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Homework Help: Pendulum period

  1. Mar 18, 2007 #1
    I wanted to calculate the period of a pendulum withould the small angle approximation. I carried out the calculation and came out with a rather ugly integration which was the same as the one wikipedia had so I assume I did it right. The answer I got was:

    T=4[root of(L/2g)]*integral of{1/[root of(cos@-cos@initial)] from 0 to @initial}

    However when I performed a numerical integration with my calculator, the answer I got was completely absurd. Any help?
    Last edited: Mar 18, 2007
  2. jcsd
  3. Mar 18, 2007 #2
    the integral should be correct. However, if you use the small angle approximation, the [itex]\cos\theta[/itex] should be gone!

    notice that for small angle,
    [tex]\cos\theta \approx 1-\frac{\theta^2}{2}[/tex]

    you'll get a nice integral.

    *notice that the integral is improper. that is, at theta=initial angle, the integrand goes to infinity. so some calculator will give you weird answers.
    Last edited: Mar 18, 2007
  4. Mar 18, 2007 #3
    but I just wanted to do it without the small angle.
    I'm just trying to figure out why I'm not getting a reasonable answer for the numerical integration.
  5. Mar 18, 2007 #4
    for a 5m string, I'm getting 258.3s
  6. Mar 18, 2007 #5
    I edited my first post. the integral is improper, some calculators give you non-sense answer for improper integrals. if you want more info, look up elliptic integral, there are some nice infos on mathworld.com

  7. Mar 18, 2007 #6


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    Big, big, big problem! Without the "small" angle restriction, a pendulum may not even have a "period"!! It is theoretically possible to place a pendulum directly upward, where it is balanced, wait for some tiny ripple of air to "knock it over" and have it come right back up to a balance again.

    I don't know how to help you because I don't know:
    1) What initial angle you used
    2) What period you got
    3) Why you think it is "absurd"
  8. Mar 18, 2007 #7
    I see, I'm only in a first year of calculus so I dont know all this stuff. I know that there is some sort of a series to do this, but how would one normally carry out this calculation?
  9. Mar 18, 2007 #8
    Well, I did the calculation with a 15 degree angle and 5 meters length and got a 258s period. By the way I only did a small angle to test it--to compare to 2pi*root of (L/g)
  10. Mar 18, 2007 #9
    One would normally use elliptic integrals. See the MathWorld page mentioned above for the mathematical intricacies. You can probably understand the page with first-year calculus, but it will take a lot of effort on your part.
  11. Mar 18, 2007 #10
    Well uh-oh.

    This is interesting--I felt like with my knowledge of calculus I could solve any summation problem like this, but it appears that I was quite wrong. Kind of frustrating really.
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