow!"
- codexb 2y ago
  I'm an American. My thoughts are that I'd prefer to spend as little time as possible in an elevator and would rather spend that time doing other things. If I don't feel the elevator moving, then it's moving too slowly and wasting my time.
bowsamic 2y ago
One thing that is strange is that we can easily imagine the first two derivatives: position we can just imagine a static point, velocity we can imagine a constant speed i.e. a straight line on a position-time graph, acceleration we just imagine a parabola, but jerk is somehow conceptually indistinguishable. The difference between a point, a line, and a parabola are stark, the third order jerk is not so easy to distinguish, instead still just looking like the parabola. I've always wondered why this is, why curves in general are perceptually similar if scaled correctly, while a straight line is so clearly different. Perhaps it is because our perceptions developed to distinguish between inertial and non inertial reference frames?
- xscott 2y ago
  I like and agree with your observation. But I think you can use conceptual tricks to get just a little further: Acceleration is "due" to a force (F=ma), so you can think of jerk as a change in that force linearly increasing over time. That doesn't help me recognize a cubic from a quadratic when looking at a small piece of it, but I can imagine an elevator ramping up it's lifting power or similar. It kind of feels like the tricks to conceptualize 4D as 3D position plus a temperature at each spot.
  - bowsamic 2y ago
    I agree the linearisation trick can be used and is often used in physics, but we must do that as a consequence of the thing I'm confused about the origin of, which is not why do we only greatly distinguish between the first few derivatives of position, but why do we only greatly distinguish between the first few derivatives of most functions? I.e. why do we have to do these tricks in the first place?
    - xscott 2y ago
      I don't have any answers, but I suspect it's because we're evolved from things that didn't need to know. Related, I sometimes wonder how many derivatives you need to go down in order to find the one that is discontinuous when you decide to make a motion. For instance: pressing the first key to type this reply, my finger didn't instantly jump from zero to non-zero acceleration (or jerk/snap) I assume. How many terms in the Taylor series for moving a muscle?
wlesieutre 2y ago
Jerk is also very important for road or rail track design. If you imagine needing to make a 90 degree bend, the "obvious" way to do it is by rounding off the corner with a circular radius. But if you do that, it means the vehicle goes from having 0 sideways acceleration to experiencing 100% of the centripetal acceleration to move an object on a circular path (a = v^2 / r) instantaneously. As an occupant of the car, that means you go from sitting comfortably to suddenly being thrown sideways. It's much more comfortable if you ease into the turn, with the track design considering the rate of change of acceleration. If the designer didn't consider jerk you would definitely notice.
- matsemann 2y ago
  That's why loops on roller coasters aren't perfect circles as well then, I guess?
  - wlesieutre 2y ago
    The forces you experience in a loop must be a bit more complicated because the turning forces in a car are perpendicular to gravity and in a loop are sometimes in-line, but yeah I would think that's why the entry and exit are a softer curve.
  - majormajor 2y ago
    Yeah, the more circular ones are pretty intense - https://guidetosfot.com/rides/shockwave/ https://guidetosfot.com/rides/shockwave/
  - rcxdude 2y ago
    In part, maybe. The bigger reason is the roller coaster slows down near the top of the loop, so even for an constant vertical acceleration you need a non-circular shape.
    - matsemann 2y ago
      Yeah, but I was just thinking that the sudden curve would jank your head quite a bit, compared to easing into it.
kovezd 2y ago
The terms are also understood in economics as prudence, and template. Albeit, not widely used.
fnordpiglet 2y ago
The terms jerk and snap while perhaps known in the rare space of elevator purchasing aren’t generally used terms in most fields. I’m surprised that’s in any way controversial ?
codexb 2y ago
Statics, dynamics, and two semesters of physics are required courses for every engineer. None of them incorporate the teaching of jerk or snap.
- midjji 2y ago
  But all of them are based on courses that define nth derivative, and the notion of continuous vs non continuous functions. What is the value of giving them names instead of just using nth derivative?

account42 2y ago

> American people want to have the gut feeling when riding a lift, especially an express lift in those high buildings. In difference in Asian countries the lift ride must be smooth as possible. They don't like to have the feeling of riding a lift at all. In Europe it is something in between How representative are these stated preferences actually of the population. I'd imagine that the individual preferences vary greatly from person to person and also change with age.

xxpor 2y ago
They're the preferences of the buying managers.
- amelius 2y ago
  If I were buying, I'd ask for a sensation of horizontal force when going up/down and see what they'd come up with.
  - peddling-brink 2y ago
    Corkscrew lift.
    - MaxBarraclough 2y ago
      I imagine a spinning lift would be easier.
      - function_seven 2y ago
        Finally an elevator that will force other occupants to respect my personal space. We’re all up against the wall during this ascent!
- deleted 2y ago
  [deleted]
matsemann 2y ago
It might not be a strict "preference", more of an expectation how things should be based on previous experience. Like, if you're used to an elevator with a bit of a jerk, an elevator taking you there just as fast but smoother might feel not as fast.
- fbdab103 2y ago
  Yet it is a captive market. If I am in a building, I only have access to a singular type of elevator. Why not always give the smoothest ride possible unless it is $0.12 cheaper for the installer, so everyone has to suffer forever.
  - failbuffer 2y ago
    You could just as well ask "why not give the fastest ride possible so everyone saves time?"
  - majormajor 2y ago
    An elevator that feels significantly different than the other elevators in the city probably results in more complaints to building management/maintenance about either "the elevator is too jerky!" or "the elevator is too slow!"
codexb 2y ago
I'm an American. My thoughts are that I'd prefer to spend as little time as possible in an elevator and would rather spend that time doing other things. If I don't feel the elevator moving, then it's moving too slowly and wasting my time.

bowsamic 2y ago

One thing that is strange is that we can easily imagine the first two derivatives: position we can just imagine a static point, velocity we can imagine a constant speed i.e. a straight line on a position-time graph, acceleration we just imagine a parabola, but jerk is somehow conceptually indistinguishable. The difference between a point, a line, and a parabola are stark, the third order jerk is not so easy to distinguish, instead still just looking like the parabola. I've always wondered why this is, why curves in general are perceptually similar if scaled correctly, while a straight line is so clearly different. Perhaps it is because our perceptions developed to distinguish between inertial and non inertial reference frames?

xscott 2y ago
I like and agree with your observation. But I think you can use conceptual tricks to get just a little further: Acceleration is "due" to a force (F=ma), so you can think of jerk as a change in that force linearly increasing over time. That doesn't help me recognize a cubic from a quadratic when looking at a small piece of it, but I can imagine an elevator ramping up it's lifting power or similar. It kind of feels like the tricks to conceptualize 4D as 3D position plus a temperature at each spot.
- bowsamic 2y ago
  I agree the linearisation trick can be used and is often used in physics, but we must do that as a consequence of the thing I'm confused about the origin of, which is not why do we only greatly distinguish between the first few derivatives of position, but why do we only greatly distinguish between the first few derivatives of most functions? I.e. why do we have to do these tricks in the first place?
  - xscott 2y ago
    I don't have any answers, but I suspect it's because we're evolved from things that didn't need to know. Related, I sometimes wonder how many derivatives you need to go down in order to find the one that is discontinuous when you decide to make a motion. For instance: pressing the first key to type this reply, my finger didn't instantly jump from zero to non-zero acceleration (or jerk/snap) I assume. How many terms in the Taylor series for moving a muscle?

wlesieutre 2y ago

Jerk is also very important for road or rail track design. If you imagine needing to make a 90 degree bend, the "obvious" way to do it is by rounding off the corner with a circular radius. But if you do that, it means the vehicle goes from having 0 sideways acceleration to experiencing 100% of the centripetal acceleration to move an object on a circular path (a = v^2 / r) instantaneously. As an occupant of the car, that means you go from sitting comfortably to suddenly being thrown sideways. It's much more comfortable if you ease into the turn, with the track design considering the rate of change of acceleration. If the designer didn't consider jerk you would definitely notice.

matsemann 2y ago
That's why loops on roller coasters aren't perfect circles as well then, I guess?
- wlesieutre 2y ago
  The forces you experience in a loop must be a bit more complicated because the turning forces in a car are perpendicular to gravity and in a loop are sometimes in-line, but yeah I would think that's why the entry and exit are a softer curve.
- majormajor 2y ago
  Yeah, the more circular ones are pretty intense - https://guidetosfot.com/rides/shockwave/ https://guidetosfot.com/rides/shockwave/
- rcxdude 2y ago
  In part, maybe. The bigger reason is the roller coaster slows down near the top of the loop, so even for an constant vertical acceleration you need a non-circular shape.
  - matsemann 2y ago
    Yeah, but I was just thinking that the sudden curve would jank your head quite a bit, compared to easing into it.

kovezd 2y ago

The terms are also understood in economics as prudence, and template. Albeit, not widely used.

fnordpiglet 2y ago

The terms jerk and snap while perhaps known in the rare space of elevator purchasing aren’t generally used terms in most fields. I’m surprised that’s in any way controversial ?

codexb 2y ago

Statics, dynamics, and two semesters of physics are required courses for every engineer. None of them incorporate the teaching of jerk or snap.

midjji 2y ago
But all of them are based on courses that define nth derivative, and the notion of continuous vs non continuous functions. What is the value of giving them names instead of just using nth derivative?