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What is Jerk and Jounce conceptually?

  1. Oct 12, 2013 #1
    Hello, I was doing some research on differentiating kinematics. For example, differentiating displacement will give you velocity, and differentiating velocity gives you acceleration.

    When you differentiate acceleration apparently you get something called "jerk" (wow what a jerk). And when you differentiate jerk, you get "jounce".

    Can someone explain to me what jerk and jounce are in like real life? What are some examples of each and how do they work?
     
  2. jcsd
  3. Oct 12, 2013 #2

    Drakkith

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    To quote wiki:

    Jerk

    800px-Jerk_Diagram.png

    Jounce

    I don't know how you would explain Jounce other than by math.
    Perhaps Jounce is the act of changing the application of the jerk? IE if you slowly start applying a force (small jerk) and then increase the rate at which you apply more force, getting a larger jerk, perhaps that itself is Jounce?

    I must say it was interesting to read up on this. Starting with a change in position over time you get velocity, then a change in velocity over time gives you acceleration, then a change in acceleration over time gives you jerk, then a change in jerk over time gives you jounce.
     
    Last edited: Oct 12, 2013
  4. Oct 12, 2013 #3
    wow

    I read some more and change in jounce is called crackle and change in crackle is called pop? (on the jounce wikipedia page) I really want to learn more about these lol.

    What if you integrated position? What happens then?
     
  5. Oct 12, 2013 #4

    Simon Bridge

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    Integrating the position is finding the area under the position-time graph.
    It has units of meter-seconds ... which do come up sometimes. It's called the "abasement".
    https://www.physicsforums.com/showthread.php?t=185121

    Take care: just because it makes sense mathematically does not mean it makes sense in physics.
    Best to start with the physics and then get the math.
     
  6. Oct 13, 2013 #5

    Drakkith

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    Only jokingly, lol.
     
  7. Oct 13, 2013 #6

    Simon Bridge

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    Heard another version of that joke in lectures, where the pattern went: jerk -> heel -> schmuck...
     
  8. Oct 13, 2013 #7
    Railroad tracks and roads ( not always ) are designed to enter from a straight section into a curve with a transition called a clothoid where the radial acceleration eases from 0 to v^2/R, in which case R is a variable from the entrance into the curve to the value at the circular curve. Thus the change in acceleration radial a is not abrupt, which would be a discomfort for passengers and could throw the cars and engine off the track. What the designers are attempting to do is produce a gradual jerk and constant jounce, which gives a smooth increase in radial acceleration. ( Or is it zero jounce, constant jerk, and linear increase in radial accleration )

    It is applied, or should be, for other transitions in curvature for machines, roller coasters in certain critical curve transitions, aircraft maneuvres most likely though I don`t know how it is applied in all cases but you don;t want to be in an aircraft enjoying your hor coffee and have it spill on you as the pilot banks a turn.

    http://en.wikipedia.org/wiki/Clothoid
    http://en.wikipedia.org/wiki/Track_transition_curve
    http://www.europakorridoren.se/spargeometri.pdf

    the last one has some interesting railway info
     
  9. Oct 13, 2013 #8
    It’s not possible to go from zero velocity to some other velocity without acceleration.

    Well, the same applies to acceleration. To get to an acceleration starting from zero acceleration, there is a change in acceleration, or jerk.

    But getting from zero jerk to some other value of jerk is a change in jerk, or jounce.

    And so it goes...

    Here is a list from my notes. Note that jounce and snap are the same. Don’t have the reference form where it came from, but wouldn’t be surprised if it came from Wiki (sorry).

    absounce: time integral of abserk
    abserk: time integral of abseleration
    abseleration: time integral of absity
    absit:y time integral of absement
    absement (absition): time integral of position

    position (displacement position

    velocity: rate-of-change of position
    acceleration: rate of change of velocity
    jerk: rate of change of acceleration
    jounce (snap): rate of change of jerk
    crackle: rate of change of jounce
    pop: rate of change of crackle
    lock: rate of change of pop
    drop: rate of change of lock
     
  10. Oct 13, 2013 #9

    HallsofIvy

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    And I suppose that change in "crackle" is "snap"?

    The fact is that we can feel force directly. And force is proportional to acceleration. Other derivatives (include speed) are felt only indirectly.)
     
  11. Oct 13, 2013 #10
    Rate of change of "snap" is "crackle".

    I don't get the point you are making.

    Say we find ourselves in an elevator being pulled by a constant acceleration. If the bottom of our elevator is hit by a much faster object and we feel it, then we have felt the jerk (and perhaps other derivatives of position) directly.

    Why do you want us to think of this as only indirectly?
     
  12. Oct 14, 2013 #11
    The higher derivatives of acceleration are used by mechanical engineers in the design of cam shafts. If you think about a normal combustion engine, it would like to maximise the area of the valve opening. At the top and bottom of the cam lobe the acceleration is changing direction very rapidly, this can lead to float and bounce of the valve.
     
  13. Sep 16, 2015 #12
    I think we can feel jerk and jounce clearly when we ride a Pirates' Ship (or Viking's ride) at an amusement park. Actually we feel jerk every day. Simply riding a car on a rough road generates change of acceleration as vibration. Your bad ride is caused by the jerk. But when you ride a Viking Ship at the park, you will feel the jounce. When the swing reaches the peak you will feel zero gravity and at the bottom you feel max cetrifugal force. The swings speed changes cause change of centrifugal force. The jerk is there but there is also change of jerk because at each location along the swing's path the feel of gravity changes. The combined effect you feel is the jouce. It is a unique feeling your body's control system very rarely experiences. And you get excitement. This may be the reason why my kid likes the swing !
     
  14. Oct 3, 2015 #13

    bcrowell

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    Acceleration without jerk is just a static load, and therefore it can never cause vibration. E.g., in a machine shop, you can damage your mill or your work if the setup starts vibrating too hard. This can only happen because of jerk.
     
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