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181 points EndXA | 21 comments | | HN request time: 1.003s | source | bottom
1. marginalia_nu ◴[] No.40727129[source]
Do these higher order derivatives say anything meaningful?

I always got the sense from physics that outside of purely mathematical constructions such as Taylor series, higher order time derivatives aren't providing much interesting information. Though I'm not sure whether this is the inherent laziness of physicist math[1] or a property of the forces in nature.

[1] since e^x = 1 + x is generally true, why'd you even need a second order derivative

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2. azernik ◴[] No.40727177[source]
Yes. That is the point of the article.

> Jerk and snap can be observed in various areas of physics and engineering. In physics and engineering jerk and snap should always be considered when vibration occurs and particularly when this excitation induces multi-resonant modes of vibration. They should also be considered at all times when a transition occurs such as: start up and shutdown; take-off and landing; and accelerating and decelerating.

> Acceleration without jerk is just a static load, and therefore constant acceleration alone could never cause vibration. In a machine shop, a toolmaker can damage the mill or the job if the setup starts vibrating. This vibration happens because of jerk and snap.

> In mechanical engineering it is important in automotive design to ensure that the cam-follower does not jump off the camshaft. It is also important in manufacturing processes as rapid changes in acceleration of a cutting tool can lead to premature tool wear and result in an uneven and rough surface finish.

> In civil engineering railway train tracks and roads should be designed for a smooth exit from a straight section into a curve, and it is common to use a transition called a clothoid, which is part of a Cornu spiral (also referred to as an Euler spiral). When a clothoid is implemented the change in acceleration is not abrupt and the levels of jerk and possibly snap are significantly reduced. If the transition between different radii of curvature is sudden, the transition is uncomfortable for passengers and potentially dangerous as it could cause the car or train to be thrown off the road or track. With good physics design engineers are attempting to produce a gradual jerk and constant snap, which gives a smooth increase in radial acceleration, or preferably a zero snap, constant jerk, and linear increase in radial acceleration. Just as road and railway engineers design out jerk and snap using the clothoid transition so, too, do roller coaster designers when they design loops and helices for the roller coasters [11, 12].

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3. fellerts ◴[] No.40727207[source]
Jerk (how fast acceleration changes) is useful. I've found being a passenger in newer electric buses to pose more challenges than ICE buses because EVs can change their acceleration so rapidly. While their maximum acceleration isn't very high, they can go from standstill to accelerating in a split second, toppling anyone standing unless they hold on to something. ICEs need more time to reach maximum acceleration. In other words, EVs jerk more.
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4. bux93 ◴[] No.40727251[source]
If you're driving along and want to stop for the traffic lights, you start decelerating. The car in front of you slams the brakes and leaves less space then you anticipated. You now need to decelerate faster. That's negative jerk. If you apply the change in deceleration instantaneously, you will also experience jerkiness in your braking (= way to remember what this derivative is called).
5. playingalong ◴[] No.40727396[source]
How do you know this is not second derivative (acceleration), but the third or higher?

Genuinely curious.

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6. short_sells_poo ◴[] No.40727411[source]
> EVs jerk more. Giggity

More seriously though, I think this might be about driver training and maybe calibrating the foot pedal. It's great that EVs have a much better torque curve, but it means the old muscle memory of opening the throttle wide at low RPMs and letting the clutch slip is simply not the way to do it (nvm that there's no clutch to operate in an EV).

7. itsoktocry ◴[] No.40727540[source]
>they can go from standstill to accelerating in a split second

Every car can go from a standstill to accelerating in a split second.

>their maximum acceleration isn't very high

What is the maximum acceleration of an EV? Do you have some numbers?

>ICEs need more time to reach maximum acceleration

I don't think what you're describing is jerk, it's acceleration (and velocity).

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8. hristov ◴[] No.40727544[source]
Here is a video of a guy that tried to automate a grinding machine by installing an electric motor. Initially the movement was very unsatisfactory, it was not smooth, or very jerky. He then received an upgraded motor that included a “jerk control” feature and the movement of his machine became smooth.

It came as a surprise to me but it seems like jerk is something that can be felt in real life.

https://youtu.be/FPhNc6GwX1o?si=8cf7wU14puB8lsaa

9. trgn ◴[] No.40727725{3}[source]
ICEs don't develop same torque at every rpm, it takes a while to get to maximum. It's noticeable in how a car speeds up.
10. readams ◴[] No.40727753{3}[source]
Jerk is by definition how fast the acceleration changes. And it's true that there is more delay in ICE engines before you get full power and thus the acceleration changes more slowly.
11. johnwalkr ◴[] No.40728036[source]
I think up to jerk is intuitive, even if the term isn't well-known. Most people have no problem figuring out how to ease-up on brakes in a car, and a hobbyist can usually figure out how to make things smoother when ramping up or down a motor.
12. glitchc ◴[] No.40728112[source]
> [1] since e^x = 1 + x is generally true, why'd you even need a second order derivative

Only true for small x (less than 1).

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13. fellerts ◴[] No.40728329{3}[source]
I know it is the third derivative specifically because a rapid _change_ in acceleration easily puts you off-balance. A change in acceleration effects a change in the forces acting on you (F=ma). When those changes happen slowly, it's easy to adapt and change your stance to neutralize those forces, thus preventing your body from accelerating relative to your frame of reference (the bus).
14. fellerts ◴[] No.40728399{3}[source]
> Every car can go from a standstill to accelerating in a split second

Going from a standstill to accelerating at say 3 m/s² is very different in a normal ICE car vs. an EV. It's anecdotal, but you must have noticed this if you've driven an EV before.

> What is the maximum acceleration of an EV?

I was talking specifically about the electric buses in my city. They don't have massive acceleration compared to, say, a Tesla.

> I don't think what you're describing is jerk

I am talking about how rapidly electric buses change acceleration. That's the definition of jerk.

15. myrmidon ◴[] No.40728502{3}[source]
Constant acceleration as bus passenger can be fully compensated by just leaning at an angle. This is not unpleasant.

But if the jerk (or higher derivatives) are non-zero, you have to change your "lean angle" quickly to avoid getting jerked around (which is obviously much more disruptive).

16. ImPleadThe5th ◴[] No.40730807[source]
I believe they have applications in missle guidance systems.

I cannot remember what it's called but essentially given a target position in space the missle uses parametric data about its current position/orientation/speed and their higher derivatives to dead reckon about where it is in regards to the target.

Anyone remember what that's called? I went on a rabbit hole with it a few years ago, it's really interesting math and programming. Everything works basically stateless except for current instrument data, last position and target position from what I remember.

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17. tomek_ycomb ◴[] No.40731346[source]
Accelerometers and gyros are used and integrated to get their higher order information. However the trick is neither sensor type is perfect so you fuse as much data as needed to get close to good and correct for drifts

Kalman filters come up a lot, maybe relevant to the terms you're looking for

18. marginalia_nu ◴[] No.40733149[source]
The joke is that physicists aren't always rigorous enough to add that caveat.
19. ordu ◴[] No.40734568[source]
> Do these higher order derivatives say anything meaningful?

I'm becoming seasick due to a jerkiness of a car. Not due to a speed or a acceleration, but it is jerk that does it for me. I watched it from my childhood, I hated trolleybuses for that: they are electric and they tend to change acceleration instantly. But I didn't understood how it works until much later.

20. midjji ◴[] No.40735594[source]
Yes. The higher derivates are useful in many cases, both as sought properties and observations. The invariances implied by relativity,(the trivial notion that the universe behaves the same regardless of where you select the center), mean that most laws are defined on the second derivative. Taylor approximations are useful to approximate something locally, but properties of the system over wider regions generally need to account for the higher derivatives. You can see this in e.g. simulating a system over time requires that the derivatives at the borders of the valid taylor approximation region to be included as diracs.

Or in other words, you can approximate exp(x) as a set of first order taylor approximations that each covers a small window to arbitrary precision, but the combination of them is still has well defined higher derivatives that are not 0.

21. rcxdude ◴[] No.40737550{3}[source]
You can work it out from the principles of the engines (electric motor can produce their maximum torque at zero speed, combustion engines generally produce very little torque at zero speed, especially when modulated through a clutch or torque converter which you need in practice. And that's before looking at turbo-lag), but it's also something you can feel fairly viscerally: jerk is how quickly the feeling of acceleration changes. In a fast accelerating ICE car you might find yourself sinking more into the back of the seat as the car picks up speed. In an EV you basically just go from still to pressed all the way back, and then you start to pick up speed (and in fact the feeling of acceleration slackens off as the EV reaches a certain speed, though this will also happen with an ICE once they reach their peak acceleration point).