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Homework Help: Physical pendulum with air friction

  1. Jan 7, 2008 #1
    1. The problem statement, all variables and given/known data

    For my control system course i need to derive differential equation and period of a physical pendulum.
    Pendulum rod's mass is m, length is l and has a spherical mass M

    2. Relevant equations

    i need relevent equatins so obviously :D

    3. The attempt at a solution

    i know that i should use stoke's equation for air firicition but i couldnt find out how.
    in some posts period of a pysical pendulum is given but i need to solve max. angle

    Thank andd sorry about my crappy englisf.İ hope i can define my question
  2. jcsd
  3. Jan 7, 2008 #2

    Learn this first, then worry about the air friction, if you're in a course that's apparently wanting you to use an advanced differential equation and you're asking for pendulum equations, you probably need to do some reading
  4. Jan 7, 2008 #3
    i know the pyhsical pendulum. İ do not need to solve differantial equation.İ just need to derive it.İ find a lot of sources about it but none of them include air friction.
    and thaks for the reply
  5. Jan 7, 2008 #4
  6. Jan 7, 2008 #5


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    You probably don't have to go back to "Navier-Stokes". Generally speaking, friction can be modeled by [itex]-k dy/dx[/itex] or [itex]-k(d^2y/dx^2)[/itex]. Which do you think is approriate here?
  7. Jan 7, 2008 #6
    i think -k.v is more approriate for my case
  8. Jan 7, 2008 #7
    If it's a control systems course I wouldn't put it past an engineering demon professor to have required it with navier stokes. But yah, if they just said model air friction, throwing in a velocity or velocity squared dependent force makes it plenty difficult I'd think
  9. Jan 7, 2008 #8

    This is the differential equation of a physical pendulum with no air friction.
    In my case there will be a force because of friction and it will be -k.v

    My problem is how to include this force to the dif. equ. and what will be the k?

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