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I Numerical Integration twice (acceleration to displacement)

  1. Nov 17, 2017 #1
    Hello everyone

    I have the following question regarding numerical integration twice from acceleration to displacement.

    Suppose that a particle has acceleration function of a = tt (which has non-elementary integral), to find the velocity it is easy as I can use Simpson's rule for numerical integration. But now I would like to find the displacement of the particle after some time, let say 5 seconds, how do I calculate it?

    Note that I need very accurate answer, so I usually integrate numerically using Excel with thousand interval.

    I prefer to use Newton-Cotes formula if possible.

    EDIT
    I tried using integration by parts, but I always reach a point where I can't continue.
    a = tt
    Let v = ∫ tt
    s = ∫ v
    = t*v - ∫ t*tt
    = t ∫ tt - ∫ t*tt

    For example, to find the displacement after t = 5 seconds
    ∫ t*tt from 0 to 5 is easy to integrate numerically
    But the problem is t ∫ tt I don't know how to do it because there is variable outside the integral.

    For those who don't know, I am using integration by parts formula for definite integral, it includes find the value of
    ( t ∫ tt ) from 0 to 5
     
    Last edited: Nov 17, 2017
  2. jcsd
  3. Nov 17, 2017 #2

    BvU

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    Hello pcmath, : welcome:

    You have ## \ a = t^t\ ## and with ##\ a = \displaystyle {dv\over dt} \ ## you get ##\ v = \int a\;dt\ = \int t^t\; dt##.

    Then , with ##\ v= \displaystyle {ds\over dt} \ ## you can write ##\ s = \int \left (\ \int t^t\; dt\ \right ) dt' ##. Written with bounds:$$
    s(t) = \int_{t'= 0}^t \left (\ \int_{t''=0}^{t' } t''^{(t'')} \; dt''\ \right ) dt' $$
    That is your double integral.
    is something I cannot agree with.

    You could do a double numerical integration using forward Euler
    $$ v(t+\Delta t) = v(t) + a(t) * \Delta t \\ s(t+\Delta t) = s(t) + v(t) * \Delta t \ $$ and you would indeed have to take very small steps because a(t) is so steep.
     
    Last edited: Nov 17, 2017
  4. Nov 17, 2017 #3

    mfb

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    If you just need this once do it with tiny integration steps (or check WolframAlpha). If you need it many times, it is worth implementing a better integration scheme - Verlet integration or one of the other methods discussed there.
     
  5. Nov 18, 2017 #4
    Thank you guys for your help, I will try it.

    This is actually correct as I have tried it. Assuming that a = x^2 then you can find s using that way, it gives the same value as integrate twice.
     
  6. Nov 18, 2017 #5

    BvU

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    Note that one successful example does not a proof constitute. It looks like integration by parts and then the question is: what is the time derivative of ##t^t##
     
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