1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor

    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?

    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook