A physics term called "Jerk"

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A physics term called "Jerk"

Postby misho » Thu Nov 08, 2018 2:58 pm

I am studying/researching instrumentation of the NASA's Orion spacecraft, and one of the displays shows readouts for H and V1 in a small Cartesian coordinates diagram, where V1 is the x axis, and H is the y axis. I am fairly certain this represents speeds - V1 being the orbital target (escape) speed, and H being the vertical speed (during a rocket launch V1 is 0, and at orbit insertion, H is zero, assuming circular orbit ) Above it are numerical readouts for H, Ḣ (H dot) and Ḧ (H dot dot)

H and H dot are vertical speed and vertical acceleration, but what is the purpose H dot dot, presumably a derivative of vertical acceleration? I looked it up, and the term for the derivative of acceleration is "Jerk". What is the purpose of this value?
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Re: A physics term called "Jerk"

Postby Julio Jerez » Thu Nov 08, 2018 3:14 pm

I would doubt very much they use derivative of an acceleration, H must be an angle or a position.
Jerk I support is a change of speed over a time, (an impulse)
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Re: A physics term called "Jerk"

Postby misho » Thu Nov 08, 2018 4:08 pm

Hi Julio!

Ok, here's the image of the portion of the display.
Image

The bottom nomenclature, it seems to me, identifies H as a vertical speed, and it shows the ascent curve. The ranges for H dot are +/-3000, and H dot dot +/- 20...

Unless... H was "height" (altitude)... then H dot would be speed, and H dot dot acceleration. That would make more sense...

Jerk I support is a change of speed over a time, (an impulse)


Jerk, as per wikipedia, is the rate of change of acceleration, which is not what an impulse is...
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Re: A physics term called "Jerk"

Postby Julio Jerez » Thu Nov 08, 2018 4:41 pm

well I never heard of that term.
in general people give names to physical parameter that are measurable, acceleration can be variable, but is a hard to measure quantity, I heard of accelerometers, but they in fact measure impulse, not acceleration.

I should say unit impulse, because is speed changes,
the rest is done but a the instrument the measure the time at which the speed is sampled.
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Re: A physics term called "Jerk"

Postby misho » Thu Nov 08, 2018 4:56 pm

Julio Jerez wrote:well I never heard of that term.
in general people give names to physical parameter that are measurable, acceleration can be variable, but is a hard to measure quantity, I heard of accelerometers, but they in fact measure impulse, not acceleration.


I know - and I was surprised as well. However, in engineering and physics terminology, it IS a real term (I'm a mechanical engineer and I never heard of it)...I thought someone was "jerking" me around :mrgreen:

Moreover, the FOURTH derivative of position is called "Jounce" (sometimes called "Snap") :roll:
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Re: A physics term called "Jerk"

Postby Julio Jerez » Thu Nov 08, 2018 5:14 pm

well yes derivatives can be calculate for any quantity that changes with respect to any other quantity that's just calculus, in physics the important part is when those derivative mean something mearurable.
take for example the ratio of the lateral speed of a vehicle to its longitudinal speed, this is call slip angle, and it would not mean anything if it was not measurable, but it turn out that the derivative of the quatity, call slip rate, is what stability controll for vehicle use the prevent car from tail spin.

similarly, the acceleration is the result of the sum of eternal forces, it is measurable,
what quatitie can be used to measure the change of rate of the acceleration other that the numerical method of finate differences.
this is where math and physic go separate ways.
some time you formulate an equation based on quantities that you can measure, and when you run it true the math, you get a range of solutions that some time are valid but some time has not physical meaning.

maybe it does mean something and has application, I just never heard of it.
btw I to am a mechanical engineer. but I got my master on analytical mechanics.
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Re: A physics term called "Jerk"

Postby misho » Thu Nov 08, 2018 6:28 pm

Julio Jerez wrote:maybe it does mean something and has application, I just never heard of it.


Yeah, same here. Perhaps there are some practical applications to this - automotive airbags come to mind... say, they don't deploy during hard braking (decceleration) but do deploy on impact (jerk/jolt)...
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Re: A physics term called "Jerk"

Postby misho » Thu Nov 08, 2018 7:46 pm

Julio Jerez wrote:btw I to am a mechanical engineer. but I got my master on analytical mechanics.


AND Comp Sci degree?? Wow!
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Re: A physics term called "Jerk"

Postby Julio Jerez » Thu Nov 08, 2018 7:59 pm

No, I am not a Comp science degree.
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Re: A physics term called "Jerk"

Postby Leadwerks » Sun Dec 02, 2018 1:09 am

I actually learned this in my biology degree. :)
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Re: A physics term called "Jerk"

Postby FSA » Sun Dec 02, 2018 4:16 pm

Just some addition:

I agree with Julio. We have that term in the automotive industry (in German its called "Ruck", meaning something like a rapid change of acceleration). The second derivative of speed is used to calculate the possibility to need an airbag during a crash, where the derivative of two independent accelerations (linear and radial) get considered the most for the calculation.
In everyday live this analogy is easy to understand.
If you start to brake your car slowly, the jerk is almost zero and you dont feel the start of the brake progress at all. But if you slam your brakes directly to the ground, the jerk would be extremly high for a short time.

EDIT: Just saw the image in the wikipedia link provided by you :roll: So just ignore this image...

Image
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Re: A physics term called "Jerk"

Postby JoeJ » Mon Dec 03, 2018 8:04 pm

Ha, this is what i'm using for almost all control problems. I did not know it has such a beautiful name :mrgreen:
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