View Full Version : Is there a term for change of acceleration over time?

Floyt

21st June 2005, 04:43 PM

Simple question: just as acceleration describes a change of velocity over time, is there a term for a change of acceleration over time? Physicists being a parsimonious bunch, it feels like there ought to be, but I couldn't come up with one.

(this from a purely use-of-language discussion with no actual applications ;) )

gtc

21st June 2005, 04:49 PM

Jerk

Not you, the term

CBL4

21st June 2005, 04:58 PM

Mathematically it is known as the third derivative. I do not think there is a common word for it but apparently is has the not-so-common word "jerk." I am pretty sure this was never used in any of my math or physics classes where we called it change in acceleration.

If A is position and B is time, then the derivative of A with respect to B is velocity. The second derivative is the rate of change of velocity, or acceleration. The third derivative, the rate of change of acceleration, is called jerk.

When your car is not accelerating, you're not being pushed back in your seat at all. When your car is accelerating mightily, you're pushed against the back of your seat. The faster the transition between these two states, the higher the jerk.

In my experience, at least 99% of all uses of the word "jerk" (with this meaning) occur in sentences of the form, "Hey, did you know that the third time derivative of position is called jerk?" The remaining one percent of the time, the word is used seriously by automobile safety engineers and the like.

The fourth and higher time derivatives of position are not used often enough for there to be a serious established word for them. Snap has been proposed for the fourth derivative, naturally followed by crackle and pop for the fifth and sixth derivatives.

http://robotics.caltech.edu/~mason/ramblings/thirdDerivative.html

ETA: Damn, gtc beat me to it. The jerk. ;)

CBL

aerocontrols

21st June 2005, 05:15 PM

I taught what 'jerk' was when I taught Dynamics.

The most common use for it is as a measure of passenger comfort (http://www.google.com/search?as_q=jerk&num=30&hl=en&c2coff=1&btnG=Google+Search&as_epq=passenger+comfort).

IndigoRose

21st June 2005, 05:25 PM

It's called "rate of onset"

Originally posted by Floyt

Simple question: just as acceleration describes a change of velocity over time, is there a term for a change of acceleration over time? Physicists being a parsimonious bunch, it feels like there ought to be, but I couldn't come up with one.

(this from a purely use-of-language discussion with no actual applications ;) )

Floyt

21st June 2005, 07:14 PM

Originally posted by gtc

Jerk

Not you, the term

Whew, nearly changed that from "parsimonious" to "acrimonious" :)

Thanks guys! This makes my Intriguing Discovery of the Day.

*goes off in search of victim to join the 99%*

eta: Waaah! The proposed terms for the derivatives of force seem to be yank, tug, snatch, and shake! This is quite as cool as collective names for animals (exaltation of larks, anyone?)

tedly

21st June 2005, 08:15 PM

Originally posted by aerocontrols

I taught what 'jerk' was when I taught Dynamics.

The most common use for it is as a measure of passenger comfort (http://www.google.com/search?as_q=jerk&num=30&hl=en&c2coff=1&btnG=Google+Search&as_epq=passenger+comfort).

I learned about jerk when we were doing graphical design of camshafts* the followers tend to bounce off if the third derivative gets excessive. As I recall it also led to really weird shapes - like square corners.

* Camshaft was a rod that held irregularly shaped pieces of metal that were used to replace a simple actuator and hundreds of lines of code. Their brutal simplicity and extreme reliability meant that they forced unemployment on thousands of otherwise employable parts jockeys.

LostAngeles

21st June 2005, 08:30 PM

Originally posted by CBL4

Mathematically it is known as the third derivative. ...

CBL

Yeah, when I saw the thread title I immediately thought, "da/dt. Third derivative."

casebro

23rd June 2005, 08:10 AM

Is "Delta V" on the wrong astral plane? Written as a triangle and the letter v, but I'm not gonna go through the mental gymnastics required to make a PC type a triangle. The symbol Delta means change, V for velocity.

Ladewig

23rd June 2005, 08:16 AM

Jerk was one of the (very) few things I remember from physics class.

So what is change of jerk over time called?

roger

23rd June 2005, 08:21 AM

Off?

davidhorman

23rd June 2005, 11:07 AM

I think it's jolt - at least that's what I remember from a documentary on rollercoasters. After that might have been jostle, but these might be informal.

Edit: okay, maybe jolt is just the British name for jerk. And "jounce" might be the next one along. But I'm sure the documentary went velocity, acceleration, jerk, jolt, jounce.

David

Rob Lister

23rd June 2005, 11:31 AM

Originally posted by Ladewig

Jerk was one of the (very) few things I remember from physics class.

So what is change of jerk over time called?

The 4th, 5th, and 6th derivatives are snap, crackle, and pop.

ceptimus

24th June 2005, 04:26 AM

I've seen the term surge used in a few books.

It fits in well with normal language, "A surge of acceleration".

I don't think it's an official scientific term, but it is sometimes used by some scientists to refer to the third derivative of position.

Soapy Sam

26th June 2005, 04:11 AM

...and the nth would be shoogle?

The Glasgow Subway is famous for it's shoogle.

Badly Shaved Monkey

26th June 2005, 05:53 AM

Originally posted by Soapy Sam

...and the nth would be shoogle?

The Glasgow Subway is famous for it's shoogle.

In case anyone needs a mental picture to visualise this motion: imagine the movement of a battered Mars Bar vigorously frying in hot lard.

I hope that is helpful. :)

Just thinking

26th June 2005, 05:40 PM

Originally posted by Soapy Sam

...and the nth would be shoogle?

Believe it or not, all these extraneous terms (acceleration to jerk to 'whatever' and so on) are displayed in simple periodic motion, such as in a swinging pendulum. As the pendulum swings to a fro the displacement, velocity, acceleration, jerk, etc. (to infinity), are all changing over time. And in such simple looking common everyday motion.

BTW ... if momentum comes from velocity (p = mv)

........... and force from acceleration (f = ma)

........... just what comes from a jerk? (? = mj)

LostAngeles

26th June 2005, 05:56 PM

Originally posted by Just thinking

Believe it or not, all these extraneous terms (acceleration to jerk to 'whatever' and so on) are displayed in simple periodic motion, such as in a swinging pendulum. As the pendulum swings to a fro the displacement, velocity, acceleration, jerk, etc. (to infinity), are all changing over time. And in such simple looking common everyday motion.

BTW ... if momentum comes from velocity (p = mv)

........... and force from acceleration (f = ma)

........... just what comes from a jerk? (? = mj)

Essence, headaches...

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